Dicing of Ultra-thin Silicon Wafers

Transcription

1 Dicing of Ultra-thin Silicon Wafers Hans H. Gatzen, Gerald Guenzel Institute for Microtechnology, Hanover University, Germany Keywords: dicing, ultra thin wafers, silicon, chipping, machining Abstract: One of the most recent semiconductor technology mass products are very thin devices like smart cards and smart labels. These devices require ultra thin wafers, while cost reasons dictate a high packaging density. For wafer separation, a combination of minimal street width and a maximal feed rate is required. For developing an optimized dicing process, dicing investigations were conducted. For the investigations, five different electroplated dicing wheels with various grain size and concentration of diamond grit and with a wheel thickness varying between 1 and were used. The feed rate was varied from mm/s to mm/s. As a result, chipping and feed rate showed a non-linear dependency with a clear local minimum of chipping. 1. Introduction Smart cards and smart labels are thin plastic devices containing a computer chip holding for instance information for medical, financial or insurance use. As smart cards are twistable, credit card sized plastic artifacts, the smart labels are made of a thin polymer film laminated in-between paper sheets or plastic foils. Smart cards and smart labels require ultra thin wafers, while cost reasons dictate a high packaging density in conjunction with an optimal usage of the wafer real estate, i.e. the accumulative kerf and chipping width, called street width, between individual devices has to be as small as possible. At the same time a high feed rate is required for an economic execution of the separating process [1]. Investigations were conducted for determining the best combination of cutting parameters for the separation of thin silicon wafers. The challenge was to find cutting conditions with both a maximal feed rate and a minimal chipping for separating very thin silicon wafers with a thickness of only. 2. Experimental Setup For the experiments with the thin wafers a precision dicing machine was used. This machine was equipped with an air-bearing spindle designed for accepting dicing wheels with an outer diameter of approx. 2 inches. To minimize the environmental influences, the experiments were carried out in a temperature controlled room at 2 C (±1 C) [2]. In order to conduct the investigations, the wheels had to be conditioned. First, the dicing wheels were trued to insure that the dicing wheel is rotating concentrically around the spindle. To do so, the dicing wheel was mounted on the spindle; afterwards, the wheel circumference was ground by a SiC cup wheel. In a next step, the dicing wheel was dressed by cutting a couple of dozen times into a SiC block to exposure the diamond grains by recessing the softer metal bond [3]. 3 µm 3 µm Fig. 1: SEM pictures of dicing wheels NiF (left) and MCT Q 2414(right)

2 For the investigations, thick silicon wafers with a diameter of 6 in were used. They were attached to a rigid substrate by a double sided adhesive tape. This sandwich was mounted directly on the vacuum chuck of the dicing machine. For each combination of wheel and feed rate, three cuts were made. The cut depth was 7 µm, with the tape scribed at a depth of 2 µm. The kerf to kerf spacing was. Due to the low hardness of the silicon the diamonds of the dicing wheel became blunt very slow. Therefor in-process dressing wasn t needed For the evaluation and to compare the dicing results, every cut kerf was divided into 6 sections. Every section had a length of mm. The size of the biggest chipping at both kerf edges was measured separately. With the measurements obtained, the arithmetic average C m and the standard deviation S were calculated. By using the values C m and S the cut quality between different cuts was compared. For these investigations, a microscope with a magnification of x was used. The grain concentration was determined with scanning electron microscopy (SEM), taking advantage of its great depth of focus and the wide magnification range of this metrology. The cutting area of all dicing wheels under investigations consisted of a homogenous diamond-metal-matrix. There were no differences in diamond concentration between the circumference and the wheel s sidewall. As a consequence, the investigations were conducted on the wheel s side wall since measurements were easier than at the circumference. For each dicing wheel under investigation, three micrographs from different areas of the sidewall were taken. In a second step, the areal grain concentration was determined. By dividing the average number of grains by the size of the measured area, this value was calculated. The average grain exposure was determined with atomic force microscopy (AFM). On each dicing wheel, three areas with a size of x each were measured and for each area five profile cuts with a distance of 2 from each other were established. µm µm Fig 2: AFM scans from MCT Q 2414 (left) and MCT Q 4614 (right) µm µm For each profile cut a reference line was found. This line divided the measured profile such that the squares of the deviations on both sides was the same. The greatest deviation from this reference line was asumed to be the grain exposure. This measurement coinsides with value R p as defined by ISO 4287/1 [4]. The value for average grain exposure was the average taken off the 1 values of each dicing wheel. 3. Machining Parameters The experiments were carried out with metal bonded dicing wheels with different grain sizes from various manufacturers. Table I provides an overview over the dicing wheels used for the investigations. TABLE I DICING WHEELS USED FOR THE INVESTIGATIONS Wheels Manufacturers Wheel width Grain size MCT Q 2414 MCT Q x.16 x 4 PD3/6 Mic NiF NiF Mitsubishi Mitsubishi Norton microkerf microkerf µm 2-4 µm 4-6 µm 3-6 µm The dicing wheels from Mitsubishi were hub-type. For this wheel type the dicing wheel is bonded to an aluminum hub serving as a carrier. The hub was mounted directly on the spindle of the dicing machine. Since all other dicing

3 wheels were hubless, they had to be mounted on the spindle with a precision pair of flanges. The thick silicon substrate was mounted on a rigid carrier using a double sided adhesive tape. This carrier, a simple blank silicon wafer, made the handling much easier. The whole package was held directly by a vacuum table on the dicing machine. The cutting speed was v c = m/s (approx. 36, rpm). The cut depth was d c = 7 µm, and the feed rates were varied in six steps by / 3 / / 64 / 8/ mm/s. 4. Results The evaluation of the cutting experiments are demonstrated in Fig Chipping [µm] Feed Rate [µm] Norton.1 x 3/6 microkerf NiF microkerf NiF MCT.1 x 4/6 MCT.1 x 2/4 Fig. 3: Results of chipping evaluation When machining thin wafers, the dependence of the chipping on the feed rate for the investigated dicing wheels can be easily deducted. For the hubtype Mitsubishi wheels, the influence of the feed rate on the chipping is weak. However, the grain size effects chipping quite strongly. For the remaining wheels, all of them hubless, the chipping depends much stronger on the feed rate and is less inflicted by the grain size. Interestingly, there is a distinctive local minimum in chipping at a feed rate of v f = 64 mm/s for most dicing wheels. The respective ratio of v c / v f is 164 at a cutting speed of v c = m/s ( 36, rpm). This effect can be observed independently from wheel width, mounting system (hub type or hub less), and grain size. Only the dicing wheel with the grain size of 2 to 4 µm does not exhibit this effect, but at a feed rate of v f = 64 mm/s the chipping decreases as well. In the graph of the microkerf NiF (grain size ) in Fig. 3 the local minimum is very strong. The wheel by Norton with approx. the same grain size of 4. shows a much weaker effect. TABLE II GRAIN CONCENTRATION, GRAIN EXPOSURE, GRAIN SIZE Dicing Wheel Grain Concentration Grain Exposure Grain Size MCT.1 x 2/4 MCT.1 x 4/6 Norton.1 x 3/ NiF NiF 4.3 grains / ² 2.22 grains / ² 4.62 grains / ² 2.26 grains / ².84 grains / ² 1.1 µm 1.21 µm 2.3 µm 2.3 µm 4.76 µm 3 µm 4.

4 In Table II the average grain concentration and the average grain exposure are evaluated. There is a linear dependency between grain size and grain concentration. The larger the grain size, the lower the grain concentration. 12 Grain Size [µm] Grain Concentration [1 / ²] Fig. 4: Dependency between grain concentration and grain size The influence of the wheel parameter grain concentration, grain exposure and grain size wheel on the chipping are quite similar. For the investigation the average of the chipping size of all feed rates was taken. The average chipping size is not proportional to the average grain size and to the average grain concentration. Figure shows the dependence between average grain size and average chipping. 3 3 Average Chipping [µm] Grain Size [µm] Fig. : Influence of the grain size on chipping Since there is a linear dependency between grain size and grain concentration (Fig. 4) the influence of the grain concentration on the average chipping is similar. Figure 6 shows the influence of the average grain exposure to the average chipping. The cut quality decreased with an increasing in grain exposure.

5 3 Average Chipping [µm] Grain Exposure [µm] Fig 6: Influence of the grain exposure on chipping. Conclusion To achieve an economic dicing process for ultra thin silicon wafers, several cutting parameters had to be coordinated. On the one hand, the wheel parameters has to be optimized. For achieving minimal chipping, the dicing wheels should have high grain concentration, small grain size and low grain exposure. Due to this parameter combination the removal per grain is low and hence the chipping is low as well. On the other hand, the cutting parameters of the dicing machine had to be optimized. The experiments demonstrated the independence of the cutting parameter from the wheel parameters. For all wheels the best cutting quality resulted at a feed rate of v f =64 mm/s with a a cutting speed of v c = m/s. This is a ratio of v c /v f = 164. In combination of all this coordinated parameters a street width of less than 32 µm was achieved. Acknowledgement This research is sponsored by the th framework Competitive and Sustainable Growth of the European Commission. Attendance for the conference was sponsored in part by the Cray Foundation, Hanover, Germany. References [1] Inasaki, I.: Dicing Of Silicon Wafers. Abstracts MicroMat 2, Berlin, Germany, Apr. 2, p. 14 [2] Gatzen H.H: Mikrobearbeitung sprödharter Kermikwerkstoffe mittels Trenn- und Profilschleifen. 9. Jahresband Schleifen Honen, Läppen und Polieren, 1999 [3] Gatzen, H.H.; Jones, G.M.; Morsbach, C.: Investigations on the Wear Profile of Dicing Wheels. Proc. ASPE 14th Ann. Meet., Monterey, USA, Oct.-Nov. 1999, pp [4] ISO 4287, Terms, definitions and surface texture parameters, 1997