Surface Grinding in Silicon Wafer Manufacturing

ti MROI-271 w Surface Grinding in Silicon n Wafer Manufacturing

< author(s) n ZJ PEI GRAHAM R. FISHER Kansas State University MEMC Electronic Materials Inc. Manhattan, Kansas St. Peters, Missouri

abstract

Silicon wafers are used for production of most microchips. Various processes are needed to transform a silicon crystal ingot into wafers. As one of such processes, surface grinding possesses the great potential of producing silicon wafers with lower cost and better quality comparing with its counterparts (lapping for wire- sawn wafers and polishing for etched wafers). In order to fully utilize the potential of surface grinding, however, some technical obstacles will have to be overcome. This paper will present these obstacles as well as some approaches to attack them.

conference

NAMRC XXIX ~ May 22-25,200l Gainesville, Florida

terms

Silicon Semiconductor Materials Silicon Wafer Lapping Subsurface Damage Sponsored by the North American Manufacturing Research institution of 8 the Society of Manufacturing Engineers * One SME Drive Society of lwmm Dearborn, MI 48121 Manufacturing Phone (313) 271-1500 Engineers www.sme.org/namri 2001 SME TECHNICAL PAPERS

This Technical Paper may not be reproduced in whole or in part in any form without the express written permission of the Society of Manufacturing Engineers. By publishing this paper, SME neither endorses any product, service or information discussed herein, nor offers any technical advice. SME specifically disclaims any warranty of reliability or safety of any of the information contained herein. SURFACE GRINDING IN SILICON WAFER MANUFACTURING

ZJ Pei Department of Industrial and Manufacturing Systems Engineering Kansas State University Manhattan, KS 66506

Graham R. Fisher MEMC Electronic Materials Inc. St. Peters, MO 63376

ABSTRACT

Silicon wafers are used for production of most 35 microchips. Various processes are needed to transform a silicon crystal ingot into wafers. As one 30 ;;; of such processes, surface grinding possesses the i great potential of producing silicon wafers with lower 25 0 cost and better quality comparing with its z counterparts (lapping for wire-sawn wafers and 2o z polishing for etched wafers). In order to fully utilize g the potential of surface grinding, however, some 15 technical obstacles will have to be overcome. This 10 5 paper will present these obstacles as well as some m approaches to attack them. 5 2

0 1. INTRODUCTION 1997 1998 Year Single crystal silicon wafers are the most important building block of semiconductors. They are used in every type of microelectronic application, including @@J Revenue + Total Area ~ computer systems, telecommunications equipment, automobiles, consumer electronics products, FIG. 1 PRICE EROSION OF SILICON WAFERS (AFTER industrial automation and control systems, and MOZER, 2000). analytical and defense systems. The falling price of silicon wafers has applied a great As seen in Fig. 1, the worldwide revenue generated pressure on silicon manufacturers to reduce the by silicon wafers in 1999 was $5.8 billion, the 4% manufacturing cost. Therefore, it is critically important increase from the revenue of 1998 but with 26% to develop new manufacturing processes, or to more silicon produced [Mozer, 20001. This indicates develop new applications for some existing processes huge price erosion. that allow manufacturing silicon wafers more cost- Subramanian, 19981, even for producing 400 mm effectively. silicon wafers rakada et al., 19981.

Surface grinding possesses the great potential of Slicing producing silicon wafers with lower cost and better I I quality relative to its counterparts (lapping for wire- sawn wafers and polishing for etched wafers). To fully Edge Profiling utilize the potential of surface grinding requires overcoming some technical obstacles. These obstacles and some approaches to overcome them Lapping will be discussed in this paper. I I This paper focuses on one of the applications of Etching surface grinding in silicon wafer manufacturing --wire- I 1 sawn wafer grinding, but will also briefly cover another application -- etched wafer grinding. Following this introduction section is a description of the surface grinding process. After that, the applications to wire- sawn wafers and etched wafers will be presented respectively. Then there will be a section on subsurface damage induced by surface grinding - an issue critical to both applications. Finally conclusions Cleaning I will be drawn.

FIG. 2. CONVENTIONAL WAFER SHAPING PROCESSES (AFTER VANDAMME, XIN AND PEI, 2. EXPERIMENTAL CONDITIONS 2000; PEI ET AL., 1999). Fig. 3 illustrates the surface grinding process. Grinding wheels are diamond cup wheels. The To turn a silicon crystal ingot into wafers with workpiece (wafer) is held on a porous ceramic chuck satisfactory quality, a sequence of processes will be by mean of vacuum. The axis of rotation for the needed. As shown in Fig. 2, this typically consists grinding wheel is offset a distance of the wheel radius of the following P/andamme, Xin and Pei, 2000; relative to the axis of rotation for the wafer. During Bawa et al., 1995; Fukami et al., 1996; Tonshoff et grinding, the grinding wheel and the wafer rotate al., 19901: about their own axes of rotation simultaneously, and Slicing (wire-sawing), to slice single crystal the wheel is fed towards the wafer along its axis. silicon ingot into wafers of thin disk shape; Edge profiling or chamfering, to chamfer Single crystal silicon wafers of ?50mm, 200mm and the peripheral edge portion of the wafer; 300mm in diameter, with (100) plane as major surface Flattening (lapping or grinding), to flatten are used. Resin bonded diamond grinding wheels the surface of the wafer; with different grit size (mesh #360, #1200, #2000, Etching, to chemically remove processing ##4000) are used. The surface grinders used include damage of the wafer without introducing Disco surface grinder (DFG840, Disco Corporation, further mechanical damage; Tokyo, Japan) and G&N surface grinder Rough polishing, to obtain a mirror surface (Nanogrinder, Grinding Machines Nuernberg, Inc., on the wafer; Erlangen, Germany). Fine polishing, to obtain final mirror surface; and During grinding, deionized (purified) water is being Cleaning, to remove the polishing agent or used to cool the grinding wheel and the wafer surface. dust particles from the wafer surface.

Surface grinding can be used for grinding wire-sawn wafers, to replace or partially replace lapping. It can also be used for grinding etched wafers, to partially replace rough polishing [Vandamme, Xin and Pei, 20001. Besides, surface grinding has also been proposed to replace etching rricard and 3

Wheel rpm Chuck rpm 3.2 Wire Saw Induced Waviness

Rotation axis - u Rotation axis Until recently, internal-diameter (ID) sawing had been of wheel of wafer the dominant slicing method for the past three decades (Werner and Kenter, 1988; Buttner, 1985). 1 Wire sawing is now fully established as the preferred method of slicing large diameter ingots. Due to its Fe&rate ’ 2 thinner kerf losses, wire sawing yields more slices per / ’ Silicon unit length of crystal ingot than ID sawing. Although Grinding the time per cut in wire-sawing is much longer than in / Wafer Wheel ID sawing, the overall throughput of wire-sawing is not I inferior to ID sawing, because wire-sawing can slice hundreds of wafers per cut while ID sawing cuts one wafer at a time.

Wire sawing induced waviness, also called long cycle FIG. 3 ILLUSTRATION OF WAFER SURFACE swelling or unevenness, has wavelength of 0.5 mm to GRINDING. 30 mm (Kato et al., 1997). It is also referred to the wavy stripes derived from wire sawing (Yasunaga et al., 1997). The generating mechanism of this waviness is not fully understood yet. This has been 3. SURFACE GRINDING OF WIRE-SAWN the main reason that it is very difficult to eliminate WAFERS waviness at wire-sawing process itself. If the subsequent processes do not remove the waviness, it will affect the flatness, especially the site flatness, of 3.1 Surface Grindinn vs Lapping the wafers. Conventionally, the wafers after wire sawing will go through lapping operations for flattening. In a typical double side lapping operation, a batch of wafers (for example, 20 wafers) is manually loaded into a lapping machine. Aluminum-oxide slurry is injected between the two metal plates, which rotate along opposite directions [Dudley, 19861.

The drawbacks of lapping operations include (1) labor intensive; (2) expensive consumable (slurry); and (3) time consuming. For example, a typical lapping operation will take about 40 minutes to reduce the wafer thickness (200mm or 300mm in diameter) by about 80 f.trn D/andamme, Xin and Pei, 20001. As-sliced Waviness Thickness variation The advantages of surface grinding over lapping are: (1) fully automatic with cassette-to-cassette FIG. 4 WAVINESS AND THICKNESS VARIATION. operation; (2) use fixed-abrasive grinding wheel rather than loose abrasive slurry so the cost of consumables per wafer may be lower; and (3) Fig. 4 shows a Magic Mirror picture of a wafer higher throughput. For example, it takes about 30- exhibiting the waviness and illustrates a cross-section 60 seconds to reduce the wafer thickness (200mm view of an as-sliced wafer with cross-section parallel to the cut direction. As shown in the figure, there are or 300mm in diameter) by about 45-75 pm. two components contributing to the “unflatness” and [Vandamme, Xin and Pei, 20001. unevenness: (a) thickness variation along cut direction, and (b) waviness. These two components One of the biggest obstacles for surface grinding to respond differently to the subsequent processes. For fully replace lapping is the wire-sawing induced example, lapping can effectively remove the waviness waviness. and thickness variation. Conventional surface grinding can achieve super parallelism with sub-pm TTV (total thickness variation), but cannot effectively remove the waviness [Kato et al., 19971

The waviness is not a problem for ID sawing. With modern ID sawing technology, the back side of the wafer is ground before the wafer is sliced off from the ingot, providing a reference plane for subsequent processes.

3.3 Inability of Surface Grinding to Remove I Conventional Wire-Saw Induced Waviness Surface Grinding j Conventional surface grinding cannot eliminate the wire-sawing induced waviness [Kato et al., 1997; Yasunaga et al., 19971. The wafer is held onto a ceramic chuck by high-pressure vacuum and thus elastically deforms to conform to the chuck surface. Although grinding can obtain very good parallelism FIG. 6 EFFECT OF CONVENTIONAL SURFACE GRINDING ON THICKNESS VARIATION. (TlV) between both surfaces, the waviness will be preserved due to the fact that the wafer will return to its original shape once the vacuum is released. This is illustrated in Fig. 5.

1 Conventional i Surface Grinding I

FIG. 5 EFFECT OF CONVENTIONAL SURFACE GRINDING ON WAVINESS.

Another problem associated with conventional surface grinding is that it will introduce additional waviness to the wafer if as-sliced wafer has LAPPING + POLISHING thickness variation along one direction. See Fig. 6. FIG. 7 EFFECTIVENESS OF SURFACE GRINDING AND LAPPING IN REMOVING WAVINESS.

Fig. 7 shows the Magic Mirror images of silicon wafers processed by a lapping operation and by a surface grinding operation. Lapping removes both sides of the wafer simultaneously while surface grinding grinds 5

one side at a time. The total removal amount is the same for lapping and grinding. The polishing operation following lapping and grinding is to provide the smooth surface required for Magic Mirror inspection. Same amount of removal is polished off for both cases. The figure shows that lapping is very effective in eliminating the waviness while surface grinding is not.

3.4 Approaches to eliminate the waviness

As discussed above, conventional surface grinding cannot effectively eliminate the waviness induced by wire-sawing operation. One practically approach to overcome this is to use surface grinding to Wax mount partially replace lapping, as shown in Fig. 8 grinding pandamme, Xin and Pei, 2000]. Surface grinding - will remove majority of the removal amount and leave the task of eliminating waviness to the subsequent lapping operation.

Slicing FIG. 9 EFFECT OF WAX-MOUNT GRINDING ON I I WAVINESS.

Surface Grinding

Lapping I I

Etching I I

Cleaning I I FIG. IO EFFECT OF REDUCED VACUUM GRINDING ON FIG. 8 SURFACE GRINDING TO PARTIALLY REPLACE WAVINESS. LAPPING (AFTER VANDAMME, XIN AND PEI, 2000).

3.4.2 Reduced chuck vacuum. Kato et al. [I9971 Several more approaches will be presented below. patented one approach to eliminate the waviness. Their approach is to grind the first side with reduced 3.4.1 Wax-mounting. One side of the wafer is chucking pressure during the spark-out time (final ground by holding the wafer onto a base plate with grinding stage). At the initial stage, the wafer is held a layer of wax in between. The wax layer absorbs onto the chuck by normal vacuum pressure and the the waviness on the back surface of the wafer. wheel is fed toward the wafer at a certain feedrate. At Then the other side is ground on the vacuum chuck the final stage, the feedrate is essentially zero, and [Kato et al., 19971. This is illustrated in Fig. 9. the vacuum pressure is reduced to such a value that 6

the elastic deformation of the wafer is substantially probably caused by the “un-uniformness” of the pad. reduced while maintaining the holding force. Then The non-uniform pattern probably does not affect TTV the second side of the wafer is ground with normal (Total Thickness Variation), but can severely affected vacuum pressure throughout the initial and final the site flatness. stages. See Fig. 10. Therefore, it is critical to develop uniform soft-pads in order for this approach to be widely used in 3.4.3 Soft-pad approach. A newly patented production. approach to this problem involves using a “soft-pad” or a resilient pad [Kassir and Walsh, 19991. When grinding the first side of wire-sawn wafer, a 4. SURFACE GRINDING OF ETCHED WAFERS perforated resilient pad is inserted in between the wafer and the ceramic chuck, hence the wafer is Surface grinding of etched wafers is also called fine not held in an elastically deformed condition. This grinding. Its first appearance in the public domain is ground surface will be the flat reference plane for through the US patent by Vandamme, Xin and Pei grinding the other side of wafer on conventional [2000]. ceramic chuck. The advantages of fine grinding of etched wafers are two-fold. One is to improve the flatness of etched wafers. Another is to reduce the removal amount for rough polishing by 2550% pandamme, Xin and Pei, 20001. The end result will be higher throughput for rough polishing and better flatness for polished wafers.

Fine grinding of silicon wafers requires using #2000 mesh (3-6 pm grit size) or finer diamond wheels. The etched surfaces to be fine-ground generally have no damage or very little damage and the surface roughness is less than 0.03 pm in Ra. [Pei and Strasbaugh, 20011

Fine grinding of silicon wafers requires high predictability and consistency, which requires the grinding wheel to have self-dressing ability, i.e., after MAGIC MIRROR IMAGE initial truing, the wheel should not need any periodic dressing by external means. In other words, there should be “a perfect equilibrium between the rate of wear of the abrasive grains and the rate of release of worn abrasive grains” [Subramanian, 19991, hence maintaining the grinding force to be relatively constant.

The other major requirements for fine grinding of silicon wafers include: a) The grinding wheel should have a reasonable THICKNESS MAP life; FIG. 11 SOFT-PAD GRINDING + POLISHING. b) The grinding force should be low and Fig. 11 shows the Magic Mirror image and constant; thickness map of a wafer ground by the soft-pad c) Surface and sub-surface damage should be method followed by a polishing operation. Once minimized; and again, the polishing operation here is to provide a 4 The ground wafers should have very good shining surface for Magic Mirror inspection. flatness. This usually means sub-pm GBIR (or TTV, Total thickness Variation). It can be seen that the soft-pad can effectively eliminate the waviness. However, it can also Due to its unique requirements, fine grinding of silicon introduce another un-uniform pattern that is wafers presents big challenges to grinding wheel 7 manufacturers, grinder machine builders and process engineers.

Experiments have been conducted to explore the effects of the grinding wheel, the grinding coolant and the process parameters in fine grinding of silicon wafers {Pei and Strasbaugh, 20011. It has been found that: . The grinding wheel has significant effects on the fine grinding process. “Softer” wheels tend to have a better self-dressing ability but higher wheel wear rate and hence lower wheel life. . Proper selection of process parameters is crucial to fine grinding of silicon wafers, as a grinding wheel that works satisfactorily under one set of grinding parameters may not work 10 20 30 40 well under another set of process Diamond Grit Size (urn) parameters. The grinding wheels tend to become “softer” under lower wheel speed and/or lower chuck speed. FIG. 12 RELATION BETWEEN GRIT SIZE AND MAXIMUM . Both the nozzle position and the flow rate of DEPTH OF CRACKS. the grinding coolant affect the fine grinding process.

6. CONCLUSIONS 5. SUBSURFACE DAMAGE INDUCED BY SURFACE GRINDING Surface grinding is potentially a more cost-effective process than lapping for flattening wire-sawn silicon To ensure high surface quality of silicon wafers, the wafers. However, its success in replacing lapping will damage layer generated by each of the machining depend on whether several technical barriers can be processes has to be removed by its subsequent removed. One such barrier is the wire-sawing induced processes. Therefore it is very important to know waviness. The approaches to overcome the barrier the subsurface damage induced by surface grinding include surface grinding followed by a lapping, wax on both wire-sawn wafers and etched wafers. mounting, reduced vacuum and “soft-pad”.

Such knowledge has been obtained through a study Fine grinding of etched silicon wafers can potentially published earlier [Pei et al., 19991. Some of the improve the throughput of polishing process and the main points are: flatness of polished wafers. Due to its unique . Subsurface cracks may exist under those requirement, fine grinding of silicon wafers presents ground surfaces, which look “fracture-free” by big challenges to grinding wheel manufacturers, microscopy observation of ground surfaces. grinder machine builders and process engineers. To . The subsurface crack depth is independent ensure the successful development of fine grinding of of the location on the ground wafer. silicon wafers, a large amount of research work . Within the tested range, the wheel rotational remains to be done. speed, chuck rotational speed and feedrate do not have significant effects on the subsurface crack depth. REFERENCES . The depth of subsurface crack on ground silicon wafers is approximately equal to half Bawa, M.S., Petro, E.F., and Grimes, H.M., (1995) of the diamond grit size used in the grinding “Fracture strength of large diameter silicon wafers,” wheel. See Fig. 12. Semiconductor International, November, pp. 115-l 18.

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