cf~ Paper International Journal of Electrical Machining, No. 8, January 2003
A New Slicing Method of Monocrystalline Silicon Ingot by Wire EDM
Akira OKADA *, Yoshiyuki UNO *, Yasuhiro OKAMOTO*, Hisashi ITOH* and Tameyoshi HlRANO** (Received May 28, 2002)
* Department of Mechanical Engineering, Okayama University, Okayama 700-8530, Japan ** Toyo Advanced Technologies Co., Ltd., Hiroshima 734-8501, Japan
Abstract Monocrystalline silicon is one of the most important materials in the semiconductor industry because of its many excellent properties as a semiconductor. In the manufacturing process of silicon wafers, inner diameter(lD) blade and multi wire saw have conventionally been used for slicing silicon ingots. However, some problems in efficiency, accuracy, slurry treatment and contamination are experienced when applying this method to large scale wafers of 12 or 16 inch diameter expected to be used in the near future. Thus, the improvement of conventional methods or a new slicing method is strongly required. In this study, the possibility of slicing a silicon ingot by wire EDM was discussed and the machining properties were experimentally investigated. A silicon wafer used as substrate for epitaxial film growth has low resistivity in the order of 0.01 g .cm, which makes it possible to cut silicon ingots by wire EDM. It was clarified that the new wire EDM has potential for application as a new slicing method, and that the surface roughness using this method is as small as that using the conventional multi wire saw method. Moreover, it was pointed out that the contamination due to the adhesion and diffusion of wire electrode material into the machined surface can be reduced by wire EDM under the condition of low current and long discharge duration. Key words: Silicon wafer, Monocrystalline silicon ingot, Slicing, Wire EDM, Contamination
1. INTRODUCTION contamination of sliced surfaces. From the above mentioned viewpoints, a new Semiconductor industry has been remarkably slicing method of silicon ingot by wire EDM developed, and higher integration of electric circuit (WEDM) is proposed. A silicon wafer used as a is rapidly making progress. Then, in the slicing substrate for epitaxial film growth has low resistivity process of mono crystalline silicon ingot, further in the order of 0.01 g .cm, which makes it possible to 4 improvements in machining efficiency and accuracy slice silicon ingot by WEDM ). Also, it is expected are strongly required, and the diameter of silicon that the length and the number of the cracks on the wafers are enlarged for increasing the number of IC machined surface might be reduced, since the l chip per one wafer ). Inner diameter blades have material removal is performed by repetition of micro been conventionally used for slicing ingot. However, craters and the machining force acting on the some problems in efficiency still exist, like relatively workpiece is extremely small. large kerf loss and large cracks of about 30~m in In this paper, the possibility of slicing silicon depth on the sliced surfaces, resulted from ingot by WEDM is experimentally discussed, and the mechanical machining. Moreover, this method is machining properties such as slicing speed and difficult to apply for large-scale wafers of 12 or 16 surface roughness are investigated. Furthermore, the inches in diameter, which are expected to be used in accuracy of sliced wafer and the contamination on l the near future ). Therefore, multi wire saw slicing the machined surface are evaluated. 2 has been gradually applied ),3). In this method, thin piano wires are fed to the ingot with slurry consisting 2. EXPERIMENTAL PROCEDURES of abrasives and cutting oil. Multi wire saw slicing has many advantages such as relatively small kerf Usually, high discharge current with short pulse loss and ability of cutting large-scale wafers and duration has been applied in the WEDM process multi wafers at the same time. However, there still used for die making, because of higher cutting remain the problems of slurry treatment and speeds, stable machining, and larger electrode wear
-21- allowed due to wire throwaway system. However, the wear of wire leads to adhesion or diffusion of 5 wire material on the machined surface ). This phenomenon is not suitable for the silicon process. 4 Authors ) have done research on machining of mono crystalline silicon by EDM before, and it was made clear that the machining speed for silicon with transistor switching circuit was much larger than that for metal mold material, since material is removed by not only heat conduction from arc column but also Joule's heat generation just below the discharge Fig.1 Experimental apparatus point due to high current density in silicon with high resistivity, which leads to large removal rate in the Table 1 Machining conditions 6 EDM of silicon ). And the electrode wear became smaller. Therefore, slicing by WEDM with transistor Electrode Molybdenum (t/J 180 pm) switching circuit is discussed in this study. It is Polarity Electrode : (-) highly expected that the contamination of wire 6 Dielectric Deionized water ( p > 10 Q. cm) material on the machined surface can be reduced. Gap voltage Ui =100 [V] Fig. 1 shows the schematic diagram of Discharge current ie = 3,12, 22 [A] experimental apparatus. In this system, the discharge Discharge duration te = 5, 10,20,40,75 [ps] current pulse made by transistor switching circuit has Duty factor r = 15 [%] a relatively long duration and a low current intensity. Speed of wire Fw = 5.0-10.0 [m/s] The used wire electrode is rewound around the Wire tension F =5.9 [N] wire-winding drum and reused repeatedly, because of the high expectation of extremely low wear in the wire electrode. In this experiment, molybdenum wire of 180llm in diameter is used as the electrode. P-type 3. SLICING PROPERTIES BY WEDM mono crystalline silicon ingot of 0.01 g . cm in resistivity is used as the workpiece, and machining 3.1 Effects of discharge current and duration fluid is deionized water whose resistivity is about 106 Fig.2 shows the variations of removal rate with g . cm. Main machining conditions are shown in discharge duration for various discharge currents. Table I. The experiment using conventional WEDM Silicon ingots of 10mm(a) and 40mm(b) in thickness with condenser circuit is also done for comparison. are used here. As can be seen form the figures, the In this case, large discharge current for 1st cut and removal rate increases with an increase of discharge small one for 2nd cut are applied. current in both cases of thickness. And the removal
c 100 c 200 ·E ·E t\i- Mo(-)/si, t=10mm ...... ie= 3A t\i- Mo(-)/si, t=40mm ...... ie= 3A E Fw=10m/s ~ie=12A E Fw=10m/s ~ie=12A E 75 -A-ie=22A E 150 -A-ie=22A en en > > 50 100 Q) caQ) ca"- "- ca 25 ca 50 > > 0 0 E E Q) Q) a: a: 20 40 60 80 00 20 40 60 80 Discharge duration te jJ.s Discharge duration te jJ.s (a) t=10mm (b) t=40mm Fig.2 Variations of removal rate with pulse duration for various discharge currents
- 22- rate takes maximum at about 5-10)ls and 20)ls, in the E 60 :::t Mo(-)/Si, t=40mm cases of 10mm and 40mm in thickness respectively, Fw=10m/s N just like die-sinking EDM process. The effects of 0::: workpiece thickness on removal rate are described (J) 40 (J) 2 ID later. In conventional WEDM, it was 104mm /min c::: 2 .s::. under 1st cut condition, and 1Omm /min under 2nd Cl ::l cut one. It was made clear that the removal rate of e 20 ID ~ ~i.=3A mono crystalline silicon by relatively long duration ~ ~ l~ie=12A 't: -A-ie=22A and low intensity current with transistor switching ::l Cl) circuit was almost the same as that in conventional 0~5~~~1~0-----2~0--~4~0~~~8~0~ WEDM with condenser circuit. Discharge duration t e J.1.S Fig.3 shows the variations of surface roughness. The surface roughness increases with the increases of Fig.3 Variations of surface roughness with pulse discharge current and discharge duration. On the duration for various discharge currents other hand, it was 21.5)lm for 1st cut and 7.6)lm for
2nd cut in conventional WEDM. It is reported that c::: 80~------' the surface roughness is about 20)lm in multi wire ·E Mo(-)/Si 7 ie=12A saw slicing applied so far ). Furthermore, the ~ Fw=10m/s E 60 removal of crack layer is actually performed after the slicing process, and the removed thickness is more ~ than 30)lm. Considering these results about the 40 removal rate mentioned before, large discharge ID ~ current and not so long pulse duration are suitable ~ 20 conditions, which leads to large removal rate and o small surface roughness. E ~ 0L-5~~~10----~--~-4~0~~~8~0~ 3.2 Effects of thickness In actual slicing process, workpiece thickness Thickness mm always changes, since silicon ingot has a circular Fig.4 Variations of removal rate with thickness cross section. Then, the effects of workpiece for various discharge durations thickness were discussed. The relationships between removal rate and workpiece thickness are given in 80 Fig.4. For short discharge duration, removal rate E ,. :::t Mo(-)/si -.-te= SJLS .... te=40JL doesn't vary so much with the thickness. On the ie=12A 0 te=10JLs ···.··te=7SJL -.. -t =20JLs other hand, it increases with the thickness for long N Fw=10m/s e 0::: 60
discharge duration. When the thickness is small and (J) •...... •••...... ••.••...... ••....••...... •.•...... •...•.....• the machining area is not so large, the electrical ~406 6 6 6 c::: discharges with long duration are difficult to disperse .s::. • --- -A------_ .... ___ -. Cl uniformly. Therefore, the machining performance is ::l o 0 0 0 8 e 20 not stable, and the removal rate becomes smaIl ). ID •• •• ~ Additionally, in the case of 80mm or less in 't: ::l thickness, larger removal rate can be attained with Cl) 20)ls pulse duration. Similar tendency was obtained Thickness mm under other discharge conditions. Fig.5 shows the variations of surface roughness Fig.5 Variations of surface roughness with with thickness. As can be seen from the figure, the thickness for various discharge durations surface roughness becomes a little smaller with the thickness, even under the same discharge duration. These variations are guessed to be related to stability larger with speed of wire in all cases of workpiece of machining performance, as mentioned above. thickness. And the effects of speed of wire on the 3.3 Effects of speed of wire removal rate increases with the thickness. From the The variations of removal rate with speed of result, high speed of wire leads to smooth flow of wire are shown in Fig.6. The removal rate becomes machining fluid in the gap, and the exclusion of
- 23- c:: 100 'E c;;.. E E 75 en > 50 Q)
ca"- CO > 25 Fig.8 Cross section of sliced surface 0 E Q) CC 0 5 7.5 since thinner crack layer leads to the shortening of Speed of wire Fw rnls time to remove the layer. Fig.6 Variations of removal rate with speed of 4.2 Contamination of machined surface wire for various workpiece thickness In order to investigate the contamination of sliced surface, XPS (X-ray Photoelectron Spectro scopy) analysis was carried out. Fig.9 shows the debris and bubbles become easy. Then, the results of analysis. The contamination layers such as machining perfonnance becomes stable by better oxidized layer and carbonized layer, which had been dispersion of electrical discharges. Consequently, the generated by atmosphere after machining, were removal rate becomes larger. removed by argon ion etching in the measurement chamber before analysis. As shown in the figure, 4. SURFACE INTEGRITY oxygen exists on the machined surface in both cases. It is considered that the machined surface is oxidized 4.1 Observation of machined surface by electrical discharge in deionized water. Of course, Fig.7 shows SEM micrographs of sliced surface. contamination by oxygen is not desirable in It can be seen that the size of crater on the surface semiconductor manufacturing process. However, it is becomes larger with discharge duration, which agree allowable to some extent, since a few processes for with the variation of surface roughness shown in removing oxygen are actually perfonned after the Fig.3. Also small holes can be observed. They are slicing process. In the case of conventional WEDM, guessed to be generated by the exclusion of copper and zinc from wire material are found on the evaporated silicon bubble from the inside. Moreover, surface. Copper is one of the most undesirable many wrinkles can be seen near the periphery of materials in semiconductor manufacturing process, craters, which might be micro crack. since it tends to diffuse into the inside of silicon. Then, cross section of sliced surface was Therefore, conventional WEDM is never applicable. observed. Fig.8 shows one of the longest cracks On the other hand, molybdenum of wire material generated in sliced surface. The crack length is about hardly adheres to the machined surface in the case of 20llm as shown in the micrograph. On the other hand, new WEDM. It is considered that adhesion or in cases of ID blade and multi wire saw slicing, they diffusion of wire material can be reduced because of are about 20-30llm. Therefore, this method is discharge current wavefonn made by transistor effective for high efficiency manufacturing of wafer, switching circuit and high melting point of
Fig.7 SEM micrographs of sliced surface
- 24- Number of scan:2 Number of scan:5 3000 15000 .------=O:----ie=---12-A-,--, CZl o § 12000 te=40~s 0 u 2000 9000 0 t:= eu u 0 ~ 0 1000 800 600 400 200 0 800 600 400 200 Binding energy (eV) Binding energy (eV) (a)Conventional WEDM with condenser circuit (b)WEDM with transistor switching circuit Fig.9 Results of XPS analysis of sliced surface
(a)Before machining (b )After machining Fig.10 SEM micrographs of wire before and after slicing
.- E molybdenum wire. g 150 Fig.10 shows SEM micrographs of wire before Mo(-)/q,=1S0mm Si 0 ie=22A, te=20 J1S and after slicing. Stripes are seen on the surface of Fw=10m/s wire before slicing. In the case of wire after slicing, Q) 100 .;:~ adhesive can be observed, and the color turned to silver from black. Then, component analysis of the -0 c: 50 wire surface by EPMA (Electron Probe Micro -Q) E Analysis) was carried out, and the results showed the Q) (,) existence of silicon on the surface. Therefore, it was ca c.. 40 60 80 100 120 140 clarified that these adhesives were re solidified Cl) silicon from the workpiece. Also from this result, it CS Machining time T (min) can be expected that the electrode wear and the Fig.ll Displacement of wire in slicing of 6" ingot contamination are very small due to the discharge current waveform made by transistor switching circuit and high melting point of molybdenum wire. many wafers are sliced at once can be realized like 5. SLICING OF 6 INCH SILICON INGOT wire saw slicing, this WEDM slicing method is applicable as a high efficiency slicing method of Next, slicing of a 6 inch silicon ingot was tried. silicon ingot. Fig.ll shows the relationship between displacement Fig.12 shows the cutting speed and the removal of wire and machining time. As shown in the figure, rate with machining time calculated from the slicing speed decreases with an increase of previous figure. Also from this figure, it can be seen workpiece thickness, and it takes about 140 min to that the removal rate is almost constant at any time, slice 6 inch silicon ingot. That is, the average slicing which indicates that the machining performance was speed is about 1.1 mmlmin. In the case of multi wire always stable. Therefore, stable slicing is expected to saw, it is 0.2-0.3mm1rnin9). If multi slicing, in which be realized in the case of large wafer such as 12 or 16
- 25- 5 250 [J.lITl ] '2 '2 30 Mo(-)I
inch ingots. like to express their thanks to Sin-Etsu Handotai Co.,Ltd. and Sodick Co., Ltd. for their help through Fig.13 shows TTV (Total Thickness Variation) this research. and Warp of the 6 inch wafer sliced by WEDM. Both are the most important dimensional parameters of a REFERENCES wafer, that is, TTV is the difference between the maximum and minimum thickness, and Warp is 1) T. Abe : Silicon Crystal Growth and Wafer defined as the difference between the maximum and Processing, Baifukan (1996) 13. 10 minimum distances from a reference plane ), as 2) K. Makino and Y. Kanemichi : Slicing by Multi shown in the figure. Both values, in the case of Wire Saw, J. of the Society of Grinding Engineers, WEDM, are almost the same as those in the case of 41, 1 (1997) 16. multi wire saw. In addition, the kerf loss was about 3) Y. Ban: Silicon LSI and Chemistry, Dai-Nippon 250llm, which is also almost the same value as in Tosho (1993) 85. multi wire saw. 4) Y. Uno, A. Okada, Y. Okamoto and H. Nakanishi: Fundamental Study on EDM of Single Crystalline 6. CONCLUSIONS Silicon, J. of JSPE, 63, 10 (1997) 1459. 5) N. Saito: Principle of EDM and its Application, (1) Wire EDM has a possibility as a new slicing Gijutsuhyoronsha, (1979) 26. method of mono crystalline silicon ingot. 6) T. Saeki, M. Kuniede, M. Ueki and Y. Sato : (2) Contamination due to adhesion and diffusion of Transient Workpiece Temperature Analysis in the wire electrode material to the machined surface EDM Processes of High Electric Resistance can be reduced by WED M with discharge Materials Considering Joule Heating, J. of JSPE, current waveform made by transistor switching 62, 3 (1996) 443. circuit and high melting point of molybdenum 7) K. Makino and K. Kinutani: Development of Wire wire. Saw for Slicing Large Diameter Silicon Wafers, J. (3) The accuracy of wafer by this slicing method is of the Society of Grinding Engineers, 42, 1 (1998) almost the same as that by multi wire saw 22. method. 8) N. Saito, N. Mohri, T. Takawashi and M.Furuya: (4) Higher efficiency slicing is possible, compared EDM Technology, Nikkankougyoshinbunsha, with multi wire saw, if a multi type WEDM (1997) 38. slicing can be realized. 9) M. Kojima, A. Tomizawa, J. Yakase, H. Hattori and M. Mitani: Development of High Precise and ACKNOWLEDGEMENTS Efficient Slicing Technology by using This study was supported by Industrial Unidirectional Multi-Wire-Saw, J. of JSPE, 56, 6 Technology Research Grant Program from the New (1990) 99. Energy and Industrial Technology Development 10) F. Shimura: Semiconductor Silicon Crystal Organization (NEDO) of Japan. The authors would Technology, Academic Press, (1989) 192-195.
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