Microfabrication - ERNETsuresh/memscourse/proflectures/lecture3.pdfMicrofabrication Process Flows...

MicrofabricationProcess Flows

G. K. [email protected] EngineeringIndian Institute of ScienceBangalore, INDIA

February 19th, 2005http://www.mecheng.iisc.ernet.in/~suresh/memscourse/proficience33.html

Presented to Proficience #33 class in Jan.-May term of 2005

Lecture 3

G.K. Ananthasuresh, Indian Institute of Science, Jan. 2005

2Equipment and Process Flows

Basic equipment

Pressure sensor process steps

Process steps for comb-drive

Process steps for in-plane hinges

MUMPs (foundry process)

Resonant pressure sensor

PennSOIL

Electronics integration and packaging


3Basic process steps

PhotolithographyOxidationThin-film depositionSelective etching through a maskWafer bondingDopingEpitaxial growthThick-film depositionDicing, die-bonding, and wire-bonding


4Silicon wafer making

(from “Fundamental of Microfabrication” by Marc Madou)


5Oxidation



6Physical vapour deposition (PVD)



7Chemical vapour deposition (CVD)



8PECVD (plasma-enhanced)



9Process design issues

Geometry of the deviceAvailable equipment and skillsSystem partitioningProcess partitioningContamination and cleaning requirmentsMaterial property controlProcess accuracyMechanical and thermal stabilityAlignment featuresPlanning the devices on the waferPost-fabrication (packaging)


10A typical process flow

Begin with substrate waferRCA clean with HF dipGrow oxidePhotolithography (alignment)Etch alignment marks into oxideStrip photoresistPhotolithography (piezoresistors)Boron implantingStrip photoresistRCA clean without HF dipDeposit nitrideBackside photolithography (diaphragm)Etch nitride and oxide from backKOH etchPhotolithography (vias)Plasma etch nitride and oxide through viasStrip photoresistPre-metal cleanEvoporate aluminiumPhotolithography (aluminum)Strip photoresistSinter for annealing contactsTest piezoresistor value at the wafer levelDie separation with sawingMount in packageWire-bondSeal packageTest and calibration

For Motorola’s pressure sensor


11MIT’s wafer-bonded pressure sensor


12Process description of MIT-pressure sensor

Sources:“A Merged MEMS-CMOS Process Using Silicon Wafer Bonding”, L.

Parameshwaran, C. Hsu, and M. A. Schmidt, IEDM 95, pp. 613-616.“Silicon Pressure Sensor Using A Wafer-Bonded Sealed-Cavity Process,” L.

Parameshwaran, A. Mirza, W. K. Chan, and M. A. Schmidt, Transducers 95, pp. 582-585.

The process begins with two (100) oriented silicon wafers, one of p-type and another of n-type. A 10 um thick n-type layer is grown epitaxially on the p-type silicon wafer. A shallow circular pit is etched into the n-type silicon wafer using plasma etching. The two wafers are then cleaned and are brought into contact (the shallow pit on the n-type wafer is in contact with the n-type epi layer of the p-type wafer) and annealed at 1100 ºC for one hour. This enables the bonding of the two wafers. Next, the p-type wafer is thinned down by grinding and polishing, and then electrochemically etched so that the etch stops on the n-type epi layer. This leaves a circular membrane of uniform thickness of 10 um. At four locations along the periphery, U-shaped piezoresistors are formed by a masked doping (e.g., using ion implantation). A subsequent metallization step creates the electrical connections (e.g., using lift-off patterning). A hole is opened at the back using KOH for connecting to the pressure source and the device is packaged.


13Process flow for MIT’s pressure sensor


16Electrostatic comb-drive actuator

(first comb-drive, Tang et al., 1989)


17Comb-drive linear electrostatic microactuator


18Process steps for comb-drive


19Process steps for comb-drive (contd.)


20Mask #1


21Mask #2


22Mask #3


23Mask #4


24In-plane hinge using surface micromachining


25 In-plane hinge using surface micromachining (contd.)


26Floating in-plane hinge

Clemens and Howell, 1999


27Mask sets for floating in-plane hinge

Axis of the floating jointAxis of the substrate joint


28Parts of an in-plane floating hinge

Hub of the substrate hinge

Body with substrate and floating hinges at the either end

Body with the floating hinge on one end


29Rotated in-plane hinge


30Making elements of mechanisms

A surface micromachined hinge(Kris Pister, Berkeley)

Substrate hinge


31Floating hinges

Masklayout


32The need for planarization step in MEMS

Cronos MUMPs process with three structural layers

The geometry and topography of the latter layers get increasingly more complicated. Leads to the problem of stringers (loose pieces upon release) too.


33MEMS foundry

PolyMUMPsMUMPs

MCNC Cronos Uniphase MEMSCap


34 Visualize device from a verbal description of the process

Being able to draw the process flow diagrams from a description.

Shallow pits were etched into n-type substrates, and p-type deflection electrodes were diffused in the above pits, followed by fusion bonding of a second wafer above the first. The top wafer was then ground and polished down to a thickness of 6 um. A passivation layer was then formed on the top wafer and sensing piezoesistors were formed using ion implantation, after which contact holes for metallization to connect to the diffused deflection electrodes were etched. Bond pads and interconnect metallization were then deposited and patterned, followed by etching of the diaphragm from the back of the wafer. Finally, two slots were etched next to the beam to release it over the buried cavity.

(Petersen et al., 1991)


35Process flow

Wire bond


36“Verbalize” the process steps from a device cross section

“Verbalizing” a process from a cross-section and then visualizing it geometrically.

What process was used to make this?

A micro-diaphragm pressure sensor (Sugiyama et al., 1986)

N-type (100) Si

PolysiliconSilicon nitrideAluminium

Top view

(IEEE Int. Electron Devices Meeting, 1986, pp. 184-187)


37Visualizing a process flow

Deposit a nitride layer using LPCVD

Etch the nitride layer to leave a square window

Deposit polysilicon using LPCVD

Deposit a thick nitride layer

Deposit polysilicon piezoresistors

Deposit another nitride layer

Etch nitride layers

Etch polysilicon and silicon using KOH

Seal using a nitride layer using plasma CVD

Deposit Al using vacuum evaporation and pattern


38Order of the process steps is important!

We cannot etch the pyramidal pit first. This is because the subsequent layers conform to the pit’s surface and it will not be possible to get a membrane.


39SUMMiT V

Oxide1

Oxide2

Oxide3

Oxide4

CMP

CMP

Poly0Nitride (0.8 um)

Thermal oxide (0.63 um)

A gear train on a moving platform.

Sandia Ultra-planar Multi-layer Micromachining Technology

Figures: courtesy of Sandia National Laboratory


40 PennSOIL(Univ. of Pennsylvania Silicon-On-Insulator Layer process)

Bulk micromachining process based on Silicon on Insulator (SOI) wafersA single releasable structural layer with selectively doped regions

Moulton and Ananthasuresh, Sensors and Actuators A, 90 (2001), pp. 38-48.

Silicon

SiO2

SiliconDoped


41

•Four inch wafers are diced into pieces for use in 2”equipment

•A membrane is formed out of the thin layer of the SOI wafer by a KOH etch from the back of the wafer

PennSOIL (contd.)


42

•An intermediate non-structural silicon etch must be done for front-back alignment

•Selective doping is accomplished by applying dopant material on top of an oxide mask

PennSOIL (contd.)


43

•The dopant is driven in at high temperature in the furnace

•Devices are formed by etching through the silicon membrane. This is done with a plasma etch and NiChromemask.

PennSOIL (contd.)


44Devices made with PennSOIL (a bulk process)

1 mm

Silicon

SiO2

SiliconDoped

Linear actuator

Three dof platform

Single actuator


45MIT microengine

MIT Microengine (Source: Epstein, 2003)

Several wafers patterned with deep features are bonded together.


46Integration of electronics

Even though MEMS are touted as “chip-scale integrated systems”, the issue of integration of electronics on the same chip is open to debate.

Some integrate, some do not.Some invent new fabrication techniques in order to integrate.Some do not integrate even though their process may allow it.It is all in $ (Rs).

Nevertheless, inclusion of electronics is essential for the full functionality of MEMS.

Almost all MEMS devices have output signals in electric form (a few have optical output); thus electronics is indispensable.


47 Ways of integrating/interfacing electronics with MEMS

Complete integration in fab in a single chipiMEMS of Analog DevicesSandia’s embedded integrative process

Off-chip electronics and integration in a packageFlip-chip with analogous electrical connection padsSpecial techniques

CMOS-based micromachiningMotorola’s process for their MAP sensor

In any case, system partitioning is a big issue with any MEMS device.


48iMEMS: Completely integrated process

Analod Devices developed iMEMS process for their inertial MEMS sensor.It combines CMOS with surface micromachining.

Mechanical components753 um x 657 um

1940 um

1940

um

One of its version consisted of…13 for electronics and11 for mechanical components.

It combines…MOS transistors,Bipolar transistors,And polysilicon structures.


49Steps in iMEMS process

Interconnect region between the circuit and sensor element formed.

After deposition and patterning of the structural polysilicon.


50iMEMS process steps (contd.)

Opening of contact holes and metallization in the circuit part.

After releasing the mechanical structure.


51Sandia’s embedded integrative process

Mechanical Electronic

Mechanics first approach.

Protects the mechanical parts and carries CMOS.


52Package-level integration

Motorola’s accelerometer

The accelerometer’s MEMS chip does not contain any electronic components.All electronics are in the Application Specific Integrated Chip (ASIC).The interface between the two is via the wire bonds.

Divide and rule!


53Flip-chip process

MCNC/Cronos/JD Uniphase/MEMSCAP MUMPs

MEMS-chip

Electronic chip (ASIC)

Solder bumps for interface

Modularity helps.Both MEMS and ASIC processes can be standardized for multiple users.


54Motorola’s special integrated process

Motorola’s MAP sensor

Motorola developed a special bipolar transistor process for (100) wafer while (111) wafers are traditionally used for it.


55Components of packaging

Motorola’s Manifold Pressure Sensor


56What is packaging?

Interface to the macro worldMounting Fluidic fittings for microfluidic devicesSample introduction for bioMEMSOptical windows for MOEMS

Thermal managementProtection from environmentDamage due to handlingDirectly impacts the ability to calibrate and test


57Why is packaging so important?

It is important for IC chips but more important for MEMS

MEMS need to work in environments that are much more differentPackaging could affect the sensitivity, noise, and robustness of the MEMS device—more so than IC chipsEach MEMS device needs a different a strategyIt accounts for 45% of the total cost; 20% for calibration and test


58Strategies for packaging

Design the MEMS device and its package at the same timeDecide on:

System partitioningMinimize the integrated electronics.

InterfacesThermal expansion, bio inertness/compatibility, chemical and fracture resistance, etc. of the interfaces

Specifications and parasiticsCapacitances, resistances, and inductances that may affcetthe devicePerformance, temperature and pressure ranges, reliability, sensitivity, noise

before detail design is doneSimulate with the packaging effects included


59 Electronics specific to capacitive accelerometers

Capacitance extraction circuitsShould be properly conditionedSufficient amplification

Sandia’s three-axis accelerometer

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