JPH01123000A - Crystal reinforcing method and apparatus - Google Patents

Abstract

PURPOSE: To strengthen crystals so as to make the crystals useful for a laser medium by applying an epitaxial layer material, which generates strains in a specific range when the material is applied to the crystal to this crystal.

CONSTITUTION: The crystal to be strengthened is prepd. and the epitaxial layer material which generates the strains of 0.01 to 0.3% when applied to this crystal is selected. The selected epitaxial layer is applied on the crystal. The basic requirements are that the inter-lattice plane spacings of the epitaxial crystal material are slightly larger than the inter-lattice plane spacings of the substrate crystal to be strengthened. The one adequate method for achieving such things is to dope the epitaxial material by a doping agent having the inter-lattice plane spacings larger than those of the unsubstd. crystal. The strains on the substrate surface are most adequately in a range of 0.01 to 0.3% and about 0.1% is optimum.

COPYRIGHT: (C)1989,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to the properties of crystals, and more particularly to methods and apparatus for strengthening crystals, particularly laser crystals.

Single crystals are used in a variety of applications, such as in the electronic and optical industries. In optical technology, single crystals are used as laser media, laser amplifiers, harmonic conversion, and other applications. A well-known example of a laser medium single crystal is YAG
, Gadolinium Gallium Garnets 1~ (GGG),
Gadolinium Scandium Garnet (GSG)
G).

Currently, much of the research in lasers is to further develop high average power lasers. Such a laser must not only have high energy in a given laser pulse, but also a high repetition rate of those pulses. So far, laser media have mainly been of two geometries.

That is, there are rods for relatively small sizes and disks for relatively large sizes. The rods were crystalline or glass, and the larger ones were made from glass.

Due to its low thermal conductivity, glass cannot quickly remove the heat generated during laser operation. Most crystals are also inherently stronger than glasses and often have other desirable spectroscopic properties. Therefore, crystals are suitable materials as new laser media.

High-average cool-state design to minimize harmful optical distortion and maximize surface area available for cooling.
For lasers, the slab geometry is superior to the traditional rod geometry. The slab has a large surface area to remove heat from sides of the amplifier medium that are not in the laser beam, and can be formed from crystalline material. Current designs for high-average-power slab lasers use single-crystal garnet as shown in Figure 1.
It has the geometry of a rectangular slab with dimensions of the order of XlX10X20.

By using single crystal laser media in high average power lasers, superior beam quality and high power can be obtained. The fundamental constraints on power output are thermal conductivity as well as component strength. High-power devices require high tensile strength and good durability of component parts. Most large solid-state laser media of the prior art were typically made of glass, and smaller components were fabricated in YAG or glass rod geometries. When strength problems arise in glass ray media,
A method of increasing strength and abrasion resistance has been discovered, published by J.E.
, HeriOn “°Development of high-intensity solid-state laser materials”, UCRL-93160Abst, Su
Advances in L
a5er 5science, Engineering 1 Editor W, C, St
awlley and u, tapp, Amer, In5
t,ofPhys,Proceedings,
14B, pp 234-237 (198B), compressive ion beam sputtered films have been deposited onto glass substrates. Attempts to extend compressible layer technology to single crystals
.

11 Ioki et al., “Strengthening of A-203 by Ion Implantation”, Journal of Mat
erials 5science Letters,
3. Ion bombardment has previously been used, as reported in pp 1099-1101 (1984).

Since new crystalline laser media are expected to operate at relatively high average powers, the laser media will be under greater applied stress than ever before. These stresses result from temperature gradients within the slab. Absorption of photovoltaic or other bombing energy causes heating of the entire slab. By intensely cooling the slab surface with high-speed fluid, the thermal gradient within the slab is brought to a steady state. Thermal gradients create biaxial tensile stresses on the slab surface, the magnitude of which can necessarily approach or exceed the strength of the component.

Therefore, by strengthening the single crystal laser medium,
It is possible to achieve 1q higher average output. Until now, one approach to reinforcement has been to
e) to eliminate or minimize the damage. In the past, it has been shown that subsurface damage can be removed by acid etching methods, as well as by grinding and polishing manufacturing steps to remove residual material. This method is described in J. E., HeriOn, “Strengthening of Solid-State Laser Materials”, Appl. Phys.
Lett, 47.

pl) 694-696 (1985),
It definitely helps increase the crystal strength by 15 times. However, this method has hitherto been difficult to implement satisfactorily because it has not been possible to preserve the initial surface throughout the handling, installation, and use of the laser.

Therefore, there is a long-standing, unmet need for enhancement of single crystal laser media. If this need is met, lasers with significantly higher average powers are possible.

SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a method and apparatus for strengthening crystals.

Yet another object of the present invention is to provide a method and apparatus for strengthening crystals to make them useful in laser media.

It is a further object of the present invention to increase the durability of a reinforced laser medium so that it remains hardened during use.

The above object is achieved according to the invention by applying a monocrystalline epitaxial layer to the surface of the single crystal to be strengthened, by selecting the material of the epitaxial layer in such a way that the surface is under compressive force. achieved.

Currently, epitaxial layers are
It is applied to crystals for e memory) and cathode ray tubes. Although unstrained layers are preferred for these applications, the present invention uses intentionally highly strained epitaxial layers to specifically induce compressive forces and strengthen the crystal.

<Description of Preferred Practical IM Embodiments> When a crystalline material is selected to be grown on the surface of a crystal in order to place the crystal in a compressed state according to the present invention, the lattice spacing of these crystals is Use relative size mismatch.

The basic requirement is that the lattice spacing of the epitaxial crystal material is slightly larger than the lattice spacing of the substrate crystal to be strengthened. One preferred way to accomplish this is to dope the epitaxial material with a dopant that has a larger lattice spacing than in the unsubstituted crystal. The epitaxial material can be the same or similar material in that regard to the substrate. The strain on the substrate surface is between 0.01% and 0.3%.
% range was found to be best, with approximately 0.1% being optimal. Strain is defined as the characteristic length of the corresponding epitaxial layer minus the characteristic length of the substrate divided by the characteristic length of the substrate. The characteristic length can be the average crystal lattice spacing.

The following is a description of epitaxial layers grown in accordance with the present invention and used to provide a detailed description of the present invention. The substrate was a 111-oriented GGG wafer fabricated with a diameter of 1 inch and a thickness of 0.020 inches, and both surfaces were polished to reduce surface damage and a final colloidal silica polish was applied. These are bubbles and bubbles in the electronics industry.
It was the same type of substrate used for epitaxial growth of iron garnet for memory devices. These wafers have small orientation flats in the 112 and 110 directions, less than 5 dislocations over the central 80% area, and 'Ic over the central 80% area.
Specified as having less than 2 defects per I2, a flatness of 6 or more fringes using green light (546 nm) over the central 80% area, and a taper of less than 0.0015 inch across the diameter. It was done.

Although tests have so far been performed on these substrates, this application can be performed on slab surfaces of laser media as well.

Epitaxial growth was performed in a class 10o clean hood. The substrate was thoroughly cleaned with a mild alkaline cleaning solution and rinsed with ionized water before growth. Rotate the board held horizontally (20Orpm)
A supercooled lead oxide flux (see Table 1) was used for liquid phase epitaxial growth using a ) isothermal immersion technique.

For more details on this well-known technique, see Il, J.
Lcvinstcin et al., Appl, Phys, L
ett, 19, 486 (1971).

Neodymium-substituted GGG to induce compressive surface stress
was grown on a pure GGG substrate. Since neodymium has a larger ionic radius than gadolinium, introducing Nd at the position of Gd in garnet gives it a larger lattice constant. With the molten composition (Table 1), a growth rate of about 1 micron per minute could be obtained when cooled by 15°C relative to the growth temperature of 895°C. A layer of approximately equal thickness is grown on each side of the substrate. Epitaxial growth under these conditions resulted in a strain mismatch of approximately 0.061% between the epitaxial layer and the substrate, giving the desired highly compressible surface of the stressed 200) iPa.

The characterization of thickness and strain is given as follows.

The thickness of the layer is 7.06°, assuming that an equal thickness layer is grown on each side of the substrate due to the weight increase of the growth layer.
Determined using a density of /cc. Experience with valve memory layers shows that the thickness variation from side to side is minimal (approximately 5%)
It was shown that Measurement of lattice constants of substrates and layers,
BOnd diffractometer (W, L, Bond, ACta
crystal, 13°814 (1960): A
31, 698 (1975)). CuK sub-alpha radiation was used to excite the (888) reflection and the diffraction pattern of the substrate was superimposed on the pattern of the epitaxial layer. Since diffraction measurements are affected by strain induced during growth, these measured lattice constants were corrected for strain using the following equation: where (da/a) was undistorted Nonconformance, (ds/S'
) is the measured mismatch and the Poisson's ratio V is assumed to be 0130, a typical value for gallium garnet.

Equi-biaxial tension along the bottom surface
The substrates were destroyed in a ball-on-disk-on three-ball jig giving a 3-ball jig.

As a control sample, an untreated specimen from the same polishing lot was also destroyed. For this destruction jig, J, B, Wac
Htman, Jr. et al., “Biaxial Bending Test of Ceramic Substrates”, J. Of HaterialS, 7.
Stress is related to load by the relationship given by Dp 188-194 (1972). All samples were broken at a loading rate of 0.1 mm/min.

Two techniques were used to evaluate the wear resistance and in-use flaw development of the epitaxial layer. Damage due to careless cleaning or handling was modeled by abrading the substrate using 6 micron diamond paste applied on a cloth pad with ethylene glycol.

Each sample for this treatment was placed on a pad and
A mass of KCI was loaded and the pad was routed 10 times along the ring (6 cm radius). The sample was then thoroughly washed with ethyl alcohol. This process models damage caused by particularly careless cleaning of laser components or by wear due to particles in the coolant.

Another area where second model wear techniques have been developed is
This is a simulation of damage that typically occurs from laser operation. It has been found that dirt on the slab surface sometimes absorbs enough light that the dirt evaporates explosively, creating small surface pores. To model this type of damage,
A Vickers microhardness indentation at a 2N load is used, which is sufficient to generate radial cracks from the corners of the indentation and cause surface defects similar to damage holes in the specimen. It's going up. In the test, the substrate was indented in its center with a single indentation with a 2N load, and the indentation was caused to fail under tension on the curved surface.

The results of the above tests are shown in Tables 2 and 3. The thickness of the epitaxial films and their compressive stress measurement results are shown in Table 2. The results of the destructive test are the third
It is shown in the table and graphically in FIG. The well-polished bare substrate was found to be very strong (average strength = 3010 HPa'). This is likely due to the characteristics of the advanced polishing process required to produce a sufficiently low surface damage to grow a dislocation-free epitaxial layer on a good quality bubble memory substrate. Growth of the highly strained epitaxial layer reduced the strength of the substrate to 2040) IPa. Increased tension just below the epitaxial layer/substrate interface, induced by compressive stress in the layer, is responsible for this strength reduction.

However, it is noteworthy that this intensity value (204
0 HPa) is approximately five times greater than the strength required for the laser medium in current designs.

The results in Table 3 for wear clearly demonstrate the advantages of the present invention. Wear due to 6 micron diamond and 2N
Indentation with a Vickers microhardness diamond indenter under load did not substantially reduce the strength of the epitaxial layer substrate. The abrasion process weakens the bare substrate to a point where it cannot be used with current laser designs.

Below are the results of epitaxial layer deposition for GGG reinforcement shown in Table 1.

Table 1 Composition of (Nd, Gd)3Ga5o12Fa solution Oxide
Heavy! (g) mole mole fraction P
bO466,2002,088g 0.89
83B2039.322 0.1339
0.0576Ga20313.296 0.0
709 0.0305Gd2038.000
0.221 0.0095Nd2033
．． 182 0.0095 0.0041
Total 500.000 2.325
2 1.0000 (The growth temperature was 895°C. However, the growth of these particular films can occur over a range of 815 to 915°C.) Table 2 (Nd, Nd on Gd3Ga5O12) Gd)3Ga50,2
Properties of compressed layer 1 1.24099 1.23979
39.7 5.3 0.1132 +, 24
099 1.23979 33.9 4.5
0.1133 L24099 1.23979
31.7 4.2 0.1134 1.24099
1.23979 43.7 5.8 0.1
135 +, 24109 1.23985 42
．． 6 5.7 0.1176 1.24109
1.23985 37.6 5.0 G, 11
77 1.24+09 1.23985 33.2
4.4 0. +178 +, 24120 1
．． 23991 49.3 6.6 0.1229
+, 24120 1.23991 43.4
5.8 0.12210 124131 1.2
3997 35.9 4.4 0.12711
1.24077 1.23968 463 3
9.6 0.10312 1.24067 1.2
3962 334 28. ! IO,099
131,240451,2395023,03,10,
0891/I 1.24099 1.23980
42.8 5.7 0.11315 1.240
24 1.23939 28.9 3.9 0
．． 08016 1.24067 1.23962
32.5 4.3 0.09917 12397+
1.239N 37.1 4.9 0.
The thickness of 061a and W4 is calculated from the density of 7.068g/-. Correction of distortion to the lattice constant was performed based on Poisson's ratio of 0.30.

bJ has 102 triangular defects per d.

No defects are visible in C1FIJ.

d. These layers were grown on a 6 micron diamond worn substrate. Except for #14,
The diffraction peaks are all very broad, resulting in “facets” (
raced) ” indicates growth.

The layer also showed a frosty white appearance of a facet growth layer. This must be the result of abrasion of the substrate with 6 micron diamond. This is because the strains grown in these layers are too low for spontaneous facet growth, and the test layers on standard highly polished substrates showed no facet growth. It is.

Table 3 Strength polishing of substrate None None None
6 3010 Lab W! 5 0.1
None 6 204G polishing
None None 6 micron wear 2560
Polish @ 50.16 micron polish 2
1745 Polishing None None
2N push 2157 grind @ 5
0.1 2N push 2 161
06 micron polishing None None None
43106 Micron Lab 9 5 0.
1 None 43906 micron polishing None None 2N indentation 1102
6 micron polishing 5 0.1 2N indentation 1171 Table 4 (Ca, Sn, Y) 3Ga501. , (D composition oxide mole fraction mole grams Pb
OO,807g2 2.58847 57
7.71811Ga OO,042850,1373
225,73885SnO20,004290,013
732,06926Ca OO,004290,013
73-0,77009B i OO,134640,
43141201,01947 total
3.20427668 811.74
551013 (growth rate is approximately 111/min at 925°C
It was planned that a strain of 0.1% strain would be generated on GGG. Growth can occur at temperatures ranging from 905 to 945°C. ) Table 5 (Composition oxide of Y3 (Sc, A Emperor) 501□ Mole fraction Mol Gram PbOO,9
02822,15423480,80200Ai203
0.01703 0.04065 4.144
37SC2030,000430,001G2 0
．． 14014B2030.07524 0.179
52 12.49820 total
2.38611000 500.00
000000 (planned to have a growth rate of about 1.5 microns/win at 1070°C and to produce a layer with 0.1% strain on YAG. Growth was performed at 1050 to 1090°C.
can occur within a range of )

[Brief explanation of the drawing]

FIG. 1 is an illustration of a "zigzag" slab amplifier geometry, where the slab is pumped through a large surface and the extracted beam zigzags through the slab by total internal reflection. FIG. 2 is a graph showing the strength of GGG substrates with and without a suppressive epitaxial layer as a function of surface abrasion treatment.

Claims (1)

[Claims] 1. When a crystal to be strengthened is prepared and applied to the crystal, 0
．． A method of strengthening a crystal comprising selecting an epitaxial layer material that produces a strain of between 0.01 and 0.3% and applying said epitaxial layer to said crystal. 2. A method according to claim 1, wherein the distortion produced is substantially 0.1%. 3. The method according to claim 1, wherein the crystal is YAG. 4. The method according to claim 1, wherein the crystal is GGG. 5. The method according to claim 1, wherein the crystal is GSGG. 6. A method according to claim 1, wherein the epitaxial layer is applied in a lead oxide flux. 7. A method according to claim 1, in which the epitaxial layer is applied by means of a molecular beam. 8. A crystal having a surface, and the crystal having a surface of 0.01 to 0.
A crystal strengthening device comprising an epitaxial layer crystal on the surface formed of an epitaxial layer material selected to be placed in a state of 5% strain. 9. Claim 8 wherein the strain is essentially 0.1%
Device according to section.