FIELD OF THE INVENTION

The present disclosure relates to a method for purifying crystalline sapphire.

BACKGROUND

Sapphire is a material that has extensive technological applications owing to its mechanical, optical and electrical properties. These unique characteristics of sapphire have promoted the demand for crystalline sapphire across many different industries. The applications of sapphire include: its use as substrates for light-emitting diodes (LEDs) and silicon-on-sapphire (SoS) devices, in bar code scanner windows to resist scratches, in smartphone touch screens, for optical windows and domes used to protect infrared, visible, and ultraviolet sensors, and optical fibers used in medicine. In most of the applications, the requirements are of high crystalline perfection, high temperature stability, and low chemical reactivity ' ^{1 "71} . Because the properties of sapphire are strongly influenced by intrinsic and extrinsic defects and impurities' ^7"101 , it is necessary to control their concentration and type, as well as to remove them as required.

SUMMARY

The present disclosure relates to a method for purifying crystalline sapphire using microwave processing to induce a migration of the impurities in sapphire crystals to one or more surfaces. The impurities are subsequently removed.

An embodiment provides a method for purifying a sample of sapphire, comprising:

irradiating a sapphire sample with microwave radiation with power in a range from about 0.3kW to about 300kW, at a frequency in a range from about 300 MHz to 600 GHz for a selected period of time to cause microwave thermal and field induced migration of impurities to one or more internal interfaces at which the impurities are trapped and neutralized and/or to one or more exterior surfaces; and

for impurities on the one or more external surfaces, removing the impurities.

There is provided a system for purifying a sample of sapphire, comprising:

a microwave generating device for irradiating the sample of sapphire inducing a migration of impurities towards a surface of the sample of sapphire; and

a means for removing the impurities from the surface of said sample of sapphire.

A further understanding of the functional and advantageous aspects of the disclosure can be realized by reference to the following detailed

description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:

Figure 1 depicts the Capacitance-Voltage (C-V) curves of sapphire layers on semiconductor wafers.

Figure 2 depicts how the variance of radiation time affects the migration of copper ions. DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described with reference to details discussed below. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. The drawings are not to scale. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well- known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosure.

As used herein, the terms, "comprises" and "comprising" are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms, "comprises" and "comprising" and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

As used herein, the term "exemplary" means "serving as an example, instance, or illustration," and should not be construed as preferred or advantageous over other configurations disclosed herein.

As used herein, the terms "about" and "approximately", when used in conjunction with ranges of dimensions of particles, compositions of mixtures or other physical properties or characteristics, are meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present disclosure.

As used herein, the phrases "gettering" and "gettering agents" and "gettering sites' means a process in which impurities in a certain region of the silicon wafer are reduced by moving and localizing them in predetermined locations (called gettering sites) from where they can be removed or neutralized. Typical gettering agents include, but are not limited to,

phosphorous, aluminum-hydrates, sulfur-oxides and any combination thereof.

As used herein, the phrase "capacitance-voltage (CV) curves" refers to a technique where the capacitance of a dielectric material is measured using a metal-insulator-semiconductor (MIS) structure as a function of an applied voltage. The capacitance is measured by applying a direct current (DC) voltage across the structure with an alternating current (AC) signal whose frequency varies from 10 kHz to 10 MHz.

As used herein, the phrase "atomic layer deposition (ALD)" refers to a technique where films are deposited one atomic layer at each cycle allowing the control of growing layers with thicknesses of just a few nanometers.

The present disclosure is a method for purifying crystalline sapphire by using microwave processing to induce migration of impurities to the surfaces of the crystalline sapphire where they can be removed by mechanical methods (e. g., polishing) and/or wet processes (e. g., chemical etching) or gettering. In this method, the use of microwaves provides an efficient means for inducing the migration of impurities. This occurs through coupling of microwave energy to the carrier system which causes an internal electric field as well as induced local heating. These effects cause migration of impurities to surfaces from which they can subsequently be removed.

The utilization of microwaves for inducing migration of impurities within the material to surface sites can improve energy consumption in at least one of the steps of obtaining sapphire end product (e.g., substrate, laminate) and can subsequently lower the costs involved with purifying sapphire.

Broadly speaking, the present method provides a method for purifying a sample of sapphire by irradiating the sample of sapphire with microwaves emitted by a microwave generating device inducing a migration of impurities within the sample of sapphire to a surface of the sample of sapphire and removing the impurities from the surface of the sample of sapphire.

A first embodiment concerns the effect of microwaves on copper metal ions migration in sapphire layers deposited on GaAs. As test samples, sapphire films are deposited on GaAs wafers by the ALD method as discussed above. The samples were divided into smaller pieces to be submitted to microwave processing. For each sample, a thin copper layer was evaporated on top of the sapphire film forming a metal-insulator-semiconductor (MIS) structure that will be tested via C-V measurements. The microwave processing comprises exposing samples to microwave radiation for controlled times in a microwave cavity. The samples are characterized before and after microwave processing by C-V measurements across the MIS structure.

The microwave cavity of the microwave generator can be designed to select different microwave modes and the magnetron can be shaped to produce different frequencies. Considering that the penetration depth of the microwaves depends inversely on the frequency, having access to cavities with different frequencies would allow impurities at different depths inside the crystal to be reached and moved with more control. The microwave power absorbed on the surface of a pure sapphire crystal is very low because the angle (tan5) is less than 10 ^"4 at room temperature ^{[11 ~12]} , but it increases with the microwave frequency in a stronger dependency than linear It's important to consider as well that imperfections in the crystal structure such as impurities and micro- structural defects will contribute to the losses, increasing the microwave absorption. Systems built with frequencies in the range between 2GHz and 600 GHz, with power between 0.3 and 300kW would allow processes in sapphire with different wafer thicknesses or particle sizes.

The cavity can be designed with a called stirrer or scatters that would allow a higher uniformity of the microwave beam for volumetric heating.

Another option is the use of a focusing mirror to concentrate the beam in a smaller region when surface heating is needed or small areas. Also, the possibility of having the microwave beam passing more than once through the sample in order to improve the absorption could be considered in the design. In addition, cavity walls with cooling systems would help avoid overheating.

Figure 1 shows capacitance-voltage (C-V) curves of sapphire layers on semiconductor wafers: before migration of copper ions (squares) and after migration of copper ions under microwave processing (circles). Purification of copper from sapphire was investigated by studying the migration of copper through sapphire under the influence of microwaves. Capacitance-voltage (C- V) measurements were used to characterize the diffusion process. High quality sapphire films were deposited on GaAs substrates using atomic layer deposition (ALD). A copper film was evaporated on the surface of the sapphire forming a metal-insulator-semiconductor structure, known as a MIS structure. The migration of copper metal ions through the sapphire film under microwaves influence was monitored by C-V. In the study shown in Figure 1 , a sample was microwave processed for 60s, with a power of 3kW, and at a frequency of 2.45 GHz.

The capacitance is measured with the application of a DC bias plus an oscillating (AC) voltage with a frequency that can be varied. The capacitance measured at a bias of -0.2V with an AC voltage of 0.010 V as a function of frequency is shown for a sample with a sapphire layer of 22nm of thickness. The curve obtained before the microwave treatment shows the expected behavior for a MIS structure. Its capacitance reduces as the frequency increases. There is a large change in the measured capacitance at higher frequencies after microwave treatment (full circles curve) when compared to the control sample (full squares curve) before exposure to the microwaves. Even in such a short time, the microwave radiation caused a migration of copper ions through the sapphire layer.

The measured capacitance at this range of bias has the contribution of both sapphire film and semiconductor capacitances. The semiconductor capacitance originates from the process of generation-recombination of minority carriers due to the application of the AC signal. As the frequency of the AC signal increases past a threshold, the recombination-generation process will not be able to respond fast enough. As a consequence, the capacitance gets smaller. The curve measured after exposing the sample to microwaves for 60s at a power of 3kW shows the opposite behavior; as the frequency increases, the capacitance rises too. This can be explained as follows: the copper ions migrate in the film to the interface between sapphire and semiconductor forming an interfacial layer of ions. For practical purposes, this layer reduces the thickness of the insulator (sapphire) film increasing its capacitance.

The contribution from the semiconductor capacitance is also decreased due to ions at the interface. Because this effect depends on the times involved in the process of generation and recombination inside the

semiconductor (slow), and of applied voltage that is inversely proportional to the frequency of oscillation (fast); it is more pronounced at higher frequencies. Summarizing, the thickness of the insulator (i.e., sapphire) is effectively reduced by this layer of diffused ions. Since the capacitor area and dielectric constants are invariant, the effective sapphire thickness can be calculated for different applied frequencies. A calculation based on the variation of capacitance caused by the migration of copper ions gives a value of 15nm for the effective thickness of the dielectric film (sapphire) at a frequency of 10MHz. This result shows that Cu ions can migrate inside a sapphire layer under the influence of microwaves.

In another study, the effect of exposure time to microwave treatment was studied. Pieces of GaAs with sapphire film on the surface and a copper thin layer on top forming a MIS structure are positioned inside a microwave cavity, one at time, and the power is switched on for 15, 30, 45 and 60s at a power of 3kW. Samples are measured by C-V method before and after the microwave processing. Figure 2 illustrates the variation of the effective thickness due to migration of copper ions in sapphire layers deposited on GaAs as a function of the time of microwave processing. Microwaves were applied to samples with a 1 7.8 nm thick sapphire layer at a power of 3kW and for processing times of 15, 30, 45, and 60s. A copper layer was deposited on the surface of the sapphire layer to work as a contact and as a source of metallic ions. Based on the change in capacitance, the effective thickness was calculated corresponding to each processing time. In the figure, the results of these calculations are presented, showing the variation of the effective thickness as a function of the microwave exposure time, normalized to the effective thickness obtained under the lowest microwave exposure time. As the exposure time to microwaves increases, more Cu ions accumulate at the interface reducing the effective thickness of the dielectric layer (sapphire). In conclusion, Cu ions migrate through the sapphire film under the influence of microwaves. Above a certain threshold, the amount of ions that migrates is proportional to the microwaves exposure time.

The results seen in Figure 2 show that the migration of copper ions increases with the exposure time for the time range considered in an

exponential manner suggesting the existence of activation energy for the whole process to occur, likely related to the emission of copper ions from the metallic contact ^[13] .

In another embodiment of the method for purifying sapphire, a susceptor such as a silicon carbide plate is disposed beneath a sapphire sample for processing. In this case, the high microwave absorption of silicon carbide results in a significant fraction of microwave power being absorbed by the susceptor which consequently heats up. With samples to be processed disposed on the susceptor, heat is transferred to the sapphire samples by both conduction and radiation. This embodiment thus enables a thermal bias to be effected during processing, to further enhance the migration of impurities during microwave processing.

The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

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