10.1149/1.2980313 The Electrochemical Society 300mm Wafer Stain Formation by Spin Etching K. Sato a, S. Mashimoto a, and M. Watanabe a a Process Development, SEZ Japan, Inc., Hongo, Bunkyo-ku 1130033, JAPAN Stain film (porous silicon) formation by immersion is reported. The Stain film formation on 300mm wafers by surface treatment is accomplished using SPIN ETCH. It is found that stain film has resistivity. Introduction Porous silicon can be created by anodic oxidation. It can be created also with Stain etching (HF + Oxidizer) solution (1). Porous silicon formed by a Stain etching solution is referred to as Stain film in this report. A thin chip is used with 3D packaging. Wire bond wiring is arranged at very narrow chip interval. It is expected that in the future the insulating technology for a chip, especially the insulating technology on the back of the chip, will become more important. Since the Stain film can be formed at room temperature and shows insulation, it may serve as insulating technology in a future packaging process. When formation of the insulated film on the back of a chip is achieved, by the conventionally used immersion etching method, a medium contacts both sides of the wafer. By the spin etching method used in this experiment, only one side of the wafer can be etched alternatively. This advantage is used in defining conditions to form the Stain film by spin etching. Experimental An outline of spin etching is shown in Fig. 1. The wafer is placed on the chuck. The device side faces downward, toward the upper side of the chuck. N2 gas is introduced from the chuck upper surface, causing the wafer to float slightly. Therefore the device side of the wafer does not actually contact the chuck. The N2 gas also has a role which protects the device side during etching. While the wafer has floated on the chuck, only the edge of the wafer touches the chuck pin. The roles of the chuck pin are to fix the wafer to the chuck, synchronize the wafer rotation with that of the chuck, and prevent slipping of the wafer relative to the chuck. Up to three kinds of medium can be used. The inside of the chamber of Fig.1 consists of four levels from bottom to top. The chamber supports independently 3 kinds of media (one in each of the bottom 3 levels) and rinse water (in the top level). Recycle or drain (single pass) is possible for each medium. The chamber is connected to the exhaust line. As a sample for the Stain film formation, P type (100) silicon wafer 300mm in diameter was used. In order to examine the Stain film formation conditions, the medium used was a mixture of HF (49wt%) and HNO3 (70wt%). The formed Stain film thickness was determined by cross-sectional SEM (Hitachi S-5200). Stain film character was investigated by infrared absorption measurement (FTIR), and compared with the data of the anodic oxidation film. After investigating film character, the insulation performance of the Stain film was investigated. 303
DI Water N2 Process Chamber Level 4 Chuck Exhaust Level 3 Level 2 Etchant 3 Etchant 2 Level 1 Etchant 1 N2 Drain Figure 1. Spin Processor Fig.1 Spin Processor Result and Discussion Results of changing medium ratio are shown in Fig. 2. The chemical ratios 1 HF:HNO3=100:1,2HF:HNO3=50:1,3HF:HNO3:H2O=25:30:29,4HF:HNO3=50:1. 5 HF:HNO3:H2O=1:3:5 were indicated for comparison(1)(8). In consideration of the point that the Stain film of uniform thickness is formed, composition of 1 was chosen as a medium for spin etching. HF:HNO3=50:1 HF:HNO3:H2O=25:30:29 2 HF:HNO3:H2O = 1:3:5 3 (*1) 0 100 Bumpy Stain Surface HF:HNO3=100:1 1 Flat Stain Surface Stain Layer Si Layer 100 HF wt% 5 3 2 1 0 HNO3 wt% H2O wt% 4 0 100 Stain Layer Si Layer HF:HNO3=1:250 4 Fig.2 Stain film morphology Fig.2 Chemical depending Setup on stain etch composition Figure 2. Stain film morphology depending on stain etch composition In 1the bottom layer shown in the photograph is the Silicon substrate, and the Stain film of about 150nm thickness appears on it. The surface of the Stain film is flat. With the photograph of 2, like the photograph of 1, the bottom layer is the Silicon substrate, and an it top is the Stain film. However, it is important to note the unevenness of the surface of the Stain film differs from 1. In 3, H2O is added to HF:HNO3. In the 304
medium with added H2O, a unique form of unevenness is made to the surface of the Stain film as shown in the photograph of 3. 5 is the ratio quoted from literature for reference. 4 is nitric acid rich. Generally the composition with nitric rich acid ratios is used as a Silicon etching solution. In this case, a Stain film is not formed although unevenness can occur on the silicon substrate. For spin etching, the parameters shown as (a), (b), and (c) in Table I correspond to the 300mm wafer photographs in Figure 3. TABLE I. Spin etching parameter Parameter (a) (b) (c) Wafer rotation speed [rpm] 700 700 700 Dispenser swing [mm] ± 15 ± 15 ± 75 Medium Temperature [C] 25 30 30 (a) (b) (c) Figure 3. Spin etching wafer In a photograph (a), Silicon color is seen. Photograph (b) and (c) show a change of color. When such a change of color is seen, it is checked from prior SEM observation that the Stain film is formed in the surface. From this, it can be judged that the Stain film is formed in the portion where surface discoloration has occurred. With parameter (a), based on the color it can be concluded that no Stain was formed. Wafer rotation speed is 700rpm. With the center of a wafer specified as 0mm, the position of the dispenser supplying the medium is varied by ±15mm from the center (back and forth motion). This is intended to improve the uniformity of the process. In parameter (a), medium temperature was 25 degrees C. For parameter (b), when temperature of medium was changed into 30 degrees C, Stain was formed in the wafer perimeter. However, the center of a wafer still has silicon color. For parameter (c), the motion range of the Dispenser was expanded to ±75mm from ±15mm. As a result, the Stain film was formed all over the wafer. The picture of cross-sectional SEM is shown in Fig.4. As a result of measuring five points at equal intervals in the direction of the perimeter from the wafer center, they were thickness 221nm and variation ±6.2% (3sigma). 305
Figure 4. SEM cross section image (Wafer center) The FTIR measurement result is shown in Fig.5. Comparing with IR data of an anodic oxidation film, it turns out that the peak of Si-O-Si combination is seen at 1072cm -1 (3)-(7)(9). For comparison, the spectrum for porous silicon is shown in reference (4). (1072) Si-O-Si Figure 5. IR absorption spectra of stain film The insulation performance of the Stain film was also checked, using spin etched wafer (c). Since the Stain film was formed only on one side of the wafer, an aluminum electrode 1mm in diameter was attached to the Stain film side. Figure 6 shows the I-V measurement result where the Si substrate was set up as the + electrode and the stain film as the - electrode. Refer to SEMI M51-0303 for the voltage impression method. Different points on the wafer showed different I-V characteristics, which were divided into two groups as shown in Fig.6. 306
(B) (A) Figure 6. I-V characteristics of stain film Part (A), the solid line, indicates insulating character, whereas part (B), the dotted line, indicates insulation behavior is not seen. In order to understand this difference between (A) and (B), the difference in their IR spectra was checked. The result is shown in Fig.7. 1060cm-1 Figure 7. Delta Absorbance The left side vertical axis of Figure 7 shows the absorbance for (A) and (B), while the right side axis represents the delta of (A) minus (B). The results indicate Si-O-Si structure, while the difference between (A) and (B) indicates a peak of. absorption at 1060cm-1. CONCLUSIONS Although Stain films are conventionally formed by anodic oxidation or immersion type etching, it has been shown that spin etching can also be used to form Stain films. IR measurement confirmed the Stain film formed by spin etching has an absorption peak at 1072 cm -1, indicating Si-O-Si. It was also confirmed that this Stain film has insulation 307
performance. Depending on the character of the Stain film, there is a possibility the Si-O- Si combination in the Stain film is a cause of the insulating behavior. References 1. L.A.Jones, Progress in Surface Science, vol50, 283 (1995). 2. M.I.J. Beale, Journal of Crystal Growth, 75, 408 (1986). 3. M.A.Vasquez, REVISTA MEXICANA DE FISICA, 53, 6 (2007). 4. H.D.Fuchs, Physical Review B, 48, 11 (1993). 5. M.S.Brandt, Solid State Communications, 81, 4 (1992). 6. D.-Q. Yang, Journal of Applied Physics, 98, 114310 (2005). 7. S.Liu, Physical Review B, 49, 15 (1994). 8. D.Dimova, Thin Solid Films, 297, 9-12 (1997). 9. Sonaly.Cruz, Journal of Electro Chemical Society, 152, 6 (2005). 308