The present invention relates to systems and methods for “wafering” or cutting materials such as silicon or glass for electronic, photovoltaic, or other uses.
In photovoltaics, thirty-five percent of silicon module costs come from wafer manufacturing, and two-thirds of those costs are directly related to the process of wafering and silicon feedstock yield. The standard technique for wafer cutting uses a wire saw set with an abrasive material that wears away the silicon under the wires. The typical production involves 160-180 μm wafers with 120 μm wire—so the material loss, also known as kerf-loss, can be up to the thickness of the wafers. Besides the loss of material other undesirable wafer characteristics are obtained with this method such as thickness variations, warp, or saw marks.
Different “kerfless” techniques have been proposed as alternatives for the sawing technology. Some of the proposed solutions have involved conducting crystal growth directly in wafer form. The main disadvantages with this technology are the lack of control of the crystal structure or defects and the lack of the inherent advantage of impurity segregation present in bulk crystal growth.
Spalling is another alternative kerfless technology in silicon which capitalizes on the crystalline and mechanical properties of silicon since it has well defined crystal planes and it is an ideal brittle solid through which cracks can propagate easily. However, this technology carries several difficulties for industrial applications. In the case of some harder materials such as sapphire, its structure and mechanical properties make spalling an unfeasible option as a wafering technique. For semiconductors wafering, there are some severe problems related to the quality and efficiency of the wafers produced by spalling.
Another disadvantage in this process is the high temperatures used to produce the cleaved wafer. In some experiments, the substrate needs to be heat up till 600-800 C in order to be able to activate the spalling mechanism upon cooling. That range of temperature degrades the properties of the substrate through defects and diffusion of species inside the silicon bulk.
In those experiments performed at lower temperatures, the lifetime of the carriers in spalled wafers is much lower compared to the original substrate. Also, the surface of the produced wafers is not regular and could lead to a total thickness variation of 100% with respect of the wafer thickness. These irregularities, where both cleaved wafer and parent substrate show rough surfaces, will cause a lower efficiency in the final cell performance.
A revolutionary process for cutting silicon that eliminates slurry and wires while doubling the yield of a silicon ingot could become a disruptive technology and make solar energy more competitive with other fossil fuel technologies. Along with photovoltaics, this technology will open a new market for those technologies that manufacture wafers or thin films with the current slurry or wire sawing techniques.
However, some alternatives to the current sawing technique have too many difficulties to become that disruptive technology that could wafer any kind of material at a low cost. Specifically, in the spalling technique, the main problem relies on the total lack of control of crack propagation that derives in low quality surfaces and the inability to wafer hard materials such as sapphire.
The success of these kerfless wafering techniques relies on the control of the crack propagation through the material. The systems and methods describes below provide kerfless cleaving of a wafer from a substrate by using ultrasound to control the propagation of an premade crack.
In one embodiment, the invention provides a method of wafering a material using controlled crack propagation. A first stress in applied to the material such that the first stress is below a critical point of the material and is insufficient to initiate cracking of the material. A controlled ultrasound wave is then applied to the material causing the total stress applied at a crack tip in the material to exceed the critical point.
In some embodiments, the frequency and/or amplitude of the controlled ultrasound wave is adjusted to maintain a substantially constant and relatively low crack velocity. In other embodiments, applying the controlled ultrasound wave includes applying a first ultrasound wave at a first frequency and applying a second ultrasound wave at a second frequency. Neither wave alone generates enough stress at the crack tip to propagate cracking of the material. However, the two waves are controlled such that, when they periodically align and are applied to the crack tip of the material in phase, the resulting stress is sufficient to propagate cracking of the material.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
The initial load L (as illustrated in
Alternatively, the initial load could be applied to obtain a stress that is just above a critical value for the material 101. This initial stress causes spontaneous crack propagation in the material 101. The ultrasound source 103 is then controlled to modulate the stress at the crack tip to remain at a value just above the critical value.
In some embodiments, such as illustrated in
After crack propagation begins, the speed and direction of the crack propagation can be regulated by controlling the amplitude and frequency of the ultrasonic waves and, in some implementations, the position of the ultrasound source. The frequency is controlled to provoke tensile stress at the crack tip such that the acoustic frequency causes bonds in the crack plane to oscillate and stretch. The amplitude is set to create a focalized stress center at the crack tip, which ultimately controls the speed of crack propagation.
Even though the system is able to control the propagation of the crack through the material, in some implementations, the interaction of reflected waves with the crack tip could cause undesirable or uneven results. In order to prevent this, mitigating measures are applied to prevent the waves emitted by the crack tip and by the ultrasound source from being reflected back into the material when they reach the edges of the specimen. In some implementations, this is achieved by backing the solid with a damping or absorbing material of an appropriate acoustic impedance. As such, when the waves reach the edge of the solid, they are transmitted into the absorbing materials and completely damped. Consequently, no waves are reflected back to the material that could alter smooth propagation of the crack. Because the reflected wave's frequency will depend on the velocity of the crack and the properties of the solid such as the elastic modulus, the type of damping material may vary for different types of solids.
The propagation of the crack can be regulated in a number of different ways using the ultrasonic source.
It is noted that the flowchart presented in
Finally, the table below illustrates examples of values determined for different materials as glass, silicon, or sapphire. In these examples, equation (1) below can be used for the critical stress necessary to start a crack propagation:
where E=Young modulus; γ=Surface energy; a=Initial crack length.
In some implementations, the surface roughness of the material at the location of the propagated crack depends not only on the velocity of the propagating crack but also on the value of the stress intensity factor, K. The stress intensity factor is indicative of a state of the stress field around a propagating crack tip and it can be modeled using finite element analysis, FEA. As the crack propagates, the K values can go below a critical value, KIC, and arrest the crack front. Low-roughness areas will be produced on the material surface where the K-value is only slightly above the critical value KIC. Therefore, in some implementations, a substantially uniform, low-roughness surface can be created by operating the ultrasound source to maintain a difference be K-value and the KIC that is as low as possible.
After the crack starts propagating, the speed of the crack accelerates until it reaches a constant value or is decelerated (either naturally or due to modulation applied by the ultrasound source). In some implementation, this acceleration is not desired because, as discussed above, a constant velocity provides a smoother surface at the crack. Accordingly, in some implementations, the controller is configured to operate the ultrasound source to maintain a crack velocity within certain defined limits and to maintain a K-value that is slightly above the KIC. Applying ultrasound waves with higher frequencies will provide a higher effect on the propagation than those ultrasound waves at lower frequencies. Also, the effect that the ultrasound wave will have on the crack propagation can vary depending on the position of the ultrasound source with respect to the crack tip. Accordingly, in some such implementations, the velocity, stress intensity, and trajectory of the crack propagation can be regulated by controllably adjusting parameters of the ultrasound waves including, for example, the frequency, amplitude, shape, and power.
Thus, the invention provides, among other things, a system and method for wafering a material (such as glass, silicon, or sapphire) using controlled crack propagation by applying sound waves. Various features and advantages of the invention are set forth in the following claims.
This application is a continuation of U.S. patent application Ser. No. 16/083,276, filed on Sep. 7, 2018, which is a 35 U.S.C. § 371 national phase application of PCT Application No. PCT/US2017/021418, filed on Mar. 8, 2017, which claims the benefit of U.S. Provisional Application No. 62/305,399, filed Mar. 8, 2016, entitled “SOUND-ASSISTED CRACK PROPAGATION FOR SEMICONDUCTOR WAFERING,” the entire contents of each application being incorporated herein by reference.
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20210053251 A1 | Feb 2021 | US |
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Child | 17094524 | US |