ailure Analysis of Sub-Micrometer Devices and Structures Using Scanning Thermal Microscopy

A. Majumdar, K. Luo, J. La i and Z. Shi Department of Mechanical and Environmental Engineering

University of Califomia Santa Barbara, CA 93106-5070

Abstract - We have developed a new technique of scanning thermal microscopy (SThM) to simultaneously measure surface morphology and temperature fields with a spatial resolution of 0.01 pm. Short-circuit defects in transistors as well as hot spots in sub-micrometer metal lines have been detected. The ability to study abberrations in local power dissipation at such unprecedented high resolution makes SThM an ideal tool for probing existing and new failure mechanisms at the length scales of defects.

INTRODUCTION

Recent trends in electronic devices have suggested that the smallest feature sizes will soon shrink to dimensions below 0.2 pm. Not only does this pose new challenges in terms of failure detection and location, but the small dimensions may lead to new failure mechanisms. In addition, the sizes of defects in many failure modes, electrostatic discharge for example, are often smaller than the devices. This places additional challenges in detection and understanding of failure mechanisms. Development of new high-resolution failure analysis techniques are, therefore, essential.

Electrical failures usually occur in the form of open or short circuits in interconnects and devices. These can be introduced during the fabrication process or during operation by phenomena such as electromigration, electrostatic discharge besides residual and thermal stresses. Such device failures generate hot or cold spots which have traditionally been detected by several optical techniques [l] such as infra- red emission microscopy [21, liquid crystals [3] and photon probing methods such as optical beam induced currents I41 and light-induced voltage alterations [ 5 ] . For infra-red emission microscopy, the hot-spot temperatures need to be about 200 O C and, hence, the wavelength of the infra-red radiation that is dominant in the radiant energy is about 7 pm [6]. Therefore, the best spatial resolution of this technique is about 10 pm. Temperature-sensitive liquid crystals (LC) have been used at 1 pm spatial resolution to identify hot spots by changes in color and polarization of the reflected light [3].

Optical beam induced currents have also been used to detect device failures at spatial resolutions as high as 0.3 pm [4]. All these optical techniques are limited in their spatial resolution by the diffraction limit and, hence, 0.3 pm is perhaps the limit for visible optics. Although vacuum-based techniques involving electron and ion beams do have higher spatial resolution, they are expensive and often destructive and cannot be used under ambient operating conditions. Fabrication of devices in the sub-0.2 pm range suggests a strong and urgent need for the development of new techniques for detecting and locating electrical failures in current and future devices. In view of this, we have developed the technique of scanning thermal microscopy (SThM) which we have successfully used for failure analysis with an unprecedented spatial resolution of 0.01 pm.

SCANNING THERMAL MICROSCOPE

Under an applied voltage bias, open or short-circuit electrical failures as well as voids in metal lines, create aberrations in local power dissipation. Measurement of the resulting temperature distribution can be used to locate and even probe the mechanisms of failure. The goal of developing the SThM is to push the resolution of thermal imaging as high as possible.

The SThM is based on the atomic force microscopy (a) [6], but with a specialized probe. The AFM (sec Fig. 1) consists of a sharp tip mounted on a cantilever beam. The sample is placed on a piezoelectric actuator which can move in the x , y (lateral) and z (vertical) directions. Cantilever deflections due to tip-sample interatomic forces are detected by an optical lever system. The differential photodetector signal, (A-B)/(A+B), is used to control the z-motion of the piezo-scanner to keep a constant cantilever deflection or tip- sample force. Deflections as small as 0.1 A can be detected. This, along with a spring constant, k = 0.1 N/m, results in a force resolution, A F = k6 = N. Such low forces can produce single atom contact which gives the AFM the ability to obtain surface images at atomic resolution [7].

0-7803-2753-51961$5,00 1996 IEEE 342

Photodetector

Ck

Laser Beam I Cantilever

Sharp Tip -Sample

Fiemoelect ric Actuator for x-y-s motion

Fig. 1 Schematic diagram of the atomic force microscope

The idea behind the SThM is to place a temlxrature sensor at the end of a cantilever probe tip and scan a heated surface to simultaneously obtain topogiraphical and thermal images. Several probe designs have been developed. The first dlesign, shown in Fig. 2(a), was of a wire-thermocouple probe where the two wires were electrochemically etched and then the sharp points were welded to make a junction that was usually about 10-20 pm in diameter [8,9]. The wires were used as the cantilever for force sensing, and an aluminum foil was used or reflecting the laser. The thin-film probes, shown in Fig. 2(b), were fabricated on commercially-available SiN, cantilever probes that are commonly used for high-resolution AFM. The probe consisted of Iwo metal films separated by an insulating oxide film, with a thermocouple junction formed between the metal films only at the probe tip. A novel nanofabrication technique has been developed to make the thermocouple junction size about 0.1-0.3 pm in diameter, and is described elsewhere [10,11]. The use of both these probes for thermal imaging is now described.

DEFECT DETECTlON IN MOSEYS

Figure 3(a) shows a 25 pm x 25 pm topographical image of a Si-MOSFET, obtained by the wire thermocouple probe shown in Fig. 2(a>. The draiin, the gate and the source are clearly visible and the gate length was found to be 3 pm. The device was covered with a passivation layer consisting of 1 pm-thick silicon dioxide and 1 pm-thick silicon nitride films. The grain structure observed in the topographical images was those of the silicon nitride layer. The thermal image iin Fig. 3(b) was obtained under a source-to-drain voltage bias, Vds = 15 V, and gate bias, V,, = 5 V. The unusually high drain bias was chosen so as to get a good thermal signal from our probe. The drain current was 2.2 mA resulting in a power dissipation of 33 mW. The gate appeared to have the highest temperature

Alumel Silicon Nitride

mounted on thin-film thermocouple thermocouple junction junction

Fig. 2 Probe designs for scanning thermal microscopy: (a) wire thermocouple probe with a diamond tip; (b) thin- film thermocouple probe with junction size of 0.1-0.3 pm located at the end of a SiN, AFM probe tip.

because this is the region of the highest electric field and, therefore, produced the highest power dissipation.

We intentionally created an electrostatic-discharge failure by human touch. The failed device produced current-voltage (I-V) characteristics shown in Fig. 4. These I-V characteristics are typical of those obtained after an electrostatic discharge failure, and has been observed before [12]. Under the same biasing conditions, the currents were much higher than the original device, thus suggesting a short-circuit leakage. Figure 3(c) shows the thermal image of the device after ESD failure, under Vd, = 10 V, V,, = 10 V and power dissipation of 200 mW. It is clear from the thermal image that the hot spot due to the failure occurred at the base of the gate and between the gate and the drain.

It is evident from Figs. 3, that the thermal images contain a negative image of the grains that are visible in the topography. The grain boundaries seem hotter than the inside of the grain. It is conjectured that this may be due to the fact that when the tip goes into the valley created by the grain boundary, the sample-to-tip thermal conductance increases due to exposure of a larger surface area. Nevertheless, the fact that grain boundaries can be imaged suggested that the spatial resolution of these wire probes was about 0.5 p [13].

TEMPERATURE DISTRIBUTIONS OF SUB-MICROMETER METAL LINES

The thin-film thermocouple probes, shown in Fig. 2(b), were developed with the intent of increasing the spatial resolution beyond 0.5 pim- To demonstrate this, we chose a magnetoresistive (MR) reading head [ 141, schematically shown in Fig. 5(a). The MR element material was NiFe and the element was 0.03 pm wide on the head or air-bearing surface, 5 p long and 2.5 p thick (i.e. into the page). The

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(a) (b) (C) Topographical Image Thermal image Before Failure Thermal Image After Failure

5 10 13 20 Ut?) 5 I C 15 2 l C I l T l 5 ' 0 ' 5 2!. , - Height Range 2 pm Temperature Range 32.7-32.8"C Temperature Range: 34 5-36 aC

Fig. 3 graphical and thermal images, before and after ESD failure, of a Si-MOSFET. Scanning thermal microscopy shows ilure to occur between the gate and the drain close to the gate interconnect.

5

0 1 2

4

6

A

2 4 6 8 10 12 o-Source Voltage, V ds (V)

Fig. 4 Current-voltage ( I - V ) characteristics of the Si- MOSFET after ESD failure

element was enclosed between two soft-adjacent MR layers and 0.1 pm thick alumina films on each side. The ~ o ~ g r a ~ h ~ c a l image in Fig. 5(b) shows long grooves formed during surface polishing of the MR head. Although the MR element cannot be seen in the topography, the hot spot created by running an electric current of 12 mA, is clearly visible in Fig. 5(c). The current density was 1.6 x 10" A/m2. Using tlae resistivity of the NiFe as 2.2 x 10-7 a-m, the power density was estimated to be 5.6 x 1015 W/m3 and total power dissipation to be 2.1 mW. The cross-sectional temperature profile in Fig. 5(c) suggests the full-width-half-maximum of the temperature peak to be 0.25 pm and the half-maximum

temperature rise to be 0.33 O C . This suggested a temperature gradient of 2.6 x lo6 K/m. Using 36 W/m-K as the thermal conductivity of alumina and the measured temperature gradient, the power dissipation was estimated to be 2.3 mW, in close agreement with that from electrical measurements. The maximum temperature rise was estimated to be about 20 OC.

Failures in such metal lines occur due to electromigration under high current density and elevated temperatures as well as due to thermal stress-induced void formation and film delamination. Thermal stresses are induced both by the temperature rise above the ambient as well as the temperature gradient. We demonstrate the ability of the SThM to measure both these quantities at the length scale of the metal line.

Due to higher heat transfer in the valleys of the surface, the thermal image in Fig. 5(c) shows that the temperature appeared hotter in the valleys than on the hills. Corresponding to the groove indicated by the arrow in Fig. 5(b), a narrow ridge in the thermal image can be found. The cross-sectional temperature profile clearly shows the ridge. The lateral distance between the bottom and the top of the ridge was measured to be 8 nm, which is close to the estimated of spatial resolution from noise measurements [10,11]. This is the first direct evidence of thermal imaging at such high spatial resolution.

CONCLUSIONS

Aberrations in local power dissipation provide signatures for defects as well as characteristics of failure mechanisms. The ability to resolve temperature distributions at the length scale of the failure can therefore provide key insight into the

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Fig. 5 (a) Schematic diagram of the magnetoresistive (MR) reading head with 0.03 pm-wide NiFe line; (b) AFM topographical image, 1 pm x 1 pm in size; (c) SThM image and temperature profile under cuiTenh density of 1.6 x 10l1 A/m2

failure mechanism. We demonstrate an unpreoedented ability to measure temperature distributions with sub- 10 nm spatial resolution by scanning thermal microscopy. This technique has the potential to become an useful tool for future failure analysis.

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I 0 0 . 2 5 0 . 5 0 0.75 pm

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