The present invention in general relates to substrate and substrate production, and in particular to a process and system for high throughput production of sapphire substrates.
Sapphire, which is composed of aluminum oxide (Al2O3), may be found naturally or may be manufactured for industrial or decorative purposes in large crystal boules. The remarkable hardness of sapphires has led to the use of this material in practical applications including infrared optical components, such as laser rods and waveguides; high-durability transparent windows; wristwatch crystals and low-friction movement bearings; optical fibers; and thin electronic substrates, which are used as an insulating substrates for solid-state electronics and integrated circuits, such as light emitting diodes (LED) and silicon on sapphire (SOS) devices that also require a high conductivity of heat that sapphire can provide. Additionally, where chemical-resistance is needed for optical and industrial components; sapphire is well suited because of its inertness to chemical etch.
One application of synthetic sapphire is optics in which a high level of transparency is required. Sapphire has high transparency to wavelengths of light between ultraviolet and infrared (150 nm (UV) to 5500 nm (IR)), but is also extraordinarily resistant to abrasion and scratches, and is significantly stronger than such as compared to silicate glass windows or lenses and has significantly larger window of transparency than other optical material. Sapphire has a value of 9 on the Mohs scale of mineral hardness and is the hardest natural substance next to diamond (with a value of 10). Sapphire has an extremely high melt temperature (2030° C.) and is unaffected by all aqueous-based chemicals except some very hot acids, caustics, and fluorides.
Transparent sapphire substrates or other products are made from high-purity sapphire boules, typically seeded in the a-crystal orientation that have been grown and then cored at specific crystal orientation, typically along a crystal axis plane, for example; the a-plane. The cores are sliced, typically by wire sawing, into substrates with approximately the desired thickness and ground or lapped to remove the saw-damaged surface, and finally polished to the desired surface finish. Sapphire can be polished to a wide range of surface finishes depending on the application. For standard optical windows to provide minimum birefringence the a-plane (1120) is chosen, for LED applications, typically the c-plane (0001) is chosen.
The use of sulfuric acid in a combination with phosphoric acid has been known since at least the early 2000's to be an effective etchant for Al2O3, and in particular the c-plane (0001) crystal orientation of sapphire. Early use of the combination of H2SO4 and H3PO4 or H3PO4 alone was to decorate the sapphire surface due to the preferential etching of the sapphire surrounding defects—either due to metallic contamination or crystal dislocations.
As taught in U.S. Pat. No. 7,579,202, the use of combination solutions of H2SO4 and H3PO4 or H3PO4 to etch groves into a sapphire substrate that pattern the surface and increase the surface area, and thus the brightness of light emitting diodes (LEDs). Subsequent U.S. Pat. Nos. (7,781,790, 8,101,447, 8,236,591) have also utilized this chemistry combination for etching patterned sapphire substrates (PSS) into c-plane sapphire. The reason for using this chemistry was to isotropically etch the c-plane sapphire along the r-plane to form pyramids into the sapphire, using a mask. However, limitations existed with respect to temperatures. There are no examples of smoothing surfaces; the intent was to roughen the surface to obtain more surface area. The need for smooth surfaces is different for LED devices, compared to display devices, which this disclosure is targeted towards. The improvement of the single crystal sapphire material purity has improved, and the ability to smooth the sapphire without having a high density of defects has allowed the use of this chemistry for smoothing, without creating a high density of surface defects from the preferential etching, further allowing for use of this chemistry for high-transparency, low reflectance optical devices.
While the use of sapphire for glass-like surfaces is widely recognized for the reasons mentioned, the adoption of sapphire in mass production applications such as display covers for consumer devices, such as cellular phones, has met with little acceptance due to the low throughput and high costs of production associated with sapphire manufacture and processing. Furthermore, the use of H2SO4, H3PO4, and H3PO4 for thinning sapphire is presently not known for producing display devices that use sapphire windows.
Thus, there exists a need for a process and apparatus for producing sapphire-based products that have a high throughput and is cost effective, thereby allowing for the adoption of sapphire in high volume and lower cost applications.
The present invention is further detailed with respect to the following drawings. These figures are not intended to limit the scope of the present invention but rather illustrate certain attributes thereof.
An inventive method for thinning a sapphire substrate is provided and includes placing a sapphire substrate in a pre-heat tank to raise the temperature of said sapphire substrate; placing the pre-heated sapphire substrate in a wet etch tank comprising a solution including at least one of H2SO4 and H3PO4 at a temperature ranging between 200-400° C.; monitoring the time to determine when to remove said sapphire substrate from said wet etch tank to thin said sapphire substrate; and placing the sapphire substrate in a cool-down tank to lower the temperature of the sapphire substrate. One or more sapphire substrate orientations may be used in the invention, illustratively including c, r, a and m plane orientations.
An inventive system for producing sapphire substrates is also provided and includes a docking base station configured to accept docking modules and controls; and a single point for facility connections to utilities and supply lines on said docking base station. The docking modules include one or more high temperature process modules, a pre-heat module, a cooling module, and a dryer/rinse module.
Additionally, a substrate composition is provided which includes a sapphire substrate having a thickness of between 50 and 400 microns and a reflectance of at wavelengths between 380 nm and 1000 nm.
Finally, an inventive method for smoothing a sapphire substrate is provided which includes placing said sapphire substrate in a pre-heat tank to raise the temperature of the sapphire substrate; placing the pre-heated sapphire substrate in a wet etch tank comprising a solution including at least one of H2SO4 and H3PO4 at a temperature ranging between 200-400° C.; monitoring time to determine when to remove the sapphire substrate from the wet etch tank to smooth the substrate; and placing the sapphire substrate in a cool-down tank to lower the temperature of the sapphire substrate. One or more sapphire substrate orientations may be used in the invention, illustratively including c, r, a and m plane orientations.
The present invention has utility in the processing of sapphire to form substrates or other laser cut or wire sawed products. An inventive process and system is provided for high throughput or any production level of sapphire substrates using an aqueous chemical etching process. Through the invocation of processing temperatures above 200° C., processing times for thinning and etching are decreased so as to provide an overall increase in throughput. Sapphire substrates obtained from the inventive process and system can be made ultra-thin with superior reflective properties, as compared to sapphire substrates produced by conventional processes. The ability to manufacture ultra-thin sapphire substrates with embodiments of the inventive process and system allow for flexible substrates and membranes that may be curved or other otherwise contoured. Unlike conventional processing of sapphire substrates which is slow and relies on physical abrading, grinding, lapping, and/or polishing of a substrate with only limited results due to the multi-faceted surface of the sapphire substrate that leads to unevenly treated high or low spots, such as pits, scratches, and buildup of amorphous Al2O3 on the substrate, the inventive process and system avoids the need for abrasive grinding, lapping, or polishing by using substrates without abrasive polishing, etc. Certain embodiments of the inventive process use agitation to minimize these localized physical removal effects, creating a very smooth, highly transparent surface. Agitation can also increase the etching rate of c-plane sapphire. The ability of the inventive process to use unpolished or unground wire sawed wafer (“as cut”), has a surprising result of superior planarity and smoothness and thereby reducing the time of, or in some embodiments entirely eliminating the polishing step of chemical mechanical polishing (CMP). In still other embodiments, the time of, or in still other embodiments entirely eliminating sapphire thinning through grinding.
Sapphire substrate etching applications provided by embodiments of the inventive sapphire production system may include patterned sapphire substrate PSS (etching), sapphire substrate dicing, sapphire thinning, sapphire smoothing, sapphire texturing, and sapphire substrate edge rounding.
It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4 and also 4:1, 3:1, 3:2, and 2:1.
Sapphire material etching applications provided by embodiments of the inventive sapphire production system may include sapphire component manufacturing such as thinning, shaping, rounding, texturing, smoothing, scoring, substrate dicing, etc., while eliminating or reducing the amount of lapping, grinding, texturing, and finishing, such as polishing. Examples of end products include sapphire plasma tubes, touch screens, protection screens, process boats, lenses, etc.
High temperature applications for the H2SO4 and H3PO4 solutions used in certain embodiments include resist stripping, organic contamination removal, organic film removal, certain metallic contamination removal, and ceramic etching and finishing. In some embodiments of the present invention these acids are contacted with materials at temperatures between 200 and 400° C., while in still other embodiments, the temperature at which reaction occurs with a target substrate is 240 and 320° C. The volume ratio of aqueous solutions of H2SO4:H3PO4 typically varies from 0.1-10:1 and in some particular embodiments is between 0.5-3:1. Additionally, H2SO4 or H3PO4 each alone can be used for specific applications in addition to the mixtures. Typical aqueous concentration of commercially available sulfuric acid H2SO4 is about 96-98 weight percent (wt %) and phosphoric acid H3PO4 is about 85 wt %.
Observations have shown that H2SO4:H3PO4 ratio at 1:1, although having a lower etch rate than the higher ratios such as 3:1, yield a smoother surface with less pits. The physical mechanism is not known, however, it is speculated that the slower etch rate does not allow the defects to be preferentially etched, which is the source of the pitting. The defects may be crystal disruptions or metallic contamination.
Sapphire substrate smoothing (SSS) is accomplished on substrates or manufactured parts, and can be successfully performed on various planes of the sapphire work piece. The SSS decreases peak to valley roughness due to sawing applications, and for top surface smoothing and removal of saw damage and the amorphous sapphire buildup. SSS can be used for edge smoothing after cutting, to minimize or replace polishing. The SSS process can minimize peaks and pits and minimize preferential etching around defects; including both metallic contamination and crystal dislocations.
Sapphire substrate thinning performed with embodiments of the invention provide for rapid and cost effective thin sapphire substrates. The no-stress thinning inventive process has the ability to thin all sapphire planes: a, c, r, m. It is appreciated that the rate and uniformity of the thinning varies with factors including the sapphire plane, contaminant concentration, acid solution, and agitation. The thinning process is readily combined with smoothing applications. It is further appreciated that specific sapphire substrates are intentionally pitted in a controlled and uniform matter. Such pitted sapphire has applications where internal reflectance is favored such as, for example, LEDs. Furthermore, embodiments of the inventive process provide an increased etch rate with improved uniformity, as compared to lower temperature etch processes. SST may be performed after sawing where the minimal thickness is limited by dimensions of the saw. SST replaces lapping or grinding which create surface damage and stress. Lapping and grinding are limited to substrate thickness due to stress; these processes can create physical stress points in the crystalline sapphire that can propagate first into lattice dislocations and then more severely into cracks, in some cases immediately, while in other cases the cracks are latent and appear at later stages of the manufacturing process. Thinning of substrates with initial thickness of 200 to 700 μm down to 50 μm has been demonstrated with embodiments of the invention. Wafers thinned to below 80 μm in some embodiments are supported so as to inhibit creasing or rolling up due to surface tension. Thin substrates may also “float” in chemical bath, and fixtures to support thin substrates have been developed.
SSS minimizes surface roughness, and also has the ability to minimize preferential etching around defects both metallic and dislocations. By choosing the appropriate volume ratio of H2SO4:H3PO4, the crystal plane-selective etching can be attenuated. Using the addition of H3PO4 in H2SO4 at the appropriate temperature, the smoothness can be maximized and the formation of undesirable Al2(SO4)3 can be minimized. In certain embodiments, when Al2(SO4)3 is formed, it is removed by etching. The etching process assists in the solubilization of the precipitate Al2(SO4)3. One method is using phosphoric alone, H3PO4, to etch the Al2(SO4)3 by replacing the SO42− group with PO43− group, thus rendering the ions soluble in the H3PO4 solution. The process can be performed at temperatures between 120-250° C. The smoothing that occurs during the H2SO4 processing step (a 5:1 ratio for example) is retained.
It is appreciated that in specific embodiments of the present invention, H3PO4 alone, at the elevated temperatures afford comparable results. In addition to H3PO4 and H2SO4, additives include solvents, solvents that form azeotropes with water, chelating agents, surfactants, other acids, salts of acids, and combinations thereof. Such additives being stable at the process temperatures of 200-400° C.
Embodiments of the inventive process and system can be used for sapphire texturing (STX) to increase peak to valley roughness, act to replace pattern sapphire substrate (PSS) etching with a no pattern process, create a translucent frosted surface, for increased reflectance due to light reflecting off the surface, for increased surface area for bonding and other applications, to maximize peaks and maximize pits, to control preferential etching around defects including both metallic and dislocations.
Embodiments of the inventive process and system can be used for sapphire substrating, wafering, and dicing. During substrating and wafering, saw damage may be removed and edge rounding performed. During the dicing process kerf damage may be removed that results from laser sawing, as well as slag removal resulting from both diamond and laser sawing. Furthermore, partial sawing or scoring and then etching with the H2SO4/H3PO4 acid process may be used to minimize long sawing times.
An acid media is required to etch sapphire (Al2O3). A total reaction given by
Al2O3+3H2O→2Al(OH)3
Al(OH)3+3H+→Al3++2H2O where
Without intending to be bound by a particular theory, Al(OH)3, AlPO4, and Al(H2PO4)3 are soluble in the etchant solution, while Al2(SO4)3 is insoluble due to the reaction of the aluminum cation, Al3+ with the anions, SO42− or PO43− in the aqueous solution due to the high concentrations thereof in the inventive etching solutions at 200-400° C. Thus, a mixture of H2SO4 with H3PO4 is able to maintain a high temperature and thus control the boiling point and also favorably minimize insoluble products, while ensuring a wide process window. In at least one embodiment, agitation is performed to help with the chemical transport mechanism to convect the insoluble impurities away from the surface. The agitation not only improves the etch rate but also decreases the amount of Al2(SO4)3 that precipitates on the surface at high H2SO4: H3PO4 ratio, for example at 5:1.
Agitation may be accomplished by 1) mechanical action, 2) bubbling gas through the solution, or 3) by applying sonic energy. For example, bubbling gas: N2 or other inert gas (i.e., Ar, He) may be bubbled into the acid solution to cause agitation of the liquid. Typically, a gas diffuser plate is placed at the bottom of the bath, and the gas flows into the liquid, causing “boiling” of the liquid. This mechanism displaces the chemical that is close to the substrate with new chemical and removes the products of the reactant. Sonic energy agitation may use: ultrasonic energy or other sonic energy that can also cause displacement of the fluid next to the substrate. The addition of gas along with the sonic energy allows a smaller quantity of gas to be used.
Referring now to the figures,
The high temperature etch bath 100 heats the chemicals to a temperature between 200-400° C. The process tank 120 is made of materials compatible with acidic chemistry and high temperatures such as quartz, or ternary carbides of the formula Mn+1AXn, where M independently in each occurrence is Ti, Nb, Zr, Hf, Nb, Cr, Ta, V, Sc, or Mo; n is 1, 2 or 3; A independently in each occurrence is Al P, Pb, Ga, S, In As, Cd, Ge, Tl, or Al with the proviso that M and A are not the same; and X is C or N independently in each occurrence. In at least one embodiment the process tank 120 is a quartz tank. In certain inventive embodiments, auto-dosing is used to maintain constant temperature and concentrations in concert with temperature, level, and thickness sensors during sapphire processing. The bath is configured with a drain port for easy disposal of bath chemistry. In certain inventive embodiments, chemistries within the bath are recirculated and/or agitated at temperatures above 200° C. to eliminate localized etching effects. In certain embodiments, the high temperature recirculation pump 110 is a quartz-lined pump. Agitation is also done to eliminate localized etching effects due to concentration or temperature gradients and minimize bubble stiction. Chemical fume capture and control is accomplished through ergonomic design and air control management, and with a recondensor at high temperatures to minimize and capture fumes and minimize chemical usage. Both double and single sided etching may be performed in the bath as a batch process. It is appreciated a robotic agitator arm is readily controlled to move a wafer holder in various patterns of movement illustratively including vertical, horizontal, arcuate, rotary, and a combinations thereof. For single sided etching a custom carrier to protect unetched side is used.
The use of thermal management such as the temperature ramp-up 24 and ramp down management 28 in the form of pre-heat and cool-down tanks in
While a single bath is typically adequate, multiple baths can achieve multiple effects. For example, a first bath can achieve a high etch rate for thinning, while a second bath may be used for smoothing a thick substrate which requires both thinning and smoothing. In a second example, a first bath provides preliminary smoothing, while a second bath provides final smoothing for any substrate with extreme saw damage. In a third example, three baths are used, where the first bath is for a high etch rate for thinning or removal of saw damage, the second bath is for preliminary smoothing, and the third bath is for final smoothing. Other combinations of baths may be used depending upon starting substrate characteristics and desired objectives.
Without being bound to a particular theory, it is believed that large, thick pieces of sapphire have a higher propensity for breakage than small thin substrates. Thus, at least one embodiment of the present invention manages the temperature of the sapphire work piece to prevent breakage. Several methods of temperature management may be employed which are widely used in the art. In at least one embodiment air cooling is used. By way of a non-limiting example, gradually raising the post-process work piece from the high-temperature bath at a rate of 1 mm to 1 cm per minute. It should be appreciated that the larger the work piece, the slower the rate of rise, thus the slower the cooling. Due to air cooling, the work piece has chemicals coated on the surface. The work piece can be post-processed once the core is cooled to within 60° C. of room temperature. Process processing can include cleaning the precipitate Al2(SO4)3 from the surface or rinsing with water to remove the chemical or both. In at least one embodiment, leaving the work piece in the processing bath and cooling down gradually can also accomplish the same rate of cooling. Optional techniques for cooling can include multiple temperature baths, where the work piece is gradually lowered in temperature from the process temperature in 60° C. increments or smaller temperature increments. In at least one embodiment, an oven is used to accomplish the same cooling. In addition, and by way of a non-limiting example, multiple ovens or furnaces are used with gradually lower temperatures between which the work piece is sequentially transferred to obtain gradual cooling. It should be appreciated that heating a large work piece requires the same care with respect to temperature management. Heating at too high of a rate will cause breakage due to the large thermal coefficient of expansion. Heating can be accomplished in an oven, in the process bath, or on any apparatus that can uniformly heat (or cool) the work piece.
Sapphire material etching applications provided by embodiments of the inventive sapphire production system operate at elevated temperatures, and the very high temperatures of the baths do not lend themselves to normal metrology methods for concentration measurement. For example, dip probes would be prone to destruction under the high temperatures, and it is infeasible to use conventional flow cell commonly used for concentration monitors. However, the use of quartz baths in embodiments allows a line of sight for optical measurement. Absorption measurements through quartz using spectrometers and light sources are feasible for concentration determinations. In addition, in situ surface roughness may be measured, as well as in situ thickness measurement is possible.
Etching rates are dependent on concentration and temperature.
Smoothing may be performed on any sapphire crystal substrate that has undergone a grinding, lapping, or polishing step. During the polishing step, the top layers of the sapphire crystal, independent of initial crystal orientation are disrupted. This lattice disruption is noticed in a variety of single crystal substrates (J. A. Randi, J. C. Lambropoulos, and S. D. Jacobs, “Subsurface damage in some single crystalline optical materials,” Appl. Opt., 44, 2241-2249 (2005) http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-44-12-2241). These crystal disruptions affect the optical and mechanical properties of the substrate. The disrupted lattice layers on sapphire substrates can be removed using the smoothing process. In some cases, no more than 2 microns sapphire is removed. Improved reflectivity is achieved and are shown in the following table, approximately 2-4 microns of sapphire is removed in each example.
H2SO4 alone deposits Al2(SO4)3 on the surface, even with agitation and at all temperatures, this chemistry alone is not acceptable for thinning or smoothing. In addition, ratio of 5:1 H2SO4 is the cut off for forming sulfates on the surface of the sapphire. Above 5:1, for example 8:1 form the sulfate precipitation. It has been found that if the sulfate precipitation occurs, placing the substrate into high temperature H3PO4 or into a H2SO4+H3PO4 bath at lower ratios, such as 3:1, the precipitation can be removed.
The present invention is further detailed with respect to the following examples that are not intended to limit the scope of the claimed invention, but rather to illustrate specific aspects of the invention.
A patterned sapphire substrates (PSS) is processed using a wet etch composed of 66-75 volume % of 98 weight % H2SO4 and 25-33 volume % of 85 weight % H3PO4 at a temperature ranging between 250-400° C. Example results are shown in table 1.
Sapphire substrate smoothing (SSS) is processed using a wet etch composed of 30-90 volume % of 98 weight % H2SO4 and 10-70 volume % of 85 weight % H3PO4 at a temperature ranging between 250-300° C. It is noted that further reductions in sulfuric acid or phosphoric concentrations are possible. In one embodiment only sulfuric acid is used for smoothing, where no etching was observed. Example results are shown in table 2.
Sapphire substrate thinning (SST) is performed using a wet etch composed of 30-90 volume % of 98 weight % H2SO4 and 10-70 volume % of 85 weight % H3PO4 at a temperature ranging between 250-300° C. It is noted that further reductions in concentrations are possible. Example results are shown in table 3.
The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.
This application claims priority to U.S. Provisional Application No. 61/877,819 filed Sep. 16, 2013, the contents of which is incorporated herein by reference as if explicitly and fully expressed herein.
Filing Document | Filing Date | Country | Kind |
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PCT/US2014/055960 | 9/16/2014 | WO | 00 |
Number | Date | Country | |
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61877819 | Sep 2013 | US |