Background Statement for SEMI Draft Document 5409 NEW...

Background Statement for SEMI Draft Document 5409 NEW STANDARD: GUIDE FOR METROLOGY FOR MEASURING THICKNESS, TOTAL THICKNESS VARIATION (TTV), BOW, WARP/SORI, AND FLATNESS OF BONDED WAFER STACKS

Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in

reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.

Notice: Recipients of this Document are invited to submit, with their comments, notification of any relevant

patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this

context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the

latter case, only publicly available information on the contents of the patent application is to be provided.

Background

This Guide, which is targeted at the 3DS-IC community, is intended to assist in the selection and use of Bonded

Wafer Stack (BWS) metrology tools and to provide guidance in performing BWS measurements such as total

thickness variation, bow, warp, and sori. In addition, the Guide provides examples of BWS measurements for these

metrology tools and can assist wafer producers and users of BWS metrology to develop products and conduct

meaningful evaluations.

This document was developed in the Inspection and Metrology TF of the N.A. 3DS-IC Committee. The SNARF for

this was approved April 3, 2012. Draft Document 5409 was approved for yellow ballot in Cycle 1 in CY2013, by

the 3DS-IC Global Coordinating Subcommittee (GCS).

Review and Adjudication Information

Task Force Review Committee Adjudication

Group: Inspection & Metrology Task Force North America 3DS-IC Committee

Date: April 2, 2013 April 2, 2013

Time & Time zone: 8:00 AM to 10:00 AM, Pacific Time 3:00 PM to 5:00 PM, Pacific Time

Location: SEMI Headquarters SEMI Headquarters

City, State/Country: San Jose, California San Jose, California

Leader(s): Victor Vartanian (SEMATECH)

David Read (NIST)

Yi-Shao Lai (ASE)

Urmi Ray (Qualcomm)

Sesh Ramaswami (Applied Materials)

Richard Allen (SEMATECH)

Chris Moore (Semilab)

Standards Staff: Paul Trio (SEMI NA)

408.943.7041 /[email protected]

Paul Trio (SEMI NA)

408.943.7041 / [email protected]

This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact

Standards staff for confirmation.

Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will

not be able to attend these meetings in person but would like to participate by telephone/web, please contact

Standards staff.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.

Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document

development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

Page 1 Doc. 5409 SEMI

Semiconductor Equipment and Materials International

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LETTER (YELLOW) BALLOT

DRAFT

Document Number: 5409

Date: 12/13/2012

SEMI Draft Document 5409 NEW STANDARD: GUIDE FOR METROLOGY FOR MEASURING THICKNESS, TOTAL THICKNESS VARIATION (TTV), BOW, WARP/SORI, AND FLATNESS OF BONDED WAFER STACKS

1 Purpose

1.1 Control of parameters, such as bonded wafer stack (BWS) thickness, total thickness variation (TTV), bow,

warp/sori, and flatness metrology, is essential to successful implementation of a wafer bonding process. These

parameters provide meaningful information about the quality of the wafer thinning process (if used), the uniformity

of the bonding process, and the amount of deformation induced to the wafer stack by the bonding process. Total

thickness variation is also critical in certain bonded wafer manufacturing process steps, since non-planarity can lead

to problems in subsequent processing steps, including lithographic overlay and intermittent electrical contact

between metal layers in the bonded wafers. This Guide provides a description of tools that can be used to determine

these key parameters before, during, and after the process steps involved in wafer bonding.

2 Scope

2.1 This Guide provides examples of the capabilities and limitations of various measurement technologies

applicable to bonded wafer stacks (BWS) as well as their suitability for different applications.

2.2 The Guide describes metrology techniques that are applicable to both temporary and permanently bonded wafer

stacks.

2.3 This Guide is complementary to existing SEMI Test Methods for measuring these parameters on single wafers,

in some cases extending existing metrology techniques to a bonded wafer stack and in other cases describing

metrology techniques specific to a bonded wafer stack.

2.4 The Guide focuses on general measurement techniques including IR laser profiling, white light confocal

microscopy, visible and IR interferometry, capacitance, and back-pressure metrology. Each technology has unique

strengths and weaknesses—some rely on front-side illumination, others on back-side illumination. Some techniques

can measure the thicknesses of individual layers in the bonded wafer stack, and some are additionally capable of

measuring surface nanotopography.

2.5 The metrology examples provided in this Guide originated from industry experts and are believed to be

representative of tool performance as of the year 2012. However, as tool and measurement techniques continue to

evolve and improve, BWS measurement performance may surpass what is contained in this Guide. The user should

investigate metrology suppliers’ current capabilities.

2.6 The measurements described in this Guide are on bonded wafer stacks with thickness in the range of 50 to 1550

µm.

2.7 The stacks considered include carrier and device wafers and bonding layers, including cases where there are

more than two wafers in a stack. Bonded wafers may be classified as either temporarily bonded (i.e. a device to a

carrier wafer) or permanently bonded. Temporary bonding uses a temporary adhesive; permanent bonding could be

adhesive, oxide, metal-metal (e.g. Cu-Cu), or hybrid bonding. Two representative two-wafer stacks are depicted in

Figures 1 and 2. The first stack (Figure 1) is a bonded pair of 775 µm thick wafers following TSV formation and the

bonding operation. The second stack (Figure 2) is a bonded wafer stack with a top wafer thinned to ~50 µm, and

bonded on top of a 775 µm wafer using a temporary adhesive.






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Figure 1

Bonded Wafer Pair Following TSV Formation and Wafer Bonding Operation

Figure 2

Temporarily Bonded Wafer Pair--Device Wafer is Edge-Trimmed and Thinned

NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their

use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and

determine the applicability of regulatory or other limitations prior to use.

3 Limitations

3.1 While this Guide provides an overview on the use of several available techniques, it does not provide the level

of detail typically available in Test Methods. For this purpose, SEMI documents are referenced that the user may

find of interest. In addition, all suppliers may not follow SEMI standard measurement methods in their metrology

procedures.

3.2 The information in this guide does not encompass the establishment of specific a methodology for such

measurements but is only a guide to the user describing the various types of metrology including their capabilities

and limitations.

3.3 This Guide describes metrologies that have been applied to a wafer stack composed of, at most, two wafers.

Thus, the user is cautioned that the methods described may be limited with applied to wafer stacks consisting of

more than two wafers.

4 Referenced Standards and Documents

4.1 SEMI Standards and Safety Guidelines

SEMI M23 — Specification for Polished Monocrystalline Indium Phosphide Wafers

SEMI M43 — Guide for Reporting Wafer Nanotopography

SEMI M49 — Guide for Specifying Geometry Measurement Systems for Silicon Wafers for the 130 nm to 22 nm

Technology Generations

SEMI M59 — Terminology for Silicon Technology

SEMI M65 — Specifications for Sapphire Substrates to Use for Compound Semiconductor Epitaxial Wafers

SEMI MF533 — Test Method for Thickness and Thickness Variation of Si Wafers

SEMI MF534 — Test Method for Bow of Silicon Wafers






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SEMI MF657 — Standard Test Method for Measuring Warp and Total Thickness Variation on Silicon Slices and

Wafers by a Non-contact Scanning Method

SEMI MF1390 — Standard Test Method for Measuring Warp on Silicon Wafers by Automated Non-Contact

Scanning

SEMI MF1451 — Test Method for Measuring Sori on Silicon Wafers by Automated Non-Contact Scanning

SEMI MF1530 — Standard test method for Measuring Flatness, Thickness, and Thickness Variation on Silicon

Wafers by Automated Non-Contact Scanning

NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.

5 Terminology

NOTE 1: Refer to the SEMI Standards Compilation of Terms (COTs) for a list of the current Abbreviations, Acronyms,

Definitions, and Symbols.

5.1 Abbreviations and Acronyms

5.1.1 AM — acoustic microscopy

5.1.2 BWS — bonded wafer stack

5.1.3 CD — critical dimension

5.1.4 CWS — chromatic white light sensor

5.1.5 CSI — coherence scanning interferometry

5.1.6 FAMM — focus/acquire/measure/move

5.1.7 FPD — focal plane deviation

5.1.8 HVM — high volume manufacturing

5.1.9 IR — infra-red

5.1.10 ISO — international organization for standardization

5.1.11 RPD — reference plane deviation

5.1.12 SLED — superluminescent light emitting diode

5.1.13 TIR — total indicated runout

5.1.14 TS — through-silicon via

5.1.15 TTV — total thickness variation

5.1.16 WLI — white light interferometry

5.2 Bonded Wafer Metrology Overview

5.2.1 explanation of wafer metrology terms1 — the parameters primarily measured, wafer bow, warp, and sori need

some explanation. Figure 3 shows schematic diagrams representing these terms.

5.2.1.1 bow — the deviation of the center point of the median surface of a free, unclamped wafer from a median

surface reference plane established by three points equally spaced on a circle with diameter a specified amount less

than the nominal diameter of the wafer; SEMI MF23 provides a standard test method for measuring the bow of a

single wafer. Note that bow is a signed value. The method can be adapted to a bonded wafer stack.

5.2.1.2 warp — the difference between the most positive and most negative distances of the median surface of a

free, unclamped wafer from a reference plane; SEMI M59 provides a standard test method for measuring the warp

of a single wafer. Warp can be zero, even for a wafer with curvature, if the curves are mirror images of each other.

SEMI MF657 measures median surface warp using a three-point back-surface reference plane, resulting in thickness

variation being included in the warp value. The use of a median surface reference plane in SEMI MF1390

1 SEMI International Standards: Compilation of Terms (COT)






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eliminates this effect. SEMI MF1390 provides a standard test method for measuring the warp of a single wafer.

The method can be adapted to a bonded wafer stack.

5.2.1.3 sori — the algebraic difference between the most positive and most negative deviations of the front surface

of a wafer that is not chucked from a reference plane that is a least squares fit to the front surface within the fixed

quality area.

5.2.1.4 flatness — the deviation of the front surface, expressed in TIR or maximum FPD relative to a specified

reference plane when the back surface of the wafer is ideally flat, as when pulled down by a vacuum onto an ideally

flat chuck.

5.2.1.5 non-contact — metrology that allows a wafer to be measured without physical contact to the wafer surface,

preventing contamination or damage to the wafer substrate.

5.2.1.6 chromatic white light sensor — a chromatic white light sensor (CWS) is based on the principal of confocal

optics and relies on chromatic scanning. A lens is used that refracts white light differentially based on its

wavelength in order to carry out distance measurements. Resolution depends on the intensity of reflected light.

5.2.1.7 interferometry — a technique that relies on the principal of superposition of multiple beams of light to

determine the effect that a material has on the state (phase and amplitude) of the original light beam. It is this

introduced phase difference that creates the interference pattern between the initially identical waves. If a single

beam has been split along two paths, then the phase difference is diagnostic of anything that changes the phase along

the paths. This could be a physical change in the path length itself or a change in the refractive index along the path.

5.2.1.8 gravity compensation for horizontally-supported wafers — unlike thickness and TTV measurements which

are independent of how the wafer is held, bow and warp measurements are complicated not only by wafer stress but

by how the wafer is supported and by gravity. Gravity causes substantial deformation of large diameter or thin

wafers, whether they are supported at the edge or in the middle. Unless compensated, gravity will induce a large

error in warp measurements. SEMI MF1390 describes three compensation approaches. One approach is to correct

for the gravitational effect of warp measurements by inverting the wafer and measuring both the top and bottom

surfaces of the wafer. Any differences between two values at the same site are due to the effect of gravity and can

be used to correct for single-side measurements. Another approach is to use an analytical expression for

gravitational deformation and subtract it from a single-side warp measurement2. Measurements obtained on

representative wafers can also be performed and the gravity value determined.

5.2.1.8.1 In addition, the wafer can either rest on a chuck or be supported by three points (MF657).

5.2.1.9 gravity compensation for vertically supported-wafers — on tools that support the wafer vertically, effects

due to gravity are negligible and therefore do not require compensation. Bow is only measured on one side so there

is no need to invert the wafer. The manufacturing tolerance for sori is very low, such that a 300 mm diameter

silicon wafer must achieve sori in the nanometer range in order to achieve the maximum yields in semiconductor

device processing.

Figure 3

Wafer Warp, Bow, and Sori Depictions2

2 W. R. Runyan, T. J. Schaffner, “Semiconductor Measurements and Instrumentation,” McGraw-Hill, 1998, pp. 218-221.






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5.3 Measurement Considerations

5.3.1 thickness and total thickness variation (SEMI MF533) — Wafers that are excessively thin may break during

processing or have inadequate thermal mass or electrical resistance during certain processing steps. Wafers that are

too thick may jam during wafer handling. Excessive thickness variation may encounter problems during mechanical

handling. Excessive deviation from flatness may cause focus problems in photolithography steps.

5.3.2 warp (SEMI MF1390) — Warp can adversely affect the yield of semiconductor devices and can affect wafer

handling and processing. Warp may be caused by unequal stresses on the two exposed surfaces of a wafer. Warp

cannot be determined from measurements on a single exposed surface. The median surface may contain regions

with upward or downward curvature or both. In some cases, the median surface may be flat. In all cases, warp is a

zero or positive quantity. Warp is a measure of the distortion of a median surface of a wafer. SEMI MF657

measures median surface warp using a three-point back-surface reference plane. The back-surface reference plane

results in thickness variation being included in the reported warp value. The use of a median surface reference plane

in SEMI MF1390 eliminates this effect.

5.3.3 bow (SEMI MF534) — If the median surface of a free, unclamped wafer has a curvature that is everywhere

the same, bow is a measure of its concave (dished) or convex (mounded) deformation, independent of any thickness

variation that may be present. Measurement of a semiconductor wafer, the deviation of the center point of the

median surface of a free, unclamped wafer from a median-surface reference plane established by three

hemispherical points equally spaced on a circle with diameter a specified amount less than the nominal diameter of

the wafer.

5.3.3.1 sori (SEMI MF1451) — Sori solves the ambiguity of warp by measuring the difference between the

maximum and minimum distances from the front surface of the wafer to a reference plane outside the surface of the

wafer. SEMI documents MF59, M65, and MF1451 provide a standard test method for measuring the sori of a single

wafer. A wafer is supported on a small-area chuck with the front surface facing up. Both external surfaces of a

wafer are simultaneously measured by an opposed set of probes to obtain a set of values at the same x and y

coordinates of the distances between each surface and the nearest probe. The paired distances are used to construct

the median surface. Gravity correction can be performed. One half of the thickness at each point is added to the

corrected median surface to construct the corrected front surface. A least-squares reference plane is constructed

from the corrected front surface. The reference plane deviation (RPD) is calculated at each pair of measurement

points. Sori is then reported as the algebraic difference between the most positive RPD and the most negative RPD.

6 Bonded Wafer Metrology Techniques

6.1 Optical Techniques

6.1.1 Chromatic White Light

6.1.1.1 Chromatic white light sensor metrology utilizes the principle of wavelength-dependent focal length to

determine distance (Figure 4). The spectrum of light reflected from a surface generates an intensity profile as a

function of focal length that is used to determine distance to the sample surface. The peak in intensity occurs at the

optimal focal point for each wavelength. This technique is useful for determination of 2D profile, 3D topography,

planarity, roughness, and wafer contour (bow and warp). Figure 5 shows a typical set of measurement locations

used in an analysis. The resulting 2D measurement profiles for thickness, TTV, and warp for an 825 µm thick

bonded wafer are shown in Figure 6, and a wafer thickness contour map is shown in Figure 7.






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Figure 4

Chromatic White Light Sensor Used for Distance Measurements for Bow, Warp, and Topographical

Measurements

NOTE: Focal lengths are wavelength dependent, providing distance measurements.

Figure 5

Data Collection Locations Showing Profile (a-d), and Points (A-I)






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Figure 6

Profile Measurements for Thickness, TTV, and Warp for 825 µm Thin Bonded Pair Wafer

Figure 7

Wafer Map Results for 825 µm Thin Bonded Pair Wafer

6.1.2 Infra-red Interferometry

6.1.2.1 Interferometry is a technique in which light waves are superimposed to obtain information on surface

profiles or displacements between two surfaces. Infra-red interferometry is useful for thickness measurements of

substrates that are transparent in near IR light. Thickness measurements can be performed on single or multi-layer

films, including adhesion layers in BWS. At each film interface, some incident light is reflected back to a detector.

6.1.2.2 When two light waves combine, the resulting pattern is determined by the phase difference between the two

waves—waves that are in phase undergo constructive interference while waves that are out-of-phase undergo

destructive interference. Typically, a beam of coherent light is split into two beams by a beam splitter. Each beam

travels a different path and the two are recombined before arriving at the detector. The path difference, the

difference in the distance traveled by each beam, creates a phase difference between them.

6.1.2.3 For a thin transparent medium, the reflection from the top surface interferes with the reflection from the

bottom surface. For some wavelengths, the interference is constructive, and for others it is destructive. In general,

the reflected intensity follows the Fabry-Perot equation (eq. 1):






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� � � ��δ

��δ�� (1)

6.1.2.4 Where r1 is the reflection coefficient at the first surface of the cavity, r2 is the reflection coefficient at the

second surface, and δ = 2 π n l / λ, n is the index of refraction, l is the cavity thickness, and λ is the wavelength.

Because of the complex exponential, the reflected intensity is periodic in δ. Since δ is proportional to optical

frequency (c / λ), the reflected intensity is periodic in optical frequency with period c / 2nl, where n is the index of

refraction, l is the layer thickness, and c is the speed of light in a vacuum. Then the basic procedure to measure the

thickness is to: 1) measure the reflectance for a spectrum of wavelengths; 2) analyze the measured spectrum for a

periodic intensity as a function of optical frequency; 3) calculate thickness from the period and index of refraction.

6.1.2.5 The light source has a wavelength in the neighborhood of 1.3 µm, and is placed facing the back of the wafer.

The sensor measures both surfaces of the wafer interferometrically and returns a thickness measurement (Figure 8).

The fast (millisecond) and non-destructive measurements based on backside IR illumination allow immediate HVM

process feedback. The sensor is used for measuring thickness of thick wafers, thinned wafers, and etch depth of

TSVs and other features.

Figure 8

Infra-Red Interferometric Sensor Used for Distance Measurements for Bow, Warp, and Topographical

Measurements

NOTE: Focal lengths are wavelength-dependent, providing distance measurements.

6.1.2.6 Some tools utilize a dual-sided interferometer to measure wafer geometry parameters such as thickness,

shape and flatness. The wafer is held vertically between 2 sets of optics so that each surface is measured

simultaneously. The tool is mainly used by bare silicon wafer suppliers to improve their process for wafer geometry

and to qualify wafers before shipment to customers.

6.1.2.7 Wafer geometry data acquired in 3D is capable of providing nanotopography and wafer edge roll-off.

Wafer manufacturing issues can affect geometry at the edge of the wafer, causing die defocus and CMP removal

non-uniformity.

6.1.2.8 The thickness and warp of the two individual wafers in a bonded pair can be measured using interferometry

utilizing the interference between the light reflected from the wafer and the light reflected from a moving mirror.

The general scheme of such an interferometer is shown in Figure 9.






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Figure 9

Schematic View of an Interferometer System

NOTE: Beam 2, which is reflected from the bottom surface of the wafer, is delayed with respect to beam 1 reflected from the top

surface. The delay between beam 1 and beam 2 is proportional to the thickness of the wafer. In a real experiment, the incident

beam is perpendicular to the surface of the wafer.

6.1.2.9 A SLED (superluminescent light-emitting diode) with a broad line width was used and retro-reflecting

mirrors were mounted in a nano-motor stage. The dual-probe scheme for total thickness and warp measurement is

shown in Figure 10.

Fiber

10X Microscope Objective

Reference Mirror

Bottom Probe

Si Substrate

Fiber

Reference Mirror

Figure 10

Dual Probe Scheme for the Measurement of the Thickness and Warp

6.1.2.10 In the dual-probe setup, each probe has a reference and a measurement arm. Light from each arm interferes

with light from the scanning mirror to generate the first two interference signals in the upper probe. Similarly, light

from the reference arm and the back surface of wafer produces the first two interference signals in the lower probe.

The distance between the two probes can be determined using the known thickness of a block gauge. This

information in conjunction with the two signals from the upper and lower probes makes it possible to measure the

total thickness of the wafer. If the wafer surfaces are rough and/or the substrate is opaque to the incident light, the






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dual probe is a good way to measure its thickness. A typical signal collected by an interferometer system on a bare

Si wafer is shown in Figure 11. For warp measurement, the distance between the reference signal and the signal

from the front surface of a wafer into the top probe is measured. In a warp measurement, a 300 mm blank wafer is

used as the un-warped reference.

Figure 11

Typical Signal from an Interferometer System

6.1.2.11 In Figure 12, TTV maps are shown with measurement locations (left) as well as 2D TTV (center) and 3D

TTV (right) maps for an 825 µm two wafer stack. Typical wafer thickness results are shown in Figures 13-15 for a

1550 µm thick two wafer stack.

Figure 12

25-Point 2D and 3D Image Files of TTV for 825 µm Two Wafer Stack






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Figure 13

1550 µm Two Wafer Stack –Results Showing Bottom Wafer Thickness Map

Figure 14

Two Wafer Stack –Results Showing Top Wafer Warp Map

Figure 15

1550 µm Two Wafer Stack –Results Showing Top (Thinned) Wafer Thickness Map






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6.1.3 Coherence Scanning Interferometry (CSI) or White Light Interferometry (WLI)

6.1.3.1 White light interferometry3 is a non-contact optical profiling system for measuring step heights and surface

roughness in precision engineering applications. The technology utilizes a white light beam which passes through a

filter and then a microscope objective lens to the surface of the wafer. The light reflecting back from the surface is

combined with the reference beam and captured for software analysis. After obtaining data for each point, the

system can generate a 3D image (topography) of the surface. The vertical (i.e. height) resolution of this technique is

extremely good, better than 0.01 nm (0.1 Angstrom), which makes it a potentially practical tool for assessing wafer

surfaces for roughness and nanotopography. However, the lateral resolution can be limited to the spot size, in the

range of 0.35 µm. With its broad capability, it is possible to measure local step height, critical dimensions (CD),

overlay, multilayer film thickness and optical properties, combined topography and film thickness, and wafer bow.

The technology has particular strength in new advanced packaging applications for process control of through-

silicon vias, microbumps, redistribution layers, and copper pillars.

6.1.3.2 Figure 16 illustrates the principle of CSI. A broadband light source produces a narrow region of optical

interference co-located with the focus of the interferometer microscope objective. As the objective scans in Z,

orthogonal to the surface, interference is detected by the imaging camera as the measured sample comes into focus.

At focus, each pixel detects the interference peak. Analysis software produces a map of surface height variation

from the pixel-by-pixel interference peak detection. A key CSI advantage is that Z resolution is independent of

field-of-view and lateral resolution. This is not the case for confocal microscopes, where Z resolution decreases with

increasing field-of-view due to decreased numerical aperture. The minimum surface feature height measurable by

CSI depends on the manufacturer’s measurement algorithm, and ranges from ~10 nm to <0.1 nm. CSI imaging

resolution depends on the numerical aperture (NA) of the objective used and the number of pixels in the camera

array. The area, or field-of-view measured depends on the objective magnification, which may range from 2.5X-

100X. Nominal resolution and field-of-view combinations are 0.7 µm with 500 µm field-of-view to 5 µm with 2.5

mm field-of-view. Nominal resolution for CSI is two times worse than optical imaging microscopes as expected for

a given NA. CSI systems report height, whereas image microscopes report contrast. It is possible to resolve

features in an imaging microscope until contrast equals the background noise. CSI must separate height features,

which is only possible at twice imaging microscope resolution.

Figure 16

Coherence Scanning Interferometer for the Measurement of Surface Topography

NOTE: Z height is measured at each pixel as the interferometric objective is scanned in Z and the camera detects the peak of the

coherence envelope as shown.

3 Blunt, R.T., “White light interferometry – a production worthy technique for measuring surface roughness on semiconductor wafers,” CS

MANTECH Conference, April 24-27, 2006, Vancouver, British Columbia, Canada.






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6.1.3.3 CSI can measure polished and rough surfaces, as well as flatness to nm – to mm high stepped and sloped

surfaces. When combined with image analysis software, CSI can measure and position three-dimensional features

to sub-micron repeatability in X and Y, combined with sub-nanometer repeatability in Z. Typical applications are

surface roughness (commercial tools provide industry standard results, but often lack ISO specified software

bandpass filters), nanotopography, local step height, critical dimensions (CD) and overlay. The technology has

particular strength in new advanced packaging applications for process control of through-silicon vias (TSVs),

microbumps, redistribution layers, copper studs and pillars.

6.1.3.4 A major strength of CSI is ease-of-use and speed of measurement. Advanced tools incorporate autofocus

and automation capabilities to speed the measurement process. Typically no special preparation is required of the

measurement sample, if the sample is homogeneous and without film layers. Microbump/pillar measurements have

been demonstrated in production with 0.5 second Focus/Acquire/Measure/Move (FAMM) times, while maintaining

nanometer Z performance. Laboratory tools can experience one to five-minute FAMM’s primarily due to instrument

set up time.

6.1.3.5 The presence of films and films stacks are both an opportunity and technology limitation of CSI. Present

CSI algorithms are limited to reliably measuring in the presence of films to thicknesses greater than 1.5µm optical

thickness (thickness times index of refraction). Some manufacturers claim this capability down to 1-µm optical

thickness. Reported measurement results are film thickness (if the index of refraction is known), surface profile of

the top and also the bottom of the film. Advanced work is occurring to measure features in thin film structures,

improved CD and other edge of resolution features. As these technologies emerge new applications will be possible.

When films are less than the measurable optical thickness unpredictable errors occur in the measured data, and the

data is unreliable in Z height.

6.1.3.6 Measuring non-homogeneous materials limits CSI accuracy. CSI measures Z height by measuring

interferometric phase. Phase is dependent on the material measured. Insulators experience a constant 180° phase

shift, whereas conductors and semi-conductors induce varying phase shifts. These phase shifts appears as surface

topography errors up to 10’s of nanometers. It is possible to correct for these by identifying material types by

regions and region-by-region apply a correction factor. Some manufacturers have seen success correcting

algorithmically but this has seen little commercial utilization. For process control, phase change on reflection is a

constant offset and therefore can be ignored when process variation is being tracked when absolute values are not

critical.

6.1.3.7 CSI requires minimal to no surface preparation. Rapid measurements, nanometer level Z resolution and

optical level image resolution make it a preferred tool for many applications. Measurement errors due to films and

phase change of reflection of non-homogeneous materials are major limitations for applications based on

semiconductor processes.

6.1.3.8 Five sets of 49 wafer bow measurements (top and bottom) for a 1550 µm bonded wafer pair are averaged in

Figure 17, and the 3 sigma standard deviation is shown in Figure 18.






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Figure 17

Results for 49 Point Dynamic Repeat Bow Measurement on a 1550 µµµµm Bonded Wafer Pair (3σσσσ, 5 Repeats)—

Average Bow of Top and Bottom Measurement (3 Point Suspension)

Figure 18

Results for 49 Point Dynamic Repeat Bow Measurement on a 1550 µµµµm Bonded Wafer Pair (3σσσσ, 5 Repeats)—

Average Standard Deviation of Top and Bottom Measurement (3 Point Suspension)

6.1.4 Laser Profiling

6.1.4.1 The diagram in Figure 19 demonstrates the principal of laser profiling with a single optical head where the

laser is focused first on the top and then on the bottom of the wafer. The laser peak positions that are used to

determine the wafer thickness are also shown.

6.1.4.2 Two focused laser beams can also be used, one focused on the top and the other on the bottom of the wafer

stack (Figure 20). The measurement is made by moving the focus position from the surface of the top wafer to the

top surface of the bottom wafer. The laser can be focused in the objective to a spot as small as 1 µm. The dual

optical measurement system provides accurate wafer thickness measurements independent of material properties,

especially useful for patterned or bumped wafers, GaAs and other wafer types, and after back grinding and dicing.

Laser profiling provides thickness resolution of approximately 0.1 µm and has also been applied to TTV, bow, warp,

and surface roughness.






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Figure 19

Principle of Single Beam Wafer Thickness Measurement System Showing Wafer Top and Bottom Signal

Peak Positions

Figure 20

Dual-Beam Thickness Description

6.1.4.3 The plot in Figure 21 is a plot of the laser focused on the wafer surface as well as a glass surface. The

difference in the “in-focus” locations equals the thickness of the glass.

OPTICAL THICKNESS

MODULE

BEAM

EXPANDER

BEAM

SPLITTER

OBJECTIVE

MEASUREMENT

SAMPLE

SIGNAL DETECTION

MODULE

OPTICAL FIBER

DETECTOR

PINHOLE FOCUSING

LENS

BOTTOM

OF WAFER

TOP OF

WAFER






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Figure 21

Example Plot Thickness Measurement

6.1.4.4 Figures 22-26 illustrate measurements for a set of five static repeatability runs obtained on an 825 µm thick

bonded wafer pair stack using laser profiling. A 2D thickness map is shown for each set of measurements in Figure

22, as well as 3D thickness maps for the five runs (average and 3σ sigma standard deviation) in Figures Figure 23

and 24. Two-dimensional bow and warp maps are shown in Figures 25 and 26.






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Figure 22

Bonded Pair Wafer Thickness Map (Five Repeats)

Figure 23

Bonded Wafer Pair 25 Site Average Thickness Map (Five Repeats)






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Figure 24

Bonded Wafer Pair 25 Site Thickness Map (3 σσσσ Std. Dev., Five Repeats)

Figure 25

Bonded Pair Wafer Bow Map (Five Repeats)






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Figure 26

825 µm Bonded Pair Wafer Warp Maps (Five Repeats)

6.2 Electrical Technique for Measuring Proximity

6.2.1 Capacitance Displacement Metrology

6.2.1.1 Capacitive sensors can be divided into two categories based upon their performance and intended use. High

resolution sensors are typically used in displacement and position monitoring applications where high accuracy,

stability and low temperature drift are required. Quite frequently, these sensors are used in process monitoring and

closed-loop feedback control systems. Proximity type capacitive sensors are typically used to detect the presence of

a part or used in counting applications. The following describes characteristics of high resolution systems, their

operating principle and application.

6.2.1.2 The capacitance sensor shown in Figure 27 is used throughout a variety of industries to provide highly

stable, accurate measurements of displacement, vibration, position, thickness and runout. Capacitance probes are

typically modeled as a parallel plate capacitor. If two conductive surfaces are separated by a distance and a voltage

is applied to one of the surfaces, an electric field is created. This occurs due to the different charges stored on each

of the surfaces.






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Figure 27

Typical Capacitance Sensor

6.2.1.3 Capacitance refers to the ability of the surfaces to hold a charge. In a typical sensor system the probe is one

of the plates and the target being measured is the other plate. If a constant current is applied, the capacitance change

can be monitored as a linear voltage charge related to the distance between the plates (Figure 28).

Figure 28

Typical Capacitance Sensor Measurement Parameters

6.2.1.4 This distance, or gap, is a function of the area of the capacitance sensor according to the following equation

(Eq. 2):

� � ��

(2)

Where capacitance is the area of sensor, A = πr2 times the dielectric constant of air, ε, divided by the gap, d.

6.2.1.5 From this relationship, capacitance is directly proportional to the area of the sensor and the dielectric

property of the material between the sensor and target (typically air). The greater the area of the capacitance sensor

the larger the measurement range, or gap. If it is assumed that the area and the dielectric constant between the plates

remain constant for a specific probe, any change in capacitance is inversely proportional to the change in distance

between the probe and target being measured and this change is converted to a voltage for monitoring. The amount

of voltage output change for a given distance change is commonly referred to as the sensitivity of the system. For

example, if a distance change of 1 mm corresponds to a voltage output change of 10 V the sensitivity would be 1

mm/10 V, or 0.1 mm/V.






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6.2.2 Capacitance Sensors Electric Field

6.2.2.1 A typical high performance capacitance sensor consists of three basic elements: the sensor tip, the guard and

the ground shell. Figure 29 illustrates a typical capacitance probe. When a voltage is applied to the sensor tip, an

electric field is established between it and any other local conductive material. To maintain accuracy and linearity it

is essential that the electric field in the measurement area be linear, directed toward the target, and not distorted. To

protect this field, each capacitive sensor has a guard electrode. This creates an additional field around the probe

sensing tip that is driven at the same phase and voltage potential as the sensor. By being equal, the auxiliary field

protects the area from becoming warped and cancels any stray capacitance between the two elements.

6.2.2.2 Because the ground shell is at a different potential than the guard, a partial distortion of the guard field to the

ground shell occurs. Although undesirable, the distortion of the guard is acceptable as long as the sensor tip field

remains linear. For best performance, the width of the guard should be at least 2X the system measurement range.

Figure 29

Capacitance Sensor Field

6.2.2.3 Capacitance probes should be designed with sufficient guard electrodes to protect the sensing area under

normal operating conditions. This probe’s range should not be greater than the width of the guard electrode or poor

linearity will result.

6.2.2.4 In addition to improving linearity and accuracy, the guard is also used to reduce noise and external

interference. Each capacitance probe is driven by a low noise coaxial cable. The shield of the cable is used to deliver

the voltage to the guard, at the same voltage and phase. This eliminates any stray capacitance that might be created

between the center conductor and the shield of the cable, or any other part that may be close to the cable. By design,

this protection significantly reduces external influences from RFI and EMI. It is important to note that ordinary

coaxial cable usually does not provide adequate protection or shielding for the system and special cable is generally

required.

6.2.3 Characteristics of Capacitive Sensors

6.2.3.1 Non-Contact

6.2.3.1.1 Capacitive displacement sensors are non-contact by design. That is, they are able to precisely measure the

position or displacement of an object without touching it. Because of this the object being measured will not be

distorted or damaged and target motions will not be dampened. Additionally, they can measure high frequency

motions because no part of the sensor needs to stay in contact with the object, making them ideal for vibration

measurements or high speed production line applications.

6.2.3.2 Range/Standoff Distance

6.2.3.2.1 As mentioned above, the range of a capacitance sensor is dictated by the diameter, or area, of the sensor.

The larger the area, the larger the measurement range. Measurement range is typically specified starting when the

probe is touching the target. At this point the output from the amplifier is 0 V. When the gap is increased to equal

the full scale measurement range of the capacitive system the amplifier output is 10 V (Vdc). In theory, the probe can

operate anywhere between these two extremes, however, it is not recommended to operate below 10% of the gap.

Thus, the ideal operating or standoff distance is between 5 V and 7 Vdc, permitting the target to move closer to or






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further away from the probe without going out of range. Figure 30 is a simplified diagram showing range, output

voltage and recommended standoff for a typical capacitance sensor.

Figure 30

Capacitance Probe Operating Range

6.2.3.3 Resolution

6.2.3.3.1 The resolution of a displacement sensor is defined as the smallest amount of distance change that can be

reliably measured by a specific system. Capacitance sensors offer extremely high resolution and stability, often

exceeding that of complex laser interferometer systems. Because of their ability to detect such small motions, they

have been successfully used in many demanding measurement applications including computer disk drive runout,

microscope focusing and nano-positioning within highly complex photolithography tools.

6.2.3.3.2 The primary factor in determining resolution is the system’s electrical noise. If the distance between the

sensor and target is constant, the voltage output will still fluctuate slightly due to the “white” noise of the system. It

is assumed that, without external signal processing, one cannot detect a shift in the voltage output of less than the

random noise of the instrument. Because of this, most resolution values are presented based on the peak-to-peak

value of noise and can be represented by the following formula: Resolution = Sensitivity × Noise.

6.2.3.3.3 Sensitivity is simply the measurement range divided by the voltage output swing of the capacitance

amplifier, that for a fixed sensitivity the resolution is solely dependent upon the noise of the system, so that the

lower the noise, the better the resolution.

6.2.3.3.4 It is important to note that some manufacturers specify resolution based on peak or rms noise, resulting in

claims that are 2X and 6X respectively better than peak-to-peak. Although an acceptable method, it is somewhat

misleading as most users do not have the ability to measure voltage changes less than the peak-to-peak noise value.

6.2.3.4 Bandwidth

6.2.3.4.1 The bandwidth, or cutoff frequency, of a system is typically defined as the point where the output is

dampened by 3 db. This is approximately equal to an output voltage drop of 30% of the actual value. In other words,

if a target is vibrating with an amplitude of 1 mm at 5 kHz and the bandwidth of the capacitance sensor is 5 kHz the

actual sensor output would be 1 mm × 70% = 0.7 mm.

6.2.3.5 Push or Range Extension

6.2.3.5.1 Typical capacitive amplifier systems operate over a specific capacitance range, limiting their ability to

measure large motions or operate at comfortable standoff distances. To overcome this problem some suppliers use a

proprietary circuit that, with minor component modifications, can be adjusted to change the range and meet a wider

variety of customer requirements. For example, a small diameter probe with a ½ mm measurement range can be

“pushed” to have a measurement range of 1 mm or even 2 mm. This allows capacitive probes to be used in

applications where space is limited or the target being measured is small. It is important to note, however, that a

pushed probe should have a guard width sufficient enough to maintain the performance required, as mentioned






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above. Additionally, pushing a sensor also amplifies system noise, reducing probe resolution. Noise increase is

proportional to push; 2 × Push = 2 × Noise

6.2.3.6 Spatial Resolution

6.2.3.6.1 The field established between a capacitance probe tip and measured object is typically larger than the

diameter of the probe tip (Figure 31). This is because there is an epoxy gap between the tip and guard elements. The

field diameter is equal to D plus 1X the epoxy gap. When obtaining measurements, capacitance probes provide a

distance equal to the average surface location within the spot area. They are not capable of accurately detecting the

position of features smaller than the size of the spot. However, they can repeatedly measure rough surfaces. Because

of this, the probe tip should always be 25% smaller than the smallest feature targeted for measurement. Smaller

sensors can distinguish smaller features on an object.

Figure 31

Effective Spot Size of a Capacitive Sensor

6.2.3.7 Linearity

6.2.3.7.1 Capacitance sensors may have an output of 0–10 Vdc over the full scale measurement range (FSR). In an

ideal world this output would be perfectly linear and not deviate from a straight line at any point. However, in reality

there are slight deviations from linearity. Typically, linearity is specified as a percentage of the Full Scale

Measurement range. During calibration the output from the amplifier is compared to the output of a highly precise

standard and differences are noted. Some capacitance systems exceed ±0.05% FSR with some achieving ±0.01% or

better.

6.2.3.7.2 Accuracy is a function of linearity, resolution, temperature stability and drift, with linearity being the

majority contributor. Calibration reports provide data that can be used to correct for the non-linearity of a system

with inexpensive computers and correction software. Digital correction typically yields a linearity of ±0.01% or

better.

6.2.3.8 Stability

6.2.3.8.1 Stability is a function of a variety of different internal and external factors. For short term or relative

measurement applications stability is typically not an issue. However, if high accuracy is required over a long period

of time, care must be taken when designing fixtures, selecting components and specifying materials of construction.

6.2.3.8.2 Temperature is typically the biggest factor that affects stability. Temperature swings not only cause

electronic drift but can also cause fixture and probe expansion and contraction. For critical applications high-quality

capacitors, resistors and inductors specifically designed for stability to minimize the electronic drift should be used.

To minimize mechanical drift, probes can be manufactured from special low thermal coefficient materials such as

Invar. Thermal correction coefficients can also be provided and used for real-time compensation.

6.2.3.8.3 Active capacitance probe systems should never be used in high stability applications because any localized

temperature change surrounding the sensor will result in drift.






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6.2.3.9 Calibration

6.2.3.9.1 For low-end proximity sensors calibration is typically not important because linearity of the sensor is not

critical. Most high performance systems are, by design, inherently linear to approximately ±0.1% of the full scale

measurement range. Some capacitance manufacturers offer sensors with this performance, however, they are

typically not suitable for high precision applications. During calibration, the manufacturer adjusts circuit gain, offset,

and typically performs a proprietary linearity adjustment. Improved performance can be obtained by adding

adjustable break point linearization circuitry within the amplifier circuit or by providing digital correction.

6.2.3.9.2 During calibration the output of the amplifier verses position of a target is recorded. A best fit straight line

is generated based on this data. Each recorded point is then compared to the generated straight line and the percent

deviation is calculated and plotted. Based upon the results adjustments can be made to improve the deviation to

within acceptable limits.

6.2.3.10 Applying Capacitive Probes

6.2.3.10.1 Target Material and Grounding

6.2.3.10.1.1 A capacitance measurement system mimics a parallel plate capacitor with the sensor as one plate and

the target being measured the other. To create the electric field between the two plates the target must be made of a

conductive material. The composition or thickness of the target is not important, allowing them to be used in many

applications not suitable for eddy current type sensors. In fact, the surface can even be a few hundred ohm-cm.

6.2.3.10.1.2 To complete the capacitance circuit the target should be grounded back to the amplifier. For optimal

performance a conductive path is required, however, capacitive coupled targets can work well if the capacitance is

0.01 µf or higher. An example of a capacitively-coupled target is a shaft rotating on air bearings. In theory, air

bearings are non-contact but the gap between elements is small, and their area is relatively large, creating a high

capacitance path. Thousands of successful applications world-wide have been installed with this type of ground.

6.2.3.10.1.3 If the target is poorly grounded, the system is susceptible to external noise and interference. Care

should be taken when designing the ground return path.

6.2.3.11 Target Size

6.2.3.11.1 The lines of flux in the electric field established between the probe and target always leave the

capacitance sensor normal (90°) to its surface and always enter the target normal to its surface. If the target being

measured is large enough, and the sensor is within range, the field within the sensing area will be consistent and

linear. If the target is not large enough to support the field it will tend to wrap around the edge and enter normal to

the target side (Figure 32). This field distortion will create measurement errors by degrading the sensor linearity and

changing its measurement range. Because of this the original factory calibration can no longer be used and an in-

place calibration is required, however, accuracy may still be compromised.

Figure 32

Field Distortion from an Insufficient Target Size






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6.2.3.11.2 Targets that are too small to support the electric field can also provide false displacement signals from

lateral motions. Capacitive sensors are typically used to measure the gap or movement in the direction of the sensor

axis. If the target is large enough any lateral motion will not distort the electric field, however, if the field is wrapped

around the target any lateral motion will change the shape of the field causing a change in output even if the gap

remains constant. As a general rule, the target should be 30-50% larger than the capacitance sensor.

6.2.3.12 Target Shape

6.2.3.12.1 Capacitance probes will measure the average distance to the target under the area of the sensor. If a tilted

or curved target is being measured the electric field will be distorted and the accuracy compromised. When sensors

are calibrated to a flat target the output voltage is theoretically zero when the probe is in contact with the target. This

is not true when measuring curved or tilted surfaces because the surface prevents the probe from full target contact.

The result will be a shift in the zero point from its original calibration which will be reflected as an offset in the

measurement, not a sensitivity change. To overcome both issues an in-place calibration is possible to correct for the

sensitivity change, however, the sensor measurement range may be reduced. As a general rule, a curved target

should be 10 times larger in diameter than the sensing element of the capacitance sensor.

6.2.3.13 Spatial Resolution

6.2.3.13.1 Capacitance sensors have a relatively large sensing area in relationship to their measurement range. As

mentioned above, these types of sensors take an average measurement to the surface in question. If this surface has

features that are smaller than the sensing element the feature may not be detected or the sensor output may not

respond accordingly. Figure 33 shows how the probe size can affect the sensor output when measuring a stepped

object, demonstrating sharper voltage changes from a smaller diameter sensor (Sensor C).

Figure 33

Spatial Resolution

6.2.3.13.2 Similarly, the output will depend on the surface roughness. If the roughness changes over an area the

output from the capacitance sensor will change when the target translates beneath the probe because the average

distance to the surface has changed. The amount of sensor output shift will depend on the magnitude of the surface

roughness.

6.2.3.14 Environmental Conditions

6.2.3.14.1 Capacitance changes as the distance to the target changes and also depends upon the dielectric property

of the material in the gap. Because of this, it is important that there is a homogeneous, non-conductive material

between the probe and target. In most applications, this material is air, however, many times oil or some other

dielectric fluid is successfully used. If it is not homogeneous, or if the dielectric properties in the gap change, then

accuracy will be affected. This is typically not a problem for changing air properties because the effects are small.






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For instance, the dielectric constant of air changes by ~1.4 ppm/1% of relative humidity. This represents a potential

offset in the senor output of less than 1 mV with a relative humidity change of 50%. Care must be taken to avoid

dielectric changes from other materials and ensure that dirt and debris do not accumulate in the capacitive probe gap.

6.2.3.14.2 The most common environmental problem that can affect the accuracy of a capacitive sensor is

temperature. Not only do the electronics exhibit temperature drift but also expansion and contraction of the probe

and fixturing physically changes the probe gap. Custom probes manufactured of highly stable materials, such as

Invar, are available for extreme stability applications. It is also possible to adjust the temperature coefficient of

components for custom high-stability applications.

6.2.3.15 Advantages and Disadvantages

6.2.3.16 Advantages

6.2.3.16.1 As with any sensing technology, capacitive systems have both advantages and disadvantages. Perhaps

their greatest attribute is their ability to resolve measurements below one micro-inch (< 25 nm), at a fraction of the

cost of other high performance technologies. Most are "passive" by design allowing them to be used in extreme

environments while still maintaining stability. Sensors can be easily customized, allowing them to be adapted into a

variety of applications or settings. They are immune to target composition and work equally well on all conductive

targets, unlike eddy current probes. They are largely immune to ultrasonic noise, electromagnetic fields, lighting

conditions, humidity and temperature.

6.2.3.17 Disadvantages

6.2.3.17.1 Capacitance technology dictates that the probe be mounted close to the target. This increases the

probability of crashing the sensor or damaging the material being measured. Some suppliers have provisions to

extend the measurement range and standoff of the sensor, however, this distance is rarely greater than 15 mm.

6.2.3.17.2 Capacitance sensors should also be kept clean. Dirt or other foreign debris can cause an offset in the

measurement so frequent cleaning may be required depending on the application.

6.2.3.18 Applications

6.2.3.18.1 Thickness Measurements

6.2.3.18.1.1 Thickness quality control monitoring is better applied on line during the manufacturing process instead

of periodic sampling after a product has been manufactured. This way process adjustments can be made “on the fly,”

reducing or eliminating the continued production of product that does not meet specification. In some applications,

contact methods can be utilized. However, they are slow, can damage the product and are subject to wear. Non-

contact sensors are commonly used in these applications.

6.2.3.18.2 A typical application consists of two capacitance probes, one on either side of the material being

measured. The difference between the output of each sensor is directly related to the thickness of the material being

measured. By taking a differential measurement, any positional movement of the material within the probe gap is

cancelled.

6.2.3.18.3 Figure 34 shows a typical thickness setup with A and B representing the sensor outputs. The gap between

the probes, G, is equal to A + B + T. If an initial sample of known thickness, T, is placed within the gap, G can be

determined and used in future calculations. Since thickness = G – (A + B), and G is constant, the thickness can be

calculated by simply subtracting the two outputs.






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Figure 34

Typical Thickness Measurement Using Dual Capacitance Probes

6.2.3.18.4 Single-sided thickness measurements can also be successfully made using capacitive sensors if the back

side of the material being measured can be referenced to some fixed plane. Figure 35 shows a typical single-sided

measurement. From this figure it is apparent that the product thickness is directly proportional to the gap between

the probe and the surface of the material. Figures 36 and 37 show a typical bonded pair wafer bow map and raw

thickness data from edge (point 0) to center of the wafer.

Figure 35

Single Point Thickness Measurements






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Figure 36

825 µm Bonded Pair Wafer Bow Map

Figure 37

825 µm Bonded Pair Raw Thickness Data

6.3 Pneumatic Technique for Measuring Proximity

6.3.1 The pneumatic technique for measuring proximity relies on the dependence of backpressure in an orifice

conducting flowing gas toward a flat surface on the proximity of the surface to the orifice. Dual sensors allow

differential measurements.

6.3.2 Differential Backpressure Metrology

6.3.2.1 With a dual backpressure sensor, one can measure thickness (similar to a white light confocal sensor), as

well as bow and warp. The advantage of differential backpressure sensing technology is that it works nearly

independent of any surface condition (smooth or rough) or material property (conductive or non-conductive)

requiring only a low supply pressure (10 psi or less) and only CDA or N2. In this technique, back-pressure is

converted to voltage; a calibration curve is generated and the operating point is determined on the curve. A






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reference backpressure measurement is made on a gauge block prior to every wafer measurement to compensate for

ambient temperature variation effects.

6.3.2.2 An air gauge is a broader terminology used for this technology, but in a broader sense, it includes any kind

of sensing set up using a pressure sensor to detect a backpressure. Usually it uses higher pressure, has poor

resolution, and is not stable. Differential backpressure technology is part of the air gauging technology but requires

the use of specific sensor layout (Wheatstone bridge equivalent) to meet the characteristics needed for its use in the

wafer industry.

6.3.2.3 Differential backpressure sensing technology can be used to achieve non-contact, high-resolution wafer

thickness measurements. The differential backpressure sensors (PEL sensors) use clean, dry, compressed air or an

inert gas, such as nitrogen.

6.3.2.4 The PEL sensors utilize a pneumatic “Wheatstone bridge” arrangement (Figure 38) with equal flow

restriction and a low-pressure differential sensor. The sensing orifice uses a calibrated high-precision sapphire

nozzle. Any small variation of the distance between the sensing nozzle and the target is detected by the PEL sensor

and converted into a corresponding output voltage. The PEL sensor includes built-in temperature compensation and

voltage regulation to provide extremely high stability over time and resolution down to 0.02 µm.

Figure 38

Backpressure Sensor Configuration

6.3.2.5 The nominal distance between the sensing nozzle and the target is set at approximately 100 µm. This is the

optimal distance to operate within the linear portion of the backpressure response curve with the highest sensitivity.

Under normal operating conditions, the distance between the sensing nozzle and the target may vary by ±20 µm

around the preset nominal distance. With a response time of 5 ms the sensor is ideally suited for relatively fast

measurement operations of fixed targets.

6.3.2.6 Source gas is supplied to the sensor at 10 to 12 psi (PS). Any variation in the backpressure (PB) resulting

from the changes in the distance (X) between the sensing nozzle (Rx) and the gauged object (Figure 38) will

generate a change in the output voltage at a rate of 50 mV/µm. Adjusting the setting of the valve (RV) provides a

wide range of settings and sensitivities that can vary the pressure reference (PR).

6.3.2.7 Measurement System

6.3.2.7.1 Figure 39 shows a block diagram of the sensor subassembly and the associated interconnecting electronic

and pneumatic controls. The servomotor in the measurement head is a voice coil that moves the probe relative to the

sample such that the backpressure reading falls on the calibration curve in the linear working region.






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Figure 39

Measurement System Block Diagram

6.3.2.7.2 The Encoder Interface conditions the signal from the Encoder, which determines the position of the probe.

The Backpressure Sensor converts the backpressure to a voltage output to be supplied to the computer. The

computer processes all data from the measurement head as well as from the robotic handling system and the Y-

Theta or X-Y stage to control the three component subassemblies.

6.3.2.8 Thickness Calculation

6.3.2.8.1 The system employs a measurement sensor on the underside of the workstation as well as the topside.

From the drawing in Figure 40 the thickness, t, of a sample is calculated in Equation 3 as:

t = t0 + (D-D0) – (a-a0) – (b-b0) (3)

Where:

t0 is the thickness of the gauge block,

D and D0 are the relative positions of the probe,

a0 is the distance from the top probe tip to the gauge block,

a is the distance from the top probe tip to the sample,

b0 is the distance from the bottom probe tip to the backside of the gauge block, and

b is the distance from the bottom probe tip to the back of the sample.






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Figure 40

Probe Measurement Technique

6.3.2.8.1.1 An example of a thickness map using differential back-pressure monitoring is shown in Figure 41. The

mean thickness is 872 µm.

Figure 41

Thickness Map Obtained by Differential Back Pressure Monitoring

6.3.2.8.1.2 An example of a warp map using differential back-pressure monitoring is shown in Figure 42. The warp

values are for an 875 µm two wafer stack.






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Figure 42

Warp Map Obtained by Differential Back Pressure Monitoring

6.4 Acoustic Microscopy for Measuring Bonded Wafer Thickness

6.4.1 A-Mode Scanning

6.4.1.1 Acoustic data can be collected at the smallest X-Y-Z region defined by the limitations of the given acoustic

microscope. An A-mode display contains amplitude and phase/polarity information (Figure 43 on left) as a function

of time of flight at a single point in the X-Y plane, as shown in Figure 43 (Figure 43 on right).

Figure 43

Example of an A-Mode Scan and Spectrum

6.4.2 B-Mode Scanning

6.4.2.1 Acoustic data can also be collected along an X-Z or Y-Z plane versus depth using a reflective acoustic

microscope, as shown in Figure 44 on the left. A B-mode scan, shown in Figure 44, contains amplitude and

phase/polarity information as a function of time of flight at each point along the scan line. A B-mode scan furnishes

a two-dimensional (cross-sectional) description along a scan line (X or Y), as shown in Figure 44 on the right.






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Figure 44

Example of a B-Mode Scan and its Resulting Image

6.4.3 C-Mode Scanning

6.4.3.1 Acoustic data collected in an X-Y plane at depth (Z) using a reflective acoustic microscope is shown in

Figure 45 on the left. A C-mode scan contains amplitude and phase/polarity information at each point in the scan

plane. A C-mode scan furnishes a two-dimensional (area) image of echoes arising from reflections at a particular

depth (Z), as shown in Figure 45 on the right.

Figure 45

Example of a C-Mode Scan and its Resulting Image

6.4.4 While acoustic microscopes are typically used to image and assess the bond quality between bonded wafers,

the intrinsic data used to image the bond quality also contains time/distance information (Figure 46 on left). In order

to image at a particular interface of interest, the time/distance to that interface is also known at each X-Y location

and is displayed in an A-Scan waveform format. The A-Scan at each X-Y location can be processed to determine

the thickness of each layer at that point and the surface contours of the front, internal interface or back of the wafer

stack (Figure 46 on right).






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Figure 46

Typical A-Scan Waveform Display (Left) and C-Scan Showing the Bond Quality at Several Layers of a Wafer

Bond Stack

NOTE: White and light gray areas indicate delamination present at several layers

6.4.5 Thickness Measurements with AM

6.4.5.1 As mentioned above, the A-Scan waveform consists of time and material interface data at an X-Y location

on a bonded wafer. Knowing the speed of sound within a material converts the time data to a distance between one

material interface and another, as shown in Figure 47 on the left. The time/distance data can be processed to color

code the defects at different layers, as shown in Figure 47 on the right.

Figure 47

An Example of a Color Coded Profile Image That Maps the Distance from the Surface of a Part to Anomalies

at a Specific Depth Within a Stacked Wafer

NOTE: All anomalies of the same color are at the same depth and the color map indicates that the blue anomalies are closer to

the top surface, deeper green and orange being the deepest in the stack.

6.4.5.2 In addition, thickness measurement of a layer within a wafer stack can be obtained, as shown in Figure 48.

In this image, the C-scan image is shown on the left, while the A-Scan waveforms showing the full bonded wafer

thickness, the delamination region, and defective regions within the stack are shown on the right. The data shown in

Table 1 shows the time and distance measurements, based on the speed of sound of the material, made on a sample

at five points of interest.






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Figure 48

C-Scan Image of a Stacked Wafer Showing 3 A-Scan Waveforms

NOTE: Waveform #1 shows the complete thickness of the stacked wafer. Waveforms #2 and #3 show the distance from the top

surface to the delamination and anomalous regions within the wafer stack.

Table 1 Example Table of Various Thicknesses Measured at 5 Positions on a Sample.

6.4.6 Bonded Wafer Thickness, TTV, Warp, Bow, Sori, and Flatness Metrology with Acoustic Microscopy (AM)

6.4.6.1 Acoustic microscopy (AM) collects data similar to a depth finder in that it can measure the transit time for

sound from a reference point, in this case the transducer, to the first surface it sees. The contour of that surface over

the X-Y area scanned can then easily be displayed in a simple color coded map or process further to provide specific

warp, bow, sori or global flatness data. In Figures 49 and 50, examples are provided of both types of data displays

for 3D measurement of wafer stack surface contours.

Waveform Thickness (ns) Thickness (µm)

1 87 108

2 81 101

3 60 75

4 70 87

5 69 86






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Figure 49

Example of an AM Surface Flatness Color-Coded Display with Data Overlaid

NOTE: In this case the wafer stack is bowed 140 µm from center to edge.

Figure 50

Example of a 3D AM Surface Flatness Map with the Data Processed and Displayed.






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6.4.6.1.1 A summary of the techniques described in this guide is shown in Table 2 showing whether they are a high-

volume manufacturing tool, and whether they measure the thickness of the wafer stack or the individual wafer

components. The content is based on supplier information and is subject to change.

Table 2 Summary of Techniques in Guide

NOTICE: Semiconductor Equipment and Materials International (SEMI) makes no warranties or representations as

to the suitability of the Standards and Safety Guidelines set forth herein for any particular application. The

determination of the suitability of the Standard or Safety Guideline is solely the responsibility of the user. Users are

cautioned to refer to manufacturer’s instructions, product labels, product data sheets, and other relevant literature,

respecting any materials or equipment mentioned herein. Standards and Safety Guidelines are subject to change

without notice.

By publication of this Standard or Safety Guideline, SEMI takes no position respecting the validity of any patent

rights or copyrights asserted in connection with any items mentioned in this Standard or Safety Guideline. Users of

this Standard or Safety Guideline are expressly advised that determination of any such patent rights or copyrights,

and the risk of infringement of such rights are entirely their own responsibility.

Metrology Technique HVMNon-

HVM

Measurements

Made

Capable of

Measuring

Each Wafer in

Bonded Wafer

Pair

Capable of

Measuring Bond

Layer

White Light Confocal

MicroscopeY Y

Thickness, TTV,

Bow, WarpN N

IR Laser Profiling Y YThickness, TTV,

Bow, WarpY Y

White Light

InterferometryY Y

Thickness, TTV,

Bow, WarpN N

Infrared

Interferometry/Confocal

Microscope

Y YThickness, TTV,

Bow, WarpY Y

Capacitance Metrology Y YThickness, TTV,

Bow, WarpN N

Differential Backpressure

MetrologyY Y

Thickness, TTV,

Bow, WarpN N

Acoustic Microscopy Y YThickness, TTV,

Bow, WarpY Y

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