Fine Grinding Of Silicon Wafers: Designed Experiments
International Journal of Machine Tools & Manufacture 42 (2002) 395–404Fine grinding of silicon wafers: designed experimentsZ.J. Peiaa,*, Alan StrasbaughbDepartment of Industrial and Manufacturing Systems Engineering, Kansas State University, Manhattan, KS 66506, USAbStrasbaugh, Inc., San Luis Obispo, CA 93401, USAReceived 2 November 2000; received in revised form 31 July 2001; accepted 2 August 2001AbstractSilicon wafers are the most widely used substrates for semiconductors. The falling price of silicon wafers has created tremendouspressure to develop cost-effective processes to manufacture silicon wafers. Fine grinding possesses great potential to reduce theoverall cost for manufacturing silicon wafers. The uniqueness and the special requirements of fine grinding have been discussedin a paper published earlier in this journal. As a follow-up, this paper presents the results of a designed experimental investigationinto fine grinding of silicon wafers. In this investigation, a three-variable two-level full factorial design is employed to reveal themain effects as well as the interaction effects of three process parameters (wheel rotational speed, chuck rotational speed and feedrate). The process outputs studied include grinding force, spindle motor current, cycle time, surface roughness and grinding marks. 2002 Elsevier Science Ltd. All rights reserved.Keywords: Ceramic machining; Grinding; Grinding force; Grinding marks; Material removal; Semiconductor materials; Silicon wafers; Surfaceroughness1. IntroductionMost IC (integrated circuit) chips are built on singlecrystal silicon wafers. These IC chips can be found inevery type of microelectronic applications, includingnetworking and computing (routers, modems, set-topboxes, Ethernet cards, disk drives), wireless communications (portable electronic devices, cellular phones,pagers, satellite receivers), consumer electronics (DVDplayers, home security systems, small householdappliances, smart cards), automotive electronics (GPSand navigational tools, air bag controls, anti-lockingbraking systems), industrial automation and control systems.However, in recent years the price of silicon wafershas dropped significantly. The huge price erosion can beseen from Fig. 1. The worldwide revenue generated bysilicon wafers in 1999 was US 5.8 billion, a 4% increasefrom the revenue of 1998 but with 26% more siliconproduced [1]. The falling price of silicon wafers has* Corresponding author. Tel.: 1-785-532-3436; fax: 1-785-5323738.E-mail address: zpei@ksu.edu (Z.J. Pei).Fig. 1. Worldwide revenue and area production of silicon wafers(after Mozer [1]).0890-6955/02/ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S 0 8 9 0 - 6 9 5 5 ( 0 1 ) 0 0 1 2 3 - 7
396Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404applied a great pressure on silicon manufacturers toreduce their manufacturing cost. It is critically importantto develop new manufacturing processes, or to developnew applications for some existing processes, that allowmanufacturing silicon wafers more cost-effectively.The manufacturing processes for silicon based ICs areillustrated in Fig. 2. As can be seen, surface grindinghas been (or can be) used at three different manufacturing steps in the manufacturing flow: (a) surface grindingafter slicing (wire sawing) as partial replacement of lapping; (b) fine grinding after etching as partial replace-Fig. 3.Fig. 2. Manufacturing processes for silicon based ICs (after Bawa etal. [2], Fukami et al. [3], Tonshoff et al. [4] and Vandamme et al. [5]).Illustration of wafer surface grinding.ment of rough polishing; and (c) back-grinding the backside of the wafer after circuits are developed on the frontside. Here, (a) and (b) take place inside silicon wafermanufacturers while (c) takes place inside IC manufacturers or their outside contractors.Due to its importance, surface grinding has attractedmore and more interest among investigators. Pei andStrasbaugh [6] have given a brief summary of reportedinvestigations into surface grinding of silicon wafers.Fine grinding of etched wafers first appeared in publicdomain through the US patent by Vandamme et al. [5].The advantages of fine grinding of etched wafers aretwo-fold. One is to improve the flatness of etched wafers.Another is to reduce the removal amount for rough polishing by 25–50% [5]. The end result will be higherthroughput for rough polishing and better flatness forpolished wafers.Another application of fine grinding is to ‘re-work’the background device wafers. Back grinding is normallydone by two steps: rough grinding by grinding wheelswith large size of diamond grains; and fine grinding bygrinding wheels with small (fine) diamond size. However, sometimes there is a need to re-grind the wafersthat have been background previously. And this regrinding is typically done by fine grinding only.Fine grinding of silicon wafers requires using #2000mesh (3–6 µm grit size) or finer diamond wheels. Thesurfaces to be fine ground generally have no damage orvery little damage and the surface roughness is 30 nmin Ra [6].The uniqueness and the special requirements of siliconwafer fine grinding process were discussed in the previous paper [6]. The major requirements for fine grindingof silicon wafers include:
Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404Table 1Test matrix397Table 2Variable levelsTestWheel speedChuck speedFeed-rateVariablesUnitLow level ( )High level ( )12345678 Wheel speedRev s 1(rpm)Rev s 1(rpm)µm s k speedFeed-rateTable 3Grinding force dataWheel speed Chuck speedFeed-rate Fig. 4.Maximum grinding force (N)Wafer 1Wafer 2Wafer nding marks under Magic Mirror pictures.1.2.3.4.The grinding wheel should have self dressing ability;The grinding wheel should have a reasonable life;The grinding force should be low and constant;Surface and sub-surface damage should be minimized; and5. The ground wafers should have very good flatness.This usually means sub-micron TTV (total thickness variation).The previous paper published in this journal [6]reported and discussed preliminary experimental workon the effects of grinding wheels, process parametersand grinding coolant. As a follow-up, this paper reportsa designed experimental study on fine grinding of siliconwafers. Three-factor two-level full factorial design isused in this study. The objective is to reveal the maineffects as well as the interaction effects of three processparameters (wheel rotational speed, chuck rotationalspeed and feed-rate) on such process outputs as grindingforce, spindle motor current, cycle time, surface rough-
398Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404Fig. 5.Effects on grinding force.Table 4Spindle motor current dataWheel speedChuck speedFeed-rateMaximummotor current(amp) 5.54.25.54.35.54.45.54.5ness and grinding marks.There are four sections in this paper. Following thisintroduction section, Section 2 describes the design ofexperiments and the experimental conditions. In Section3, the experimental results will be presented and discussed. Conclusions are drawn up in Section 4.Fig. 6.Effect on spindle motor current.
Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–4043992. Design of the experiments and experimentalconditionsThe experiments are conducted on a StrasbaughModel 7AF surface grinder in the process developmentlaboratory of Strasbaugh, Inc. (San Luis Obispo, CA).The grinding wheel used is a diamond cup wheel. Thegrit size is mesh #2000 and the diameter of the wheelis 280 mm. As illustrated in Fig. 3, the workpiece(wafer) is held on the porous ceramic chuck by mean ofa vacuum. The axis of rotation for the grinding wheel isoffset by a distance of the wheel radius relative to theaxis of rotation for the wafer. During grinding, the grinding wheel and the wafer rotate about their own axes ofrotation simultaneously, and the wheel is fed towards thewafer along its axis.Single crystal silicon wafers of 200 mm in diameterwith the (100) plane as the major surface are used forthis investigation.During grinding, deionized (purified) water is used tocool the grinding wheel and the wafer surface. For thisstudy, the coolant is supplied to the inner side of the cupwheel. The coolant flow-rate is 3.2 gallons per minute.Three process parameters are chosen to study theireffects and interactions:1. Wheel speed: the rotational speed of the grindingwheel.2. Chuck speed: the rotational speed of the chuck. It isthe same as the rotational speed of the workpiece(wafer).3. Feed-rate: the feed-rate of grinding wheel (spindle)towards the wafer.A 23 (three variables, two levels, eight tests) full fac-Fig. 8.Fig. 7.Comparison of grinding force and spindle motor current.Effects on grinding cycle time.torial design is used for the experiments. Detaileddescription of factorial design can be found in many textbooks such as the one by DeVor et al. [7]. The matrixof the experiments is shown in Table 1 and the variablelevels are listed in Table 2. These tests are conducted ina random order.Five output variables are observed: grinding force;spindle motor current; cycle time; surface roughness;and grinding marks.Under each test condition, three wafers are ground.Grinding force, surface roughness and cycle time dataare taken for all the three wafers. Only one data point
400Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404Table 5Grinding cycle time dataWheel speed Chuck speed Feed-rate is taken for motor current and grinding marks due to thefollowing reasons. The motor currents for the threewafers are very consistent and it is very complex andexpensive to prepare the samples for the evaluation ofgrinding marks by means of a Magic Mirror.The grinder records the grinding force automatically.The grinding force measured is the interaction forcebetween the grinding wheel and the wafer in the direction parallel to the spindle axis. It is also the directionperpendicular to the wafer surface. The maximum forceduring the entire grinding cycle is used for analysis. Themonitor of the grinder displays the spindle motor currentduring grinding and the maximum motor current valueis recorded manually. Grinding cycle time is the time ofactual grinding. It does not include the time for the wheelto approach the wafer surface and the spark-out time.Surface roughness of the ground surface is measuredalong a direction approximately perpendicular to thegrinding lines. The instrument used is a Tencor P-2 surface profiler (KLA-Tencor, One Technology Drive, Milpitas, CA). The scan length is 100 µm and scan speedis 5 µm/s for the measurement. The measurement is doneat the same X–Y coordinates for each wafer.Magic Mirror pictures are used to evaluate the grinding marks. One wafer from each test condition undergoesa same polishing process with same amount of polishingGrinding cycle time (s)Wafer 1Wafer 2Wafer 45751removal on the ground surface. Then the wafer isinspected under a Magic Mirror (Model YIS-200SP-4,HOLOGENiX, 15301 Connector Lane, HuntingtonBeach, CA). The picture does not automatically give anyquantitative description about the grinding marks. Toobtain a quantitative measure for grinding marks, all theMagic Mirror pictures are compared and each picture isassigned a number subjectively according to the severityof the grinding marks. For example, the grinding marksin Fig. 4(a) are hardly visible and thus receive a severitynumber of 0. The grinding marks in Fig. 4(b) are severeand therefore number 6 is assigned to the picture.3. Results and discussionIn this section, the test results are presented for eachof the output variables. The software called DesignExpert (Version 5, Stat-Ease Corporation, Minneapolis,MN) is used to process the data. After identifying thesignificant effects, the analysis of variance (ANOVA) isperformed for each output variable. The details of theseanalyses will not be presented here. This section willgive the geometric representations of the significanteffects along with some discussion.Table 6Surface roughness dataWheel speed Chuck speed Feed-rate Surface roughness Ra (nm)Wafer 1Wafer 2Wafer .111.718.517.711.210.711.816.812.211.316.920.6
Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404401grinding force; while at the high level of feed-rate, theincrease in chuck speed will decrease the grinding force.3.2. Results on spindle motor currentTable 4 shows the results on spindle motor current.The maximum value for the current is very consistentover three wafers ground. Therefore only one value pergrinding condition is used for ANOVA analysis.Only the main effect of wheel speed is significant.Lower wheel speed will cause larger motor current (seeFig. 6).Comparing Fig. 5 with Fig. 6, it is easy to see thatthe spindle motor current does not have the sameresponse as grinding force when the process variablessuch as wheel speed change their levels. Fig. 7 showsboth grinding force and motor current data when continuously grinding 35 wafers under a same grinding condition. It can be seen that grinding force is much moresensitive than the motor current. For instance, the grinding force increases over 50% from the first wafer to thelast wafer ground while the current increases only 5%.3.3. Results on grinding cycle timeTable 5 and Fig. 8 show the results on grinding cycletime. Feed-rate has the most significant effect on cycletime. The higher the feed-rate, the shorter the cycle time.The interaction between wheel speed and chuck speedis significant. At the low level of chuck speed, increasein wheel speed will increase the cycle time. While at thehigh level of chuck speed, increase in wheel speed willdecrease the cycle time.3.4. Results on surface roughnessThe results on surface roughness are included in Table6 and Fig. 9. The main effect of chuck speed is significant. Higher chuck speed produces rougher surface. Theinteraction between chuck speed and feed-rate is alsosignificant. The effect of chuck speed on roughness isstronger at the higher feed-rate level.Fig. 9.Effects on surface roughness.3.1. Results on grinding forceThe results on grinding force are shown in Table 3.The main effects of wheel speed and feed-rate are significant. The increase in wheel speed or feed-rate willincrease the grinding force (see Fig. 5).As also shown in Fig. 5, the interaction between chuckspeed and feed-rate is significant. At the low level offeed-rate, the increase in chuck speed will increase3.5. Results on grinding marksFig. 10 shows the Magic Mirror pictures for all thetest conditions. Also included are the test conditions andseverity number of grinding marks. The Design-Expertsoftware does not identify any significant effects, probably due to the fact that the assignment of the severitynumber for grinding marks is judgmental. This pointsout the necessity of looking into some objective ratherthan judgmental ways to quantify grinding marks.All the main effects and interactions are graphicallypresented in Fig. 11. The figure shows obvious interactions between the three variables.
402Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404Fig. 10.Results of the grinding marks.An important observation from Figs. 10 and 11 is thefollowing. With the same grinding wheel and the samegrinder, altering the process variables (wheel speed,chuck speed and feed-rate) can dramatically change theseverity of grinding marks. There exists an optimum setof process variables that can produce wafers with theleast severity of grinding marks.It is clear that the five process outputs studied hererespond differently to the change in the process variables. For example, as feed-rate increases, the cycle timedecreases (Fig. 8) and hence the throughput increases.However, an increase in feed-rate will increase grindingforce (Fig. 5). Therefore, the optimum grinding condition for one output is not necessarily good for other
Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404403Fig. 11. Effects on the grinding marks.outputs. In other words, there are no such grinding conditions under which all five outputs can be optimized atthe same time. Therefore, it is important to prioritize therequirements for the outputs.Another important point obtained from this study isthat the interactions are significant for all the outputsexcept for spindle motor current. Therefore, the optimized condition for any of the outputs (except spindle
404Z.J. Pei, A. Strasbaugh / International Journal of Machine Tools & Manufacture 42 (2002) 395–404current) cannot be achieved by changing one processvariable at a time. The variables have to be altered simultaneously for optimization.4. ConclusionsA three-factor two-level full factorial design is usedto conduct an experimental investigation into fine grinding of silicon wafers. The main effects and the two-factorinteractions of wheel speed, chuck speed and feed-rateon the process outputs (grinding force, spindle motorcurrent, cycle time, surface roughness and grindingmarks) are obtained.The following conclusions can be drawn from thisstudy:1. The interactions between wheel speed, chuck speedand feed-rate are significant. Therefore, these processvariables need to be changed simultaneously to obtainthe optimized output performances.2. The five process outputs respond differently to thechange in process variables. Therefore, these outputscannot be optimized at the same time. Compromiseand prioritization are needed for process optimization.3. Process variables have significant effects on grindingmarks. For a given grinding wheel and a givengrinder, grinding marks can be greatly reduced byoptimizing the process variables.4. Compared to spindle motor current, grinding force ismuch more sensitive to changes in the grinding process such as wheel status.References[1] A. Mozer, Plane silicon wafer technology, Eur. Semicond. April(2000) 29–30.[2] M.S. Bawa, E.F. Petro, H.M. Grimes, Fracture strength of largediameter silicon wafers, Semicond. Int. Nov. (1995) 115–118.[3] T. Fukami, H. Masumura, K. Suzuki, H. Kudo, Method of manufacturing semiconductor mirror wafers, European Patent Application, EP0782179A2, Bulletin 1997/27.[4] H.K. Tonshoff, W.V. Schmieden, I. Inasaki, W. Konig, G. Spur,Abrasive machining of silicon, Ann. CIRP 39 (2) (1990) 621–630.[5] R. Vandamme, Y. Xin, Z.J. Pei, Method of processing semiconductor wafers, US Patent 6 114 245, September 5 (2000).[6] Z.J. Pei, A. Strasbaugh, Fine grinding of silicon wafers, Int. J.Mach. Tools Manufact. 41 (5) (2001) 659–672.[7] R.E. DeVor, T.H. Chang, J.W. Sutherland, Statistical qualitydesign and control, contemporary concepts and methods, Macmillan, New York, 1992.
grinding marks by means of a Magic Mirror. The grinder records the grinding force automatically. The grinding force measured is the interaction force between the grinding wheel and the wafer in the direc-tion parallel to the spindle axis. It is also the direct
Wafer # 200 250 300 400 500 300 500 Wafer # 50 100 150 200 (a) (b) Fig. 3. Consistent versus fluctuating force when grinding silicon wafers [6]. (a) Relatively Consistent grinding force, (b) fluctuating grinding force. 0 10 20 30 40 50 0 5 10 15 20 25 30 35 Force (lbs) 0 10 20 30 40 50 0 5 10 15 20 25 30 35 Force (lbs) Wafer # Wafer # (a) (b .
Temporary Bond. Four inch silicon wafers were bonded to thin (325 µm) fused silica using BCB as the adhesive. The silicon and silica wafers were cleaned using an ultrasonic SC1 bath, rinsed in DI water and spin dried. An adhesive promoter followed by BCB was spin coated on the silicon wafers. The
From Sand to Silicon Wafers: A Process Systems view of Solar Cell Production CAPD ESI Overview, 2014 . c) Solargrade Silicon SoG d) Silicon wafers 3.Conclusions . 3 The Market Outlook . . 2006 Spot Price 200 per kg 2013 spot price 17.50 per kg .
0.2–20mm [7]. It differs from flatness (SFQR) in that for . A grinding wheel of a cup shape has a diameter larger than the wafer diameter. The rotation axis of the grinding wheel is located on the circle along which the centers of the wafers
be formed by metal-assisted wet chemical etching (MAWCE) of polished Si powder-based wafers and as-cut wafers irradiated with medium laser power, while a surface texturing on the as-cut pc-Si wafers occur, and no nanowires can form in the region subject to a liquid phase crystallization (LPC) caused by high-power laser treatments. 1. Introduction
different operations like turning, threading, tapper grinding, end drilling etc. The plant has the capacity to do most of operation except taper grinding. Presently the plant has to outsource the shaft to outside plant for the taper grinding. presently the plant have a grinding machine of G 17-22U, which means it is a universal grinding machine that can machine a job up to 220mm diameter shaft .
Grinding wheel selection, Qw, Vw Wheel dressing parameters Limitations of on-board inspection Grinding Wheel 101 Grinding wheel definitions and descriptions Dressable and Non-dressable grinding wheels Tool wear Hands on Lab or Gear Grinding Simul
business cases, using the Five Case Model – in a scalable and proportionate way. It recognises and aligns with other best practice in procurement and the delivery of programmes and projects. Experience has demonstrated that when this guidance is embedded in public sector organisations, better more effective and efficient spending decisions and implementation plans are produced. At the same .
