CN102148139B - Method for eliminating residual stress of GaN epitaxial wafer by improved laser quasi-stripping - Google Patents
Method for eliminating residual stress of GaN epitaxial wafer by improved laser quasi-stripping Download PDFInfo
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Abstract
The invention discloses a method for eliminating residual stress of a GaN epitaxial wafer by improved laser quasi-stripping, which well releases lattice mismatch stress and thermal mismatch stress. According to the method, the GaN epitaxial wafer is heated, and is stripped under the condition that the energy threshold for stripping the GaN and the sapphire substrate is lower than the energy threshold for stripping the GaN and the sapphire substrate by using a nondestructive laser stripping technology at the temperature of 200-900 ℃, or the energy threshold for laser stripping is reached, but the GaN and the sapphire interface form a net-shaped separation incomplete separation state through laser interval scanning, so that the purpose of releasing the residual stress of the GaN epitaxial wafer is achieved. By using the laser pre-stripping method under the heating condition, the residual stress can be released more effectively and more largely, and the damage is reduced.
Description
Technical field
The present invention relates to the laser lift-off technique field, be specifically related to a kind of being stripped from and carry out the accurate elimination Grown GaN crystal on Sapphire Substrate of peeling off of laser under the material situation and participate in stress, and reduce the method for injury caused by laser peeling in heating.
Background technology
With GaN and InGaN, AlGaN is that III/V nitride of master is the semi-conducting material that receives much concern in recent years; Be the core component part of light-emitting diode in the semiconductor lighting, the direct band gap of its 1.9～6.2eV continuous variable, excellent physics, chemical stability; High saturated electrons mobility etc.; Etc. characteristic, make it become laser, the most preferably material of light-emitting diode or the like opto-electronic device.
Yet because the restriction of the growing technology of GaN (gallium nitride) own, large tracts of land GaN material now is grown on the Sapphire Substrate mostly.Though Grown GaN is of high quality on the Sapphire Substrate, use also the widest, because sapphire non-conductive and relatively poor thermal conduction characteristic has limited the development of GaN (third generation semi-conducting material) based semiconductor device greatly.In order to avoid this inferior position, behind the sapphire growth GaN base device, sapphire to be removed, and changed the Si of high heat conduction, high conduction, the method for substrates such as Cu has been invented.In the process that sapphire is removed, the main method of using is exactly a laser lift-off technique.
Laser lift-off technique just is to use energy less than the sapphire band gap; But LASER Light Source greater than the GaN band gap; See through the GaN layer of Sapphire Substrate irradiation sapphire～gallium nitride junction; When the GaN through here absorbed laser energy and produces high temperature, the GaN here was decomposed into gallium and nitrogen, thereby realizes the method that GaN separates with Sapphire Substrate.
The basic principle of laser lift-off technique is exactly to utilize the difference of GaN and sapphire energy gap, and sapphire/GaN GaN extinction at the interface that makes of locality decomposes, and becomes low-melting gallium and nitrogen mutually, realizes separating of GaN base epitaxial loayer and Sapphire Substrate.
The Sapphire Substrate that short wavelength's pulse laser of incident sees through high energy gap is radiated at sapphire/GaN at the interface.Energy gap (3.4eV) at this GaN is lower than photon energy, and GaN is these light of intense absorption very, and are translated into heat, makes GaN that thermal decomposition take place, shown in accompanying drawing 1.So it is the thermal decomposition process of laser irradiation heating GaN that laser lift-off obtains the process of GaN film actual.
The residual stress problems of Sapphire Substrate the growing GaN: (origin of residual stress; The harm of residual stress), be that III/V nitride of master is the semi-conducting material that receives much concern in recent years with GaN and InGaN, AlGaN, the direct band gap of its 1.9～6.2eV continuous variable; Excellent physics, chemical stability; High saturated electrons mobility or the like characteristic makes it become laser, the most preferably material of light-emitting diode or the like opto-electronic device.
Wide bandgap semiconductor such as table 1, GaN material and sapphire material lattice constant
Shown in above-mentioned table 1, because the restriction of the growing technology of GaN own, large tracts of land GaN material now is grown on the Sapphire Substrate mostly.Though Grown GaN is of high quality on the Sapphire Substrate, use also very extensively,, because Sapphire Substrate and GaN crystal are dissimilar materialss, there are a great difference in both lattice constants and thermal coefficient of expansion.
We can see from table; Lattice constant mismatch degree between GaN and the Sapphire Substrate surpasses 14%; The thermal mismatching degree differs more than one times, and big like this lattice mismatch and heat expansion difference will inevitably cause the residual mechanical stress problem of Grown GaN epitaxial wafer on Sapphire Substrate.
Lattice mismatch can make GaN in growth, introduce lattice mismatch stress; We can understand lattice mismatch stress like this; Because sapphire lattice constant is greater than the GaN crystal; Therefore, its lattice of Grown GaN crystal can stretch along with the lattice of Sapphire Substrate on Sapphire Substrate, shown in accompanying drawing 2.
Thereby so just in the GaN crystal, produced tensile stress and gathered potential energy.Lattice mismatch stress can be described by formula (1), (2):
Lattice mismatch parameter f m; And al is the lattice constant of GaN epitaxial loayer; Asub is the lattice constant of Sapphire Substrate,
and
be in x, y plane the GaN epitaxial wafer because the strain regime of lattice mismatch.
Yet according to the growth balance theory, lattice mismatch can cause misfit dislocation, and when the ECDC growth thickness surpassed critical thickness hc, mismatch stress is intact can to discharge with the form of misfit dislocation in a large number.Akasaki and Amano once measured the critical thickness of InGaN/GaN and AlGaN/GaN, The measured results show for its critical thickness of InGaN/GaN greatly about 400nm and for AlGaN/GaN greatly about 300～700nm.
Except that lattice mismatch stress, another important residual mechanical stress is a thermal mismatch stress, and thermal mismatch stress is because the GaN crystal need grow under 1000 degrees centigrade high temperature, and growth need be reduced to the room temperature taking-up after finishing again and generation.From table 1, Sapphire Substrate is because than high nearly one times of GaN crystal coefficient of thermal expansion, therefore; When 1000 degree high growth temperatures; The Sapphire Substrate lattice can be than bigger, the same reason of GaN lattice dilatation, and growth finishes when dropping to room temperature; The ratio that the Sapphire Substrate lattice shrinks also can be much larger than the GaN crystal, owing to being tightly linked of sapphire crystal and GaN crystal makes the very big thermal mismatching compression of cooling back GaN epitaxial wafer remnants that finishes of growing.
Thermal mismatch stress can use following formula (2-4), (2-5) to calculate:
（2-5）
Thermal mismatching parameter
;
is the thermal coefficient of expansion of epitaxial loayer;
is the thermal coefficient of expansion of substrate; Temperature when T2 is end; Temperature when T1 is initial; For the GaN epitaxial growth, the thermal stress of GaN gather be since 1000 the degree growth temperature to returning to the thermal stress that room temperature is saved bit by bit.
Therefore, the residual mechanical stress of Sapphire Substrate Grown GaN epitaxial wafer mainly is made up of two parts, before critical thickness, mainly is that lattice mismatch stress is taken as the leading factor, and after surpassing critical thickness, thermal mismatch stress is the remaining main stress of crystal.This is consistent with international bibliographical information; A, b, c, d are four samples that thickness increases gradually, from figure raman spectra we can be clearly seen that sample stress is along with the thickness of sample increase has a transition process from the tensile strain to the compressive strain; Its transition point is greatly about about 750nm; This is just similar with critical thickness value that forefathers obtain, before changing residual mechanical stress mainly from the contribution of lattice mismatch stress, and transformation afterwards stress mainly contribute from thermal mismatch stress.And we also can see from figure, and when GaN epitaxial wafer thickness surpassed 750nm, residual mechanical stress mainly was a compression in its face, and this also utilizes thermal mismatching formula result calculated consistent with us.
The residual stress that the GaN epitaxial wafer gathers has very significantly influence for the GaN device performance.Such as (film) electric leakage, dislocation, point defect etc.
Summary of the invention
The objective of the invention is to overcome the shortcoming of prior art; Provide a kind of effective improved laser the accurate method of eliminating GaN epitaxial wafer residual stress of peeling off; Use the accurate method of peeling off of laser can well discharge lattice mismatch stress and thermal mismatch stress; Impact to weak and be connected or during netted connection when we are suitable for the accurate method of peeling off of laser with GaN and sapphire linkage interface; Alleviated the interaction of lattice between the two effectively, made GaN crystal obtain release, thereby reduced stress away from GaN and sapphire interface.
For reaching above-mentioned purpose, a kind of improved laser standard of the present invention is peeled off the method for eliminating GaN epitaxial wafer residual stress, the technical scheme below adopting:
The accurate method of eliminating GaN epitaxial wafer residual stress of peeling off of a kind of improved laser comprises the steps:
1., before the laser standard is peeled off; At first utilize heating apparatus will place the sample stage heating of GaN epitaxial wafer, and then the superincumbent GaN epitaxial wafer of heating placement, be heated 200～900 ℃; Preferred 650～800 ℃, be at epitaxial wafer and under this temperature sample carried out standard and peel off;
2., with the solid state laser be LASER Light Source; Using girth is 3～1000 microns; And two angular distance or longest diameter are no more than 400 microns small light spot and carry out pointwise laser scanning line by line farthest; Wherein the inner Energy distribution situation of small light spot is: the spot center energy is the strongest, to around energy die down gradually; Solid state laser can be improved solid double-frequency laser light source; Its improvement has been to improve the inner laser fluctuation situation of hot spot; With the spot center is the energy peak, to around energy die down gradually, whole hot spot internal energy is Gaussian distribution or approximate Gaussian distribution;
3., be lower than and carry out standard under the energy threshold that GaN and Sapphire Substrate are peeled off and peel off; Or reach the laser lift-off energy threshold; But between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating, reach the purpose of release GaN epitaxial wafer residual stress thus; Be specially; Use is lower than the threshold energy that GaN and Sapphire Substrate are peeled off and scans accurate peeling off normally; Between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating; Or reach the laser lift-off energy threshold, but between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating.
The present invention heats the GaN epitaxial wafer; Under 200～900 ℃; Use harmless laser lift-off technique to carry out standard under the energy threshold that GaN and Sapphire Substrate are peeled off and peel off being lower than; Or reach the laser lift-off energy threshold, but between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating, reach the purpose of release GaN epitaxial wafer residual stress thus.
And use the method for carrying out the laser prestripping under the situation of heating, release residual stress that can be more effectively to a greater extent, and reduce damage.When we are heated to 800 ℃ with the GaN epitaxial wafer; Temperature during GaN growth that it is approaching; Thermal mismatch stress between GaN epitaxial wafer and the Sapphire Substrate has been eliminated minimum state at this moment; Under such state, carry out the laser standard and peel off, reduced the damage that thermal mismatch stress release is caused in the stripping process effectively, and discharged the residual stress of thermal mismatching greatly.
Description of drawings
Shown in Figure 1 is the basic principle figure of laser lift-off technique;
Shown in Figure 2 is the sketch map that the GaN crystal on the Sapphire Substrate stretches;
Shown in Figure 3 for measuring back calculated stress release through XRD with temperature variation curve;
Shown in Figure 4 for measuring back calculated stress release through XRD with the laser single-pulse energy change curve;
Fig. 5 a is depicted as original pulse hot spot energy profile;
Fig. 5 b is depicted as light spot energy distribution map of the present invention.
Embodiment
For further understanding characteristic of the present invention, technological means and the specific purposes that reached, function, resolve advantage of the present invention and spirit, by detailed description of the present invention further being understood below in conjunction with accompanying drawing and embodiment.
Its technical characterictic is; The GaN epitaxial wafer is heated; Under 200～900 ℃, use harmless laser lift-off technique to carry out standard under the energy threshold that GaN and Sapphire Substrate are peeled off and peel off being lower than, or reach the laser lift-off energy threshold; But between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating, reach the purpose of release GaN epitaxial wafer residual stress thus.
The characteristic of the harmless laser lift-off technique described in the present invention is; With the solid state laser is LASER Light Source; Using girth is 3～1000 microns; And two farthest angular distance or longest diameter be no more than 400 microns small light spot and carry out pointwise laser scanning line by line, wherein the inner Energy distribution situation of small light spot is: the spot center energy is the strongest, to around energy die down gradually.
In the prior art, the energy in the large spot is uniformly, in hot spot edge energy generation cataclysm; So cause damage easily, original pulse hot spot Energy distribution is shown in accompanying drawing 5a, and the x axle is represented hot spot length of side direction among the figure; The y axle is represented the energy size, the corresponding spot center of x axle dead-center position.In the present invention, we have changed the inner energy distribution state of hot spot, pursuit uniformity of energy no longer simply, but considered the gradual change of hot spot edge energy, Energy distribution can be shown in accompanying drawing 5b.With respect to large spot, small light spot is realized the gradual change of light spot laser energy more easily.Progressive formation (from distinguish the gradual change in the more weak district of deep energy more by force near the spot center energy) just because of the energy of having stressed the hot spot fringe region; Improved the force-bearing situation of GaN sill, thereby realized the harmless laser lift-off of small light spot at the hot spot edge.
Solid state laser used in the present invention can be improved solid double-frequency laser light source; Its improvement has been to improve the inner laser fluctuation situation of hot spot; With the spot center is the energy peak, to around energy die down gradually, whole hot spot internal energy is Gaussian distribution or approximate Gaussian distribution.
Use the harmless laser lift-off technique described in the present invention, can improve the effect of laser lift-off significantly, be close to separating of harmless realization sapphire and GaN epitaxial wafer, and perfectly reduce the interface state of GaN epitaxial wafer when growing with Sapphire Substrate.
The GaN epitaxial wafer is heated under 200～900 ℃ described in the present invention is meant; Before the laser standard is peeled off; At first utilize heating apparatus will place the sample stage heating of GaN epitaxial wafer, and then the superincumbent GaN epitaxial wafer of heating placement, be heated 200～900 ℃; Preferred 650～800 ℃, be at epitaxial wafer and under this temperature sample carried out standard and peel off.
Laser of the present invention is accurate to be peeled off and is meant to use and is lower than the threshold energy that GaN and Sapphire Substrate are peeled off and scans normally certainly and peel off; Between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating; Or reach the laser lift-off energy threshold, but between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating.
Use the accurate stripping means of laser under the heating can alleviate the residual stress between GaN epitaxial wafer and the Sapphire Substrate effectively.As indicated above, in the epitaxial process of GaN, residual stress mainly is lattice mismatch stress and thermal mismatch stress, and lattice mismatch stress mainly is that Sapphire Substrate and GaN crystal lattices size do not match and cause.Thermal mismatch stress mainly is because both coefficient of thermal expansions are different; And the GaN epitaxial wafer is at the high growth temperature more than 800 ℃, and growth finishes after the cooling, and both lattice shrinkage rates have a great difference; Therefore checking and balance of lattice each other, residual heat mismatch stress have been caused.
Use the accurate method of peeling off of laser just can well discharge lattice mismatch stress and thermal mismatch stress.Its principle is to impact to weak and be connected or during netted connection when we are suitable for the accurate method of peeling off of laser with GaN and sapphire linkage interface; Alleviated the interaction of lattice between the two effectively; Make GaN crystal obtain release, thereby reduce stress away from GaN and sapphire interface.
And use the method for carrying out the laser prestripping under the situation about heating, can more effective release residual stress to a greater extent.And reduce to damage.When we are heated to 800 ℃ with the GaN epitaxial wafer; Temperature during GaN growth that it is approaching; Thermal mismatch stress between GaN epitaxial wafer and the Sapphire Substrate has been eliminated minimum state at this moment; Under such state, carry out the laser standard and peel off, reduced the damage that thermal mismatch stress release is caused in the stripping process effectively, and discharged the residual stress of thermal mismatching greatly.
Embodiment one:
We fix laser prestripping single pulse energy, change under the situation of heating-up temperature, see to add the thermal laser prestripping to discharging the effect of stress.
2 inches epitaxial wafers are put into sample stage, and under the situation about not heating fully, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 115uj;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 650 degrees centigrade the situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 115uj;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 700 degrees centigrade of situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 115uj;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 750 degrees centigrade of situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 115uj;
The stress situation of using XRD to measure the prestripping front and back is put into sample stage with 2 inches epitaxial wafers, is heated under 800 degrees centigrade of situation, and the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 115uj;
Use XRD to measure the stress situation of prestripping front and back;
Above-mentioned sampling test finishes, and the stress that sample discharges behind the XRD test result demonstration laser lift-off rises with temperature and obviously increases, shown in accompanying drawing 3.
Embodiment two:
We fix temperature, change under the situation of pulse laser lift-off energy, see to add the thermal laser prestripping to discharging the effect of stress.
2 inches epitaxial wafers are put into sample stage, be heated under 800 degrees centigrade the situation, the sapphire back side is towards laser scanning direction;
Do not use laser scanning, after 30 minutes, sample is taken out;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 800 degrees centigrade the situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 90uj;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 800 degrees centigrade of situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 100uj;
Use XRD to measure the stress situation of prestripping front and back;
2 inches epitaxial wafers are put into sample stage, be heated under 800 degrees centigrade of situation, the sapphire back side is towards laser scanning direction;
Use the pulse laser energy sample of 120uj;
Use XRD to measure the stress situation of prestripping front and back;
Above-mentioned sampling test finishes, and the stress that sample discharges behind the XRD test result demonstration laser lift-off increases same obviously increase with single pulse energy, and is as shown in Figure 4.
The above embodiment has only expressed part execution mode of the present invention, and it describes comparatively concrete and detailed, but can not therefore be interpreted as limitation of the scope of the invention.Should be pointed out that for the person of ordinary skill of the art under the prerequisite that does not break away from the present invention's design, can also make some distortion and improvement, these all belong to protection scope of the present invention.Therefore, protection scope of the present invention should be as the criterion with accompanying claims.
Claims (3)
1. the accurate method of eliminating GaN epitaxial wafer residual stress of peeling off of improved laser is characterized in that said method comprises the steps:
1., laser is accurate peel off before, at first utilize heating apparatus will place the sample stage heating of GaN epitaxial wafer, and then the superincumbent GaN epitaxial wafer of heating placement, be heated 200～900 ℃, be at epitaxial wafer and under this temperature sample carried out standard and peel off;
2., with the solid state laser be LASER Light Source; Using girth is 3～1000 microns; And two angular distance or longest diameter are no more than 400 microns small light spot and carry out pointwise laser scanning line by line farthest; Wherein the inner Energy distribution situation of small light spot is: the spot center energy is the strongest, to around energy die down gradually;
3., be lower than and carry out standard under the energy threshold that GaN and Sapphire Substrate are peeled off and peel off; Or reach the laser lift-off energy threshold; But between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating, reach the purpose of release GaN epitaxial wafer residual stress thus.
2. the accurate method of eliminating GaN epitaxial wafer residual stress of peeling off of a kind of improved laser according to claim 1; It is characterized in that: said step solid state laser 2. can be improved solid double-frequency laser light source; It has improved the inner laser fluctuation situation of hot spot; With the spot center is the energy peak, to around energy die down gradually, whole hot spot internal energy is Gaussian distribution or approximate Gaussian distribution.
3. the accurate method of eliminating GaN epitaxial wafer residual stress of peeling off of a kind of improved laser according to claim 1; It is characterized in that: 3. said step is specially; Use is lower than the threshold energy that GaN and Sapphire Substrate are peeled off and scans accurate peeling off normally; Between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating; Or reach the laser lift-off energy threshold, but between laser or scanning make GaN and sapphire interface form netted incomplete released state of separating.
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| CN107452842A (en) * | 2017-09-15 | 2017-12-08 | 西安交通大学 | The method that vertical stratification deep ultraviolet LED laser stripping energy threshold is reduced using antireflective film |
| CN108265329A (en) * | 2018-01-22 | 2018-07-10 | 东莞市中晶半导体科技有限公司 | Fixed-point localized laser stripping device |
| CN111243977B (en) * | 2018-11-28 | 2023-01-24 | 上海微电子装备（集团）股份有限公司 | Gallium nitride and sapphire substrate stripping device and method |
| JP7600659B2 (en) * | 2020-12-11 | 2024-12-17 | 株式会社Sumco | Epitaxial wafer defect inspection method |
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