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Raising yields of bonded silicon wafers with POU DI-water microfiltration and purification

Iqbal Bansal, M/A-Com, a Tyco Electronics company; and Bipin Parekh, Mykrolis

In the competitive semiconductor manufacturing environment, product yields have become the predominant concern in VLSI device processing. As much as 75% of yield loss can be attributed to structural defects, the majority of which result from surface particle contamination.1–3 Price’s Yield Law relates die yield (Y) to defect density (D), effective die area (A), and the number of critical mask steps (n) using a simple exponential equation:3

An example of Price’s Yield Law for a single mask step is shown in Figure 1. Yield declines rapidly with an increase in the defect density. The combined effects of multiple masking operations result in even more dramatic losses.

Figure 1: Correlation between number of defects per die and yield.

To enhance product yields, it is critical to reduce surface contamination by employing an optimized sequence of chemical cleaning and deionized (DI)-water rinsing processes. The removal of submicron particles and ionic contaminants from cleaning chemicals and DI rinsewater results in cleaner wafer surfaces, yielding improved wafer surface cleanliness and enhanced yields.

This article describes the role of pure water and cleaning chemicals, wet cleaning processes, and contamination control using advanced filtration and purification methods for producing clean bonded-wafer surfaces. Silicon-to-silicon bonded wafers offer a cost-effective substrate for thick epitaxial and inverse epitaxial layers, which have traditionally been used for manufacturing power devices and pin diodes. Test results showing the effectiveness of point-of-use (POU) purification are presented, along with wafer-level data that correlate high yields to a cleaner wafer environment.

The article presents detailed product throughput and yield data in terms of void density. It discusses the role of ultrapure DI water and ultradilute concentrations of ultrapure chemicals to produce clean silicon- wafer surfaces. In the tests highlighted in the article, the final wafer rinse was performed using ultrapure DI water, which was first filtered using a high-efficiency membrane filter that removes particles down to 0.02 µm and is purified to remove metal ions to sub-parts-per-billion levels. Detailed void data from tests both with and without POU microfiltration of DI water are compared.

Ensuring High Wafer Yields

Direct Silicon-Wafer Bonding. Direct wafer bonding is a three-step process that consists of surface preparation, contacting, and thermal annealing. The contacting step involves ultrafine alignment and face-to-face joining of two silicon wafers. Hydrophobic Si-Si bonding is a process for the direct bonding of two silicon wafers without an interfacial oxide film. A schematic diagram of the hydrophobic Si-Si bonding process is shown in Figure 2.

Figure 2: Schematic diagram of a typical hydrophobic bonding operation.

DI-Water Microfiltration and Purification. In the bonding process, the control of surface contamination is a key parameter for enhancing product yields. Surface contamination such as haze and light-point defects (LPDs) must be minimized to ensure good bonds. Before joining takes place, the silicon wafer is chemically cleaned to reduce surface contamination. The submicron particle density can be lowered greatly by performing point-of-use (POU) microfiltration and purification of DI water and applying an ultradilute concentration of ultrapure chemicals containing extremely low liquid particle counts.

High-purity POU DI rinsewater is made using a combination of a high-performance submicron particle filter and a metal-ion purifier. These devices ensure desirable performance characteristics such as excellent particle and ion removal, downstream cleanliness, and low pressure drop. The submicron particle filter discussed here is based on a charged membrane that consists of a polyvinylidene fluoride matrix and a proprietary moiety. The charged membrane removes particles by sieving as well as by electrokinetic capture and adsorption throughout the depth of the membrane. This dual mechanism enhances particle removal and enables the charged filter to remove particles smaller than the pore size of the membrane.4

The metal-ion purifier is a POU dissolved-ion purifier/filter that removes trace amounts of metal contamination from ultrapure water, organic solvents, and other chemicals used in the microelectronics industry. The purifier/filter device, assembled in a 10-in. cartridge and a disposable capsule, uses a pleated ion-exchange membrane. Also known as a radiation-grafted membrane (RGM), this part is prepared using the radiation-induced grafting polymerization method, which introduces ion-exchange groups directly and covalently onto the surface of a microporous membrane.5

The membrane purifier’s ion-removal reaction is essentially identical to that of a strong acidic ion-exchange resin. In other words, it replaces H+ on the sulfonic acid group by the positive ion in solution. However, while the purifier has ion-exchange groups on the surface of a microporous membrane through which fluids flow, conventional ion-exchange resins consist of spherical beads that contain ion-exchange groups inside the micropores, resulting in diffusional resistance to ion-exchange reactions. The purifier’s ion-exchange membrane does not limit the rate of ion-exchange reactions, enabling it to achieve high metal-removing conversion under high fluid flow rates and with a small filter size. In addition, unlike ion-exchange resins, the ion-exchange purifier membrane does not exhibit a polymer swelling effect. Hence, the metal-ion purifier offers a high ion-removal rate in a single pass, high ion-removal capacity, and the ability to remove multielement ions.

A schematic diagram of the RGM is shown in Figure 3. The removal efficiency of POU DI rinsewater generated using the metal-ion purifier is presented in Tables I, II, and III, which present data from M/A-Com, an Asian customer fab, and in-house tests, respectively. Additional details of ion removal performance and customer evaluation results appear in the literature.5 These results demonstrate that trace impurities in DI water were reduced to parts-per-trillion levels by using the 0.02-µm filter followed by a metal-ion purifier. Ion-removal rates ranged between 75 and 99%.

Figure 3: Schematic diagram of the purifier membrane structure.
Table I: M/A-Com fab data showing trace-ion removal results before and after the use of the metal purifier.
Table II: Asian customer data showing trace-ion removal results before and after the use of the metal purifier.
Table III: In-house test data showing trace-ion results before and after the use of the metal purifier.

The Prebonding Procedure

Chemical Cleaning. Prior to the joining operation, device wafers were chemically cleaned using ultradilute modified RCA SC-1 (NH4OH:H2O2:H2O) and RCA SC-2 (HCl:H2O) chemistries to remove surface haze and submicron particulate contaminants. It was not necessary to perform a chemical clean on incoming handle substrates. Both device and handle substrates were simultaneously processed through an ultradilute oxide etchant to completely remove chemical and native oxide films. The wafer substrates acted like hydrophobic surfaces—that is, DI water did not stick to them and formed tiny beads near the wafer edge. The oxide etching step was immediately followed by a drying step using a motionless dryer.6 A DI-water rinse was not performed between the oxide etch and drying steps. The DI rinsewater supplied to the dryer was purified at the point of use using the 0.02-µm-rated particle filter followed by the metal-ion purifier.

Surface-Contamination Measurement. A Model 5000 laser beam Surfscan system from KLA-Tencor (San Jose) was employed to directly measure surface haze and LPDs on device and/or handle substrates before the ambient-temperature joining operation. This system can measure LPDs down to 0.26 µm.

Submicron LPD counts must be minimized to reduce the density of voided or disbonded regions, which result in delamination. A 1-µm particle, for example, can cause voids as large as 1 cm in diameter during the direct wafer bond process.7

Analysis of Voids in the Bonded Interface. A C-mode scanning acoustic microscope (C-SAM) system from Sonoscan (Elk Grove, IL) was used to quantitatively detect the presence of microvoids, delamination, and other defects in an interface layer of bonded wafers. Using a focused low-numerical-aperture transducer, the C-SAM system applies pulsed-frequency ultrasound waves in the 10–200-MHz range to obtain acoustic images. In the test described in this article, the ultrasound media was DI water. A typical map of an acceptable annealed wafer pair is shown in Figure 4a, and a map of a failed annealed pair containing numerous small voids is shown in Figure 4b. The failed sample also contained a large void that occupied 2.1% of the total active surface area of the wafer.

Figure 4: C-SAM map of (a) an acceptable annealed wafer pair and (b) a failed pair with a large void and several smaller ones.

Data and Results

The effectiveness of submicron particle filtration and metal-ion removal is reflected in the increased cleanliness of DI rinsewater and a corresponding improvement in the yields of thermally bonded wafer pairs. To determine the effectiveness of filtering and purifying DI water, detailed itemized lot data were gathered from bonded wafers that were processed using unfiltered and unpurified DI water or POU-filtered and purified DI water.

Yields of Annealed Wafer Pairs without POU Microfiltration and Purification of DI Water. Detailed product lot yield data for wafer regions with voids are provided in Figure 5. Over a three-year period, the total number of annealed pairs tested was 3359. The overall pass yield was measured at 94%. A primary cause of yield loss was particulate contamination on the wafer surfaces prior to the joining step.

Yields of Annealed Wafer Pairs with POU Microfiltration and Purification of DI Water. Product lot yield data for wafer regions with voids are presented in Figure 6. During an eight-month period, 36 25-wafer product lots were tested, totaling 900 annealed wafer pairs. The overall pass yield was measured at 97%. A primary factor in the increased yield was the use of POU microfiltered and purified DI rinsewater.

Yields of Individual Annealed Wafer Pairs in Different Carrier Slots with POU Microfiltration and Purification of DI Water. Product lot data for bonded pairs are illustrated in Figure 7. On only two pairs (processed in carrier slots H1 4 and H2 5) did the largest void occupy 0.1% of the wafer surface area. All other 23 pairs had small voids or no voids at all. A primary factor in the absence of regions with voids was the improved cleaning that was achieved using microfiltered and purified DI rinsewater for both prejoined handle and device wafers.

Figure 5: Yields of annealed wafer pairs that were processed without POU DI-water microfiltration and purification (mean pass yield = 94%).

Figure 6: Yields of annealed wafer pairs that were processed with POU DI-water microfiltration and purification (mean pass yield = 97%).

Figure 7: Yields of individual annealed wafer pairs processed in different carrier slots with POU DI-water microfiltration and purification.

Conclusion

This article has demonstrated the effectiveness of performing POU microfiltration and purification of DI rinsewater in direct silicon-wafer-bonding applications. Ultrapure DI water, processed through a 0.02-µm filter and then run through a metal-ion purifier, yielded cleaner wafer surfaces than untreated DI water, increasing product-wafer yields. In the tests described here, pass yields increased from 94% for wafers that were processed without POU DI-water filtration and purification to 97% for wafers that were processed with POU filtration and purification.

Acknowledgments

This article is an edited, revised version of a presentation from the Semiconductor Pure Water and Chemicals Conference (SPWCC), held February 14–16, 2005, in Santa Clara, CA. Copyrights reserved by SPWCC. Reprinted with permission of SPWCC. The authors would like to thank Mark Surgent, Joel Goodrich, and Bruce Cochran from M/A-Com for their contributions, encouragement, and continued support of the work described in this article.

References

1. JM Duffalo and JR Monkowski, “Particulate Contamination and Device Performance,” Solid State Technology 27, no. 3 (1984): 109.

2. J Berger, “Sources of Contamination in VLSI Processing: A User’s Point of View,” Microcontamination 3, no. 2 (1985): 16–18.

3. P Gise, “Surface Particle Detection Technology,” in Handbook of Contamination Control in Microelectronics, ed. Don Tolliver (Park Ridge, NJ: Noyes, 1988).

4. B Parekh, K Vakhshoori, and J Zahka, “Particle Control: Filtration of High Purity DI Water for Semiconductor Manufacturing,” Ultrapure Water 10, no. 4 (May/June 1993): 53–59.

5. B Parekh, Y Hashimoto, and M Amari “Purification of Wet Etch and Cleaning Chemicals and DI Rinse Water Using High Performance Membrane Purifiers,” in Proceedings of the Semiconductor Pure Water and Chemicals Conference (San Jose: SPWCC: 2003), 46–63.

6. I Bansal, “Using Ultradilute and Ozonated DI-Water Chemistries to Clean Silicon Wafer Surfaces,” MICRO 20, no. 8 (2002): 37–41.

7. A Mirza and A Ayon, “Silicon Wafer Bonding for MEMS Manufacturing,” Solid State Technology 42, no. 8 (1999): 73–78.


Iqbal (Izzy) Bansal is a senior principal engineer at M/A-Com, a Tyco Electronics company (Burlington, MA). He is the author or coauthor of more than 35 technical publications, including scientific articles and book chapters. He also holds three patents. With extensive practical experience in various processes used in the manufacture of submicron ICs, Bansal has technical expertise in the area of microcontamination control and has received the Maurice Simpson technical editor’s award presented by the Institute of Environmental Sciences. He has also been active in the research and development of physiochemical treatment systems for DI water and wastewater treatment systems. He received an MS in chemical engineering from Clarkson University in Potsdam, NY. (Bansal can be reached at 781/564-3417 or bansali@tycoelectronics.com.)

Bipin Parekh, PhD, is senior consulting engineer in liquid applications development at Mykrolis (Billerica, MA), where he has worked since 1980. He has coauthored more than two dozen articles on separations and contamination control. He holds four patents. A member of the American Institute of Chemical Engineers and the Tau Beta Pi engineering honor society, Parekh received a PhD in chemical engineering
from the State University of New York at Buffalo in 1977. (Parekh can be reached at 978/436-6636 or bipin_parekh@mykrolis.com.)


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