Observation of defects in semiconductor-on-insulator (SOI) wafers by a nondestructive bulk micro-defect analyzer

Journal of Crystal Growth 129 (1993) 405—410 North-Holland

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CRYSTAL GROWTH

Observation of defects in semiconductor-on-insulator (SOl) wafers by a nondestructive bulk micro-defect analyzer H. Wada and K. Moriya Corporate R&D Center, Mitsui Mining & Smelting Co., Ltd., 1333-2 Llaraichi, Ageo-shi, Saitama 362, Japan Received 15 December 1992

A nondestructive detection method for near-surface defects on SO! (semiconductor-on-insulator) wafers by means of spectroscopic laser scattering topography was developed. The laser scattering from defects in/on multilayered wafers was found to be strongly dependent on the wavelength of the laser. This wavelength dependence is understood as the interference phenomena between the surface and oxide layers in the SQl wafer. By choosing the correct wavelength, the system can distinguish between defects and dust or pits on the surface of the wafer. The results obtained by this method closely coincide with measurements by ESCA, SEM and reflection spectroscopy.

1. Introduction

fine precipitates and micro-defects near the top and the interface region between Si and Si02

Lately, the yield of materials for LS1 processing has been considerably improved by advances in fabrication techniques for integrated circuits [1]. In particular, SIMOX (separation by implanted oxygen), has been found to be particularly useful within the field of SOl (silicon on insulator) processing. There are two basic steps in the preparation of a SIMOX wafer. At first, a high dose of oxygen is implanted, which forms a buried oxide layer below a thin top silicon layer. Secondly, the sample is annealed at high temperature in an inert gas atmosphere to reduce implantation damage, and to remove oxide precipitates. This treatment improves the quality of the top silicon layer. The SIMOX wafer consists of three layers. The top layer is a silicon layer about 1.0 ~tm in thickness including dislocations and oxide precipitates which increase in density towards the oxide layer. The next layer is a buried oxide layer about 0.4 ~tm in thickness and then there is a base silicon layer below this. The study of how to eliminate

layers is still being continued by several methods. In bulk silicon, intrinsic gettering (IG) [2—61 processing of Si wafers has been adopted and found to be effective in both removing oxidation induced stacking faults (OSFs) [4], and achieving effective gettering of heavy metal contamination. The bulk micro-defect (BMD) analyzer [71was developed to observe these micro-defects in Si wafers. With this system, an image of defects can automatically be obtained using laser scattering tomography. This defect image then undergoes binary image processing and defect counting, making it possible to obtain a defect density profile. The detectability of this method has been confirmed by comparisons with images taken by X-ray diffraction microscope [8,9], transmission electron microscope [10—12],and etching method [13—16].With the conventional BMD analyzer, the scattering image is observed from the cleaved surface of a Si wafer; therefore it is impossible to resolve either the conditions near the surface, or the boundary conditions of an ion implanted Si

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Elsevier Science Publishers B.V. All rights reserved

406

H. Wada, K Moriya

/ Obseriation

of defects in 501 wafers by nondestructii e BMD analyzer

wafer. For eliminating this problem, a new technique, the non-destructive BMD analyzer, has been developed, In this paper, a SIMOX wafer was examined for detecting the damage near its surface, and defects in the Si and Si02 layers, using the new non-destructive BMD analyzer system, and then these results were compared with those taken by ESCA (electron spectroscopy for chemical analysis) [17], and near infrared spectroscopy.

2. Experimental procedure and results Fig. Ia shows a block diagram of the nondestructive BMD analyzer, employing a tunable

wavelength laser. A laser beam of specified wavelength is focused onto the surface of a sample at a large induced angle, i.e. minimizing surface reflection, and the scattered light from the defects within is received by an infrared TV camera. The resulting signals are then digitized, processed by a frame memory (512 X 512 X 16 bits) and stored in a computer. This operation is repeated in steps along the surface of the wafer until the whole area has been scanned (fig. ib). The images are then processed to correct inhomogeneity and background noise of the TV camera, and transferred to the frame memory and displayed on a CRT monitor. Fig. 2 shows the cross section of a SIMOX wafer by SEM observation. From the difference

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Sample stage Y stage Vibration isolation bench

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Laser beam Image in Frame buffer

Si wafer

(b) Fig. I. Block diagram of nondestructive BMD analyzer (a) and the method for image formation (b).

H. Wada, K Moriya

/ Observation of defects in SOl wafers by nondestructive BMD analyzer

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rate of this SIMOX wafer having 1.0 ~rm thickness shows a minimum at 940 nm and a maximum

0.4km

at nm. This means that the nm beam is not1000 reflected at the surface but 940 is able to pass

Fig. 2. Cross sectional image of a SIMOX wafer observed by SEM.

through the Si and Si02 layers. The 1000 nm beam, however, is reflected at the surface, and the beam intensity in the layers is very weak. The scattering image was measured at 940 and 1000 nm. The laser beam is finely focused on the surface, at a diameter of about 2—3 ~tm. In order to confirm the existence of the defects, the observed SIMOX wafer was slightly etched, as

in darkness between the Si and Si02 layers, the thickness of the Si02 layer can be measured. In this case, there is a homogeneous zone in an O~implantedwafer. The thickness of the Si layer is about 1.0 ~itn and that of the Si02 layer about 0.4 ~.rm,this can be checked by comparison with ESCA. The thickness of the Si layer is also confirmed by the measurement of reflectance spectroscopy. As shown in fig. 3, an oscillation curve of the reflectance spectrum is obtained for the SOl wafer. The thickness of the Si layer is calculated from the interval measurement of this oscillation. In the range near the wavelength corresponding to the band gap energy of Si, the reflection

shown in fig. 4. On the left-hand side of the surface, many pit patterns were formed. On the right-hand side, some contaminations were found. Fig. 5 shows the scattering images at 940 and 1000 nm obtained by a nondestructive BMD analyzer. ln the observation at 940 nm, the boundary between the etched and the unetched layer was enhanced and the scattering images for both the etched and unetched planes were visible. On the other hand, in the observation at 1000 nm, the boundary was indistinct and only the scattering image of the etched plane was enhanced. Three contamination points were confirmed on the right side of the surface in both scattering images. The horizontal lines in both images are the raster lines of the two-dimensional laser scanning due

Silayer~ 8i02 layer

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1000

wavelength

1500

2000 (nm)

Fig. 3. Refractive index spectrum of the SIMOX wafer. Dashed region shows the experimental range in here.

408

H. Wada, K. Moriya

/ Observation

etched surface

of defects in SOl wafers by nondestructive BMD analyzer

:

unetched surface Surface contamination

Si layer

Fig. 4. Schematic view of the etched SIMOX wafer. Surface pits are related to the dislocations.

to the fluctuation of the laser power. These lines will be reduced by stabilizing the laser intensity or correcting the image intensity by the monitored laser intensity.

The wavelength dependencies of the scattering intensity of the etched and unetched parts were measured and compared with the reflective index. As shown in fig. 6, the reflective index curve

-~—--~-

etched

unetched

etched

unetched

Fig. 5. The scattering images at 940 nrn (a) and 1000 nm (b) by a nondestructive BMD analyzer; 200 ~omsquare.

/ Observation of defects in SOl

H. Wada, K Moriya

wafers by nondestructive BMD analyzer

409

100 Reflective index

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The scattering intensity of etched part

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is similar to the scattering intensity curve of the etched part; the unetched part shows an inverse relationship with the reflective index curve. From this behavior, it has been proved that the scatter-

7b show the beam intensity of 940 and 1000 nm wavelengths in the wafer, respectively. The 1000 nm beam is reflected by the surface as a result of interference between the surface and Si/SiO2

ing intensity from surface pits and defects in the layer is proportional to the intensity of reflected light and transmitted light, respectively, i.e. the strong light scattering from the defects in the layers and pits or dust on the surface are observed by 940 and 1000 nm wavelengths, respectively.

interface, and the defects in the wafer are insensitive to the light. On the other hand, the 940 nm beam passes through the surface and hence an image of the defects in both the Si and Si02 layers can be obtained. By using the difference in sensitivity between internal defects and surface pits or contamination, it is possible to distinguish between them. In this case, we obtained the image of the defects by ustng a near infrared laser. However, as shown in fig. 3, such measurement can be performed by visible laser as the interferometri-

3. Discussion Scattering behavior can be explained by the schematical drawings shown in fig. 7. Figs. 7a and light intensity

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depth

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back surface depth

Fig. 7. The path of a laser beam in the SIMOX wafer. (a) The laser beam path at 940 nrn; (b) the laser beam path at 1000 nm.

410

H. Wada, K. Moriya

/ Observation of defects in SOI wafers by nondestructi, ‘e BMD analyzer

cal oscillation can be observed at wavelengths longer than 600 nm. When the laser beam is finely focussed, it is possible to distinguish between internal defects and the surface. The penetration depth of the beam depends strongly on its wavelength, within the range of the band gap of silicon. Therefore by tuning the wavelength of the laser and hence the penetration depth, this system is able to detect nearsurface defects [18], and can be applied to multilayer devices (e.g. SO!). This ability to measure defects within a wafer, from the surface, using a tunable laser, i.e. the basis of the non-destructive BMD analyzer, is an entirely original idea within the field of Si wafer inspection.

4. Conclusion In conclusion, a new non-destructive BMD analyzer for detecting near-surface defects has been developed. In this system, a laser beam is finely focused and introduced to a Si wafer at a large induced angle to avoid the influence of surface reflectance, and hence the defects within the wafer can be viewed under a microscope. By this method we can obtain information about defects at any depth from the surface. After some modifications, observation of the smaller precipitates near the surface of an IG treated Si wafer, and the defects under LSI devices, will be possible.

References [II

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[5] Phys. GA.

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