Meeting current and future wafering challenges
03/01/2009
Philippe M. Nasch, Romain Sumi, Applied Materials Switzerland SA, Cheseaux—sur—Lausanne, Switzerland
Currently, the multi—wire slurry sawing (MWSS) technique is the leading technology for high precision machining of large cross—sectional area wafers for both the semiconductor and photovoltaic industries. This article presents current and expected future challenges in MWSS technology, especially in the context of the cost—driven double—digit growth of the photovoltaic industry.
Mono—crystalline materials — silicon, III—V compounds, carbides, quartz, ceramics, and other exotic crystals — are considerably hard (hardness of a few tens of GPa) with an elevated rigidity that renders their machining and slicing particularly difficult. After the growth of crystalline materials in the form of ingots, slicing of the ingots into wafers is the first manufacturing step for electronic device fabrication in the semiconductor industry and for main—stream solar cells manufacturing in the photovoltaic industry. Demands on wafering quality are high, including, for example, requirements of minimum material loss during slicing (i.e., high material utilization), minimum surface damage (micro—cracks), minimum topology defects (warp, bow and thickness variation), high production rates and yields, and minimum machining impacts on down—stream processes.
Photovoltaic applications
The key to large scale photovoltaic (PV) adoption is largely driven by the industry’s ability to drive down the cost per watt. For wafer—based PV, the crystalline silicon (c—Si) ingot raw material and slicing costs together amount to more than one third of the total module cost. Hence, reduction in raw material cost is a major focus for PV manufacturers. Also, due to the rapid growth of the market, the amount of silicon consumed annually for PV has now exceeded that for IC production, further exaggerating the importance of starting material and methods of cost reduction. Therefore, development of technologies for efficiently cutting thinner wafers, both to minimize the amount of silicon used per wafer, as well as the silicon lost during the cutting of the wafers, is crucial in order to drive down the solar cost per watt. Figure 1 shows the PV industry trend towards using thinner wafers and thinner wires, thus reducing the amount of silicon required to produce electricity.
 Figure 1. Industry trend to thinner wafers and smaller diameter to reduce the amount of silicon needed per output Watt (g/Wp). |
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Technology
The multi—wire slurry saw (MWSS) consists of a high—strength, high—carbon content steel wire, with diameter ranging from 250??m???80??m, that moves either uni—directionally or bi—directionally to perform the cutting action. The single wire is wound on wire—guides carefully grooved with constant pitch, forming a horizontal “web“ of parallel wires (Fig. 2). The wire—guides are rotated by drives that cause the entire wire—web to move at a relatively high speed of 5???20m/s. High flow—rate nozzles bathe the moving wires with cutting slurry ???a liquid carrier with suspended abrasion particles (e.g., of silicon carbide). Surface tension causes the wire to carry the slurry into the cutting zone, thus producing a cut. During the cutting action, either the ingot is moved vertically and pushed down through the wire web or conversely, the ingot is stationary while the wire web is pushed up through the ingot. Wire tension is maintained constant (10—80N) during the cutting process with state—of—the—art feedback control. A wire feed reel provides the required new wire and a wire take—up reel stores the used wire. The ingot diameter that can be processed by the wire saw is limited by the shaft spacing of the wire guides for the web and the vertical travel possible.
 Figure 2. Schematic of a typical MWSS arrangement. |
The MWSS cutting action is essentially that of a fast three—body lapping process, characterized by a rolling and indenting cutting mechanism. The wire, acting as a slurry carrier, applies a cyclic compressive loading. The larger abrasive grains are occasionally trapped by asperities on the surfaces of the specimen and the tool, forcing them to rotate by the parallel shear motion of these two bodies. During this rotation, the abrasive grains transmit part of the applied compressive load from one surface to the other, resulting in both surfaces being indented. The rotation of the particles enables indentation of the surface, but prevents any scratching action from occurring.
The relative penetration depth into each surface is determined by the relative hardness of the two materials. The stress pattern in the indenting zone, produced by a line contact between surfaces subjected to a compressive loading and sliding friction, is the superimposition of the stresses due to the normal and tangential forces applied by the wire. The maximum compressive stress (normal force) is found at the tip of the indenting notch where micro cracking occurs. Combined with lowering of fatigue strength due to cyclic loading, these cracks propagate as the ingot is moved in a direction transverse to the wire axis. The maximum shear stress (tangential force) is attained at a small distance from the surface. Maximum subsurface shear stress, characteristics of contact mechanics, facilitates the formation of ingot chips (kerf) on the ingot surface. Macroscopically, the removal rate of matter is proportional to the product of the contact load pressure and the wire speed. The proportionality coefficient is a wear—like coefficient, that depends upon multiple factors like the elastic properties of the materials (Young’s modulus, fracture toughness, and hardness), tribologic parameters (contact area, film thickness), viscosity of liquid carrier, and the concentration and diameter of abrasive grit.
Due to elasto—hydrodynamic interaction between the wire and the viscous slurry, a film of slurry forms in the cutting zone. This slurry film thickness increases with increasing slurry viscosity and decreases with increasing load pressure. Hence, at sufficiently high load pressures, the slurry can be expected to be completely expelled from the interface between the tool and the ingot, leading to a sudden loss of cutting efficiency. Within typical wire saw operating conditions, increasing the wire speed can increase the value of this pressure threshold through impact dynamics. On the other hand, for a given feed rate, a lower wire speed results in a higher pressure on the wire. This is due to the fact that the wire velocity and the feed rate are correlated through the material removal rate. Furthermore, there is a lower limit to the wire velocity below which the cutting efficiency drops due to insufficient hydrodynamic pressure of the slurry at the entrance slot. Then no slurry is capable of reaching the ingot internal cutting zones, resulting in a loss of process control.
MWSS evolution
The idea of cutting materials with a moving wire or cable wetted with an abrasive solution is at least as old as the ancient Egyptians, 4000 years ago. Modern wire saw technology was driven in large part by the PV industry’s need to slice wafers with minimum raw material losses. The first MWSS machines for PV wafering applications were introduced to the market in the early 1980s, thanks to the pioneering work of Dr. Charles Hauser, founder of former HCT Shaping Systems, Switzerland. These wire saws had 3 wire—guides, with the slicing web positioned in the bottom and the ingot was pushed upwards through it. Hence, after the cut, the wafers were held together inside the triangle formed by the wire—guides. However, pulling the wafers out from the web by lowering the table, even with slow wire motion, created saw marks on the wafer surfaces. Therefore, the wafers had to be removed manually while in the uppermost position. This posed a considerable challenge due to limited accessibility inside the triangle and the second generation of wire saws was built with an inverted triangle. This gave better accessibility to the operator for unloading wafers after the cutting operation.
 Figure 3. Some available MWSS slicing head configurations. |
As the ability to slice ingots side by side was developed, the cutting web structure was changed from triangular to rectangular (Fig. 3) and the number of wire—guides changed from 3 to 4, with inherent motorization adaptation. In the early 1990s, double—table machines were introduced, with the slicing head configuration consisting of two large diameter wire—guides for cutting 200 or 300mm diameter monocrystalline silicon ingots used in semiconductor applications. Today, multiple wire saw configurations are available with 2, 3 or 4 wire—guides that can simultaneously cut 1, 2, 4, or even 6 ingots.
The high wire tension causes the cutting wire web ??? comprising 500 to 1500 wires ??? to produce enormous pressure on the surface coating as well as forces on the axis and bearings. To withstand this several tons of load and allow rapid exchange after coating wear, extremely robust bearing box designs were implemented. The wire—guide diameter initially started out small (50mm), but rapidly increased to 80mm to reach 300mm today for most Si applications and even 400mm for some special applications. As the wire speed increased at the same time, innovations were made for motorization and for managing the inertia and vibration issues (80kg at 1000 rpm). Also, solutions were implemented for the cooling of wire—guides and bearing boxes, along with temperature monitoring and control.
 Figure 4. Cross—web MWSS that transforms post—growth poly—or mono—crystal ingots into parallelepiped bricks. |
MWSS technology adaptation was accelerated by three major events: The introduction of electronics in control systems that replaced electro—mechanical systems in 1982; the necessity of the PV industry to lower costs and move to high productivity in 1984; and the internet boom that necessitated the semiconductor sector to economically produce large—area wafers in 1994. It took about 10 years for the MWSS technology to be entrenched in the PV industry. In 1993, the electronic sector for silicon—based semiconductors was faced with the throughput limitations of the ID—saw for 6" and 8" wafer production. The MWSS technology, strengthened by 10 years of development for PV applications, could meet the high quality requirements for semi applications and was a natural successor for ID saws. MWSS throughput is much higher than that of ID saw technology that uses a diamond—coated rim on a circular blade for the cutting action. With the most powerful MWSS available on the market today, a single cut, lasting typically 8—12h, is capable of producing about 6000 wafers 156 ?? 156 mm2, yielding an equivalent PV surface area on the order of 150m2. Currently, almost 100% of silicon wafering for both PV and semiconductor sectors is achieved using MWSS.
Since becoming the industry standard for wafering in the semiconductor and PV sectors, the application range for the MWSS has widened. Over the past 10???15 years, wafer thickness for c—Si PV cells has been reduced by ˜50%, from 330µm to today’s typical 180—220µm thickness range. At the same time, the wire diameter has been reduced from 180—160µm to 140—120µm, and productivity of MWSS tools improved by a factor of 2 to 5.
Current development efforts for MWSS are focused on enabling even thinner wafers with thinner wires. In R&D environments, MWSS technology has already been proven to slice wafers <120??m—thick, using 80??m wire. The challenge now is for wire manufacturers to develop a production—ready thin wire, i.e., a wire manufacturing process capable of producing more than 1000km of ultra—thin wire in a single piece without any joints, cracks, or defects.
Manufacturing tools for the post—slicing processes that are capable of handling very fragile thin wafers in a stress—free manner also need to be developed. Since the trend toward thinner wafers and thinner wire is driven by the continuing need of the PV industry to reduce the amount of silicon required per watt due to the high silicon raw material costs, the breakeven point is when Si raw material usage yield (expressed in m2 of wafers produced per kg of Si) decreases due to the rate of wire breakage occurrence and the wafer fragility, both of which increases with decreasing wire diameter and wafer thickness, respectively.
Another example of the widening of the MWSS application range is a multi—wire slurry saw for PV applications that transform post—growth poly— or mono—crystal ingots into parallelepiped bricks to be then sliced into wafers in a conventional MWSS (Fig. 4). The machine is built with a 90° cross—web to simultaneously slice the squared cross—sections bricks.
Conclusion
Today, MWSS is a mature technology for high—precision slicing of hard, brittle materials. In the PV industry, MWSS has enabled a decrease in amount of raw material required to produce solar electricity, i.e., a reduction in the grams/Watt, by enabling the cutting of thinner wafers with less material lost during the sawing operation. As raw material accounts for over one third of the cost of solar electricity today, this is crucial in reducing the cost/Watt and thus allowing PV to reach parity with retail electricity prices. The introduction of the MWSS in the early 1990s for semiconductor applications facilitated the smooth progression per Moore’s law by enabling production of 200mm diameter wafers and beyond. Further, new applications for MWSS technology continue to be developed.
Philippe M. Nasch received his master’s in physics from the U. of Lausanne, and his PhD in geophysics from the U. of Hawaii, and is R&D Director at the Precision Wafering Systems unit of the Solar Business Group of Applied Materials Switzerland SA, Route de Genève 42, 1033 Cheseaux—sur—Lausanne, Switzerland; ph.: +41 (0)21 731 9100; email [email protected].
Romain Sumi received his master’s in engineering (microtechnique) at the EPFL—Swiss Federal Institute of Technology in Lausanne, and is engineering director at the Precision Wafering Systems unit of the Solar Business Group of Applied Materials Switzerland SA.