Abstract:
The present disclosure provides methods for treating a part made from a decomposable semiconductor material, and particularly, methods for detaching a surface film from the rest of such part. According to the provided methods, a burst or pulse of light particles of short duration and very high intensity is applied to the part in order to selectively heat, under substantially adiabatic conditions, an area of the part located at a predefined depth from the surface to a temperature higher than the decomposition temperature of the material, and subsequently a surface film is detached from the rest of the part at the heated area. In preferred embodiments, the decomposable semiconductor material comprises Ga, or comprises Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1 and x+y≦1.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(e) to French Patent Application Serial No. FR 1055002, filed Jun. 23, 2010. 
     FIELD OF THE INVENTION 
     The present invention relates to methods for treating parts made from decomposable semiconductor materials, and particularly, to methods for detaching surface films from the rest of the part, and even more particularly, where the material has a composition Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1 and x+y≦1. 
     BACKGROUND OF THE INVENTION 
     Semiconductors of the III-N type, that is, having the general formula Al x Ga y In 1-x-y N where 0≦x≦1, 0≦y≦1 and x+y≦1, and, in particular, GaN, have features that make them highly attractive for the optoelectronic, power and radiofrequency applications. However, the development of these applications is hampered by technico-economic limits of Al x Ga y In 1-x-y N substrates, which are not readily available. 
     III-N devices are, therefore, commonly fabricated on a substrate formed by the transfer to a support substrate of a thin film taken from a donor substrate of high-grade III-N material suitable for the intended application. The poor availability of Al x Ga y In 1-x-y N substrates is thus offset by the possibility of using a donor substrate repeatedly. 
     The S MART C UT ® process is a well-known transfer technique, whereby, in general, a dose of atomic or ionic species is implanted in a donor substrate in order to create an embrittlement area therein at a predefined depth that bounds a thin surface film to be transferred. Then, the donor substrate is bonded to a support substrate, or recipient substrate, and is fractured at the embrittlement area, thereby allowing detachment of the thin surface film now bonded to the recipient substrate. 
     However, because fracturing substrates made from III-N material requires doses of atomic or ionic species, which are five to ten times higher than in silicon, such processes typically have substantially higher costs than similar processes in silicon. 
     It can be appreciated, therefore, that it would be advantageous to develop methods for detaching thin films from substrates made from III-N material that overcome these limitations of available processes, for example, being less costly. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention provides processes for treating parts comprising a thermally decomposable material, which includes the steps of:
         application of a pulse of particle flux on a surface of the part, the duration and intensity of the pulsed flux being selected in order to selectively heat an area of the part located at a predefined depth from the surface under substantially adiabatic conditions to a temperature higher than the decomposition temperature of the material; and   detaching a surface film from the rest of the part at the heated area.       

     According to other features of this method:
         the thermally decomposable material includes Ga, and preferably includes a III-N compound such as Al x Ga y In 1-x-y N where 0≦x≦1, 0≦y≦1 and x+y≦1;   the duration and intensity of the particle flux are selected so that the half-thickness of the energy deposition profile is less than the thermal diffusion length during the pulse duration, where the thermal diffusion length is preferably approximated as the square root of the pulse duration times the thermal diffusivity;   the pulse of particle flux has a duration between 10 nanoseconds and 10 microseconds and/or an intensity between 1×10 6  and 5×10 7  W/cm 2 ;   the part is at an initial temperature that is lower than the decomposition temperature of the material at the time of application of the particle flux, for example, at a temperature between 400° C. and 750° C.;   the flux includes light particles having an atomic number between about 1 and 3 and/or H or He in a neutral or an ionic form;   the particles are applied to a dose of between about 1×10 12  and 5×10 13  particles/cm 2 ;   the heated area is returned to an ambient temperature prior to detaching the film;   the part is assembled with a support in order to obtain a composite structure after applying the pulse of particle flux and prior to detaching a film; and   a mechanical or thermal stress or a combination thereof is applied to the composite structure after assembling in order to cause the detachment of a film from the rest of the part.       

     Additional features and advantages of the present invention will be described with reference to the drawings. In the description, reference is made to the accompanying figures that are meant to illustrate preferred embodiments of the invention. It is understood that such embodiments do not represent the full scope of the invention. Further aspects and details, and alternative combinations of the elements, of this invention that will be apparent from the following detailed description to one of ordinary skill in the art are also understood to be within the scope of this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be understood more fully by reference to the following detailed description of the preferred embodiment of the present invention, illustrative examples of specific embodiments of the invention and the appended figures in which: 
         FIG. 1  illustrates a particle flux being applied to a part or substrate; 
         FIG. 2  illustrates assembly of the part on a support; and 
         FIG. 3  illustrates an exemplary profile of energy deposited by the particle flux as a function of depth. 
     
    
    
     DETAILED DESCRIPTION  
     The present invention applies to parts made from compound semiconductor materials that are liable to decompose under the effect of an energy input, especially an energy input by a particle flux. 
     The term “compound material” is used herein to refer to material consisting of at least two types of atoms. Typically, these atoms are Group III atoms, e.g., gallium, aluminum, indium, etc., and the most preferred compound material has the composition Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1 and x+y≦1. This material is typically formed by deposition on a growth support made from, e.g., sapphire. 
     The term “decomposition” is used herein to refer to a modification of the inherent structure of the material, which may, depending on the material, consist of a chemical transformation, a phase separation, an embrittlement, the creation of new phases, a localized fusion, etc. 
     The term “part” is used herein to refer not only to compound materials in the form of a wafer or other substrate that can be used in the electronic, optical or optoelectronic industry, but also to compound materials in other forms, e.g., an ingot, a ball, or any other shape, having at least one surface across which the particle flux can be applied. Preferably, the roughness amplitude of such a surface is significantly lower than the penetration depth of the particles. 
     In the non-limiting, preferred embodiment described below with reference to  FIGS. 1 and 2 , the part  1  to be treated is considered to be a substrate made from a material having the preferred as Al x Ga y In 1-x-y N where 0≦x≦1, 0≦y≦1 and x+y≦1. However, as stated above, the invention should be understood to also apply to parts present in forms other than substrates (for electronic or similar use) and to decomposable materials of other compositions. 
     According to the present invention, the part is treated by applying a particle flux  3  in the form of a burst of short duration and very high-intensity on the substrate  1 . Such a short, high-intensity particle flux is also referred to herein as a “pulse.” The pulse duration is preferably between about 10 nanoseconds and a few microseconds, for example, 10 ns. The power flux density is preferably between about 1×10 6  and a few 10 7  W/cm 2 . 
     Preferably, light particles having a low atomic number are selected, for example, having an atomic number of one, two or three, so that the energy deposited inside the material is, for the most part, extracted from the particle pulse by braking due to electron interactions and not by atomic collisions. The latter are undesirable because they might generate defects. Preferably, the particles  3  are selected from light, ionic (positively or negatively charged) or neutral particles, for example: H, H+, He, He+, their isotopes, H−, or electrons (either alone or combined together). 
     With this choice of the type of particles, and as shown in the graph in  FIG. 3 , the shape of the energy deposition profile generally displays an amplitude increasing from the surface of the substrate (depth: 0), toward a peak P at a depth approaching the target thickness. This peak is commensurately narrower, in general, as the particles have a lower atomic number and/or a higher energy. 
     If the flux burst is of sufficiently short duration, e.g., being in the above-mentioned duration range, the energy deposition can be considered as adiabatic, that is, practically without any heat transfer elsewhere than in the target area. In the absence of energy transfer from the target area, the temperature elevation profile generated by the flux in the substrate is similar to the energy deposition profile. 
     The adiabaticity criterion can be estimated rapidly by comparing the half-thickness of the energy deposition profile with the thermal diffusion length during the pulse duration, where the thermal diffusion length can be approximated as the square root of the pulse duration times the thermal diffusivity. If the half-thickness of the energy deposition profile is less than the thermal diffusion length during the pulse duration, then the adiabaticity criterion can be considered to be satisfied. 
     Consider an example where the half-thickness is 1 μm of the energy deposition profile, where the pulse duration is 400 ns, and where the thermal diffusivity is 0.1 cm 2 /s. then the thermal diffusion length is:
 
√{square root over (100·10 −9 ·0.1)}=10 −4  cm=
 
√{square root over (400·10 −9 ·0.1)}=2·10  −4  cm=2 μm
 
Since 1 μm&lt;2 μm, the adiabaticity criterion can be considered to be satisfied for this example.
 
     However, if the particle flux is not applied under adiabatic conditions, the temperature profile resulting from the energy deposition by the particles is less peaked and more spread out than the energy deposition profile. 
     It may be possible to reduce the particle flux density if the substrate is pre-heated prior to implantation to an initial temperature (also referred to as the “bottom temperature”), which is lower than the temperature at which the material starts to decompose. With such pre-heating, the ion beam is only required to provide the additional energy in the vicinity of the targeted area that is sufficient to raise the temperature from the initial temperature to at least the decomposition temperature. 
     It may also be possible to reduce the particle flux density where the profile of the energy peak stands out clearly, that is, typically with a ratio higher than 2 between the height of this peak and the surface energy level. This is typically the case with high-energy implantations. 
     The particle treatment can be carried out using machines producing particle beams that have a temporal steady-state intensity, that are highly focused, and that can be scanned at high speed on the surface of the substrate. With such beams, any one location experiences a pulse of particles depending on the beam size and scanning speed. Alternatively, particle beams can be used that are spatially uniform across the surface of the part but temporally pulsed. 
     Preferred equipment includes continuous focused beam machines and very high-power pulsed beam machines. 
     In further preferred embodiments illustrated in  FIG. 2 , a part  1  after such particle treatment can then be assembled on support substrate  5 , which acts as a stiffener, and then a mechanical and/or thermal stress can be applied to this structure at the embrittled area  2 . In this manner, a film  4  (see  FIG. 1 ) can be transferred to the support substrate  5  by detachment at the embrittled area  2 . Generally, only a low thermal budget suffices to cause the detachment of the film  4  from the rest of the support  1 . 
     The methods of this invention find particular application when the material contains gallium, which has the particular feature of having a melting point lower than 50° C. In fact, after the application of a stiffener, a film can be detached after the heated area returns to ambient temperature, because the gallium liquid phase thereby created allows such a detachment, by the application of small mechanical forces and/or a slight heat treatment. 
     For example, suitable conditions for the bombardment of a GaN substrate by H+ ions are:
         initial temperature: about 750° C.,   particle dose: about 2×10 12  particles/cm 2 ,   particle energy: about 1.5 MeV,   pulse duration: about 100 ns,   energy deposition density: about 0.5 J/cm 2 , and   ion current density: about 3.5 A/cm 2 .       

     In general, these conditions lead to an implanted dose during the treatment that is typically between a few 10 12  and a few 10 13  particles/cm −2 . It should be noted that this dose is much lower than that used to embrittle a silicon substrate for the application of a S MART C UT ® process, in which the usual dose is higher than 10 16  particles/cm −2 . 
     The word “about” (and other words of approximation or degree) are used herein to mean within acceptable and expected limits, usually commercially acceptable limits and ranges. The limits and ranges signified by these terms depend on commercial requirements (or research requirements, or the like) and can vary, but in all cases are not to be construed as imposing requirements beyond what are currently achievable given a current state-of-the-art. 
     The preferred embodiments of the invention described above do not limit the scope of the invention, since these embodiments are illustrations of several preferred aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention, in addition to those shown and described herein, such as alternative useful combinations of the elements described, will become apparent to those skilled in the art from the subsequent description. Such modifications are also intended to fall within the scope of the appended claims. Headings and legends are used herein for clarity and convenience only.