Secondly, the most generally used excimer lasers METHOD OF ANNEALING A SEMICONDUCTOR which are operated in a pulsed mode are not suitable for
completely driving hydrogen out of the film; the duraThis application is a continuation of Ser. No. tion of the laser irradiation per pulse is too short. 07/886,817, filed May 22, 1992, now abandoned. 5 Furthermore, any laser apparatus for use in the laser
annealing inevitably suffers a non-uniform laser beam BACKGROUND OF THE INVENTION output and a fluctuat}on in power output. Those attri
1. Field of the Invention butes make the hydrogen distribution non-uniform inThe present invention relates to a method for anneal- side the film upon driving hydrogen atoms out of the
big a semiconductor, in particular, to an annealing pro- 10 film. Such a film having a non-uniform hydrogen districess for obtaining a polycrystal semiconductor used in a bution therein results in a crystallized film consisting of thin film device such as an insulated gate field effect crystal grains of non-uniform grain diameter.
transistor by laser irradiation. SUMMARY OF THE INVENTION
2. Description of Prior Art
Thin films of polycrystalline silicon semiconductor 15 The present invention relates to a laser annealing
for use in a thin film device such as a thin film insulated process having overcome the aforementioned problems,
gate field effect transistor (abbreviated hereinafter as a More specifically, the present invention provides a
TFT) have been fabricated heretofore by first deposit- method for annealing a semiconductor comprising the
ing amorphous silicon films by plasma-assisted chemical steps of:
vapor deposition (CVD) or thermal CVD processes, 20 thermally annealing an amorphous semiconductor m
and then irradiating a laser beam thereto to crystallize vacuum, or inactlve atmosPhere at a temperature
the thus deposited amorphous silicon films. *** higher than a crystallization temperature of
The process of crystallizing an amorphous silicon . sa>d amorphous semiconductor; and
film by irradiating a laser beam comprises, in general, irradiating said amorphous semiconductor with a
first irradiating a low energy density laser beam to the 25 las*r ^ vacuum or inactive atmosphere after
amorphous silicon film to allow desorption of hydrogen sald thermally annealing step to crystallize said
, . . . ,. . . ,. „ , ... amorphous semiconductor,
having incorporated into the starting amorphous silicon , v . , , . , .
„. , r . ,. .. , f 1 The laser to be used in the process in general is an
film, and then irradiating a laser beam at an energy . v^-uui. ^j^.i.^It.
, „ , lL iL 6 , ., , .. . . . excimer laser, but it should be noted that the construe
density well above the threshold energy density (a mini- 3Q ^ ^ ^ ... ^ nQ ... tQ
mum energy density necessary to initiate meltmg of ^ me ^ ... ^ type Qf ^ C£m fee
si icon). _ employed in the process.
A laser beam having a sufficiently low energy should ^ ^ usgd hous semiconductor, but
be irradiated to the amorphous silicon film for the pur- nQt Hmiti Js a si]icon semiconductor. In the descri
pose of releasing the hydrogen being incorporated in 35 tion of the present invention hereinafter, however, a
the film because, if a beam of a high energy density silicon semiconductor is used for purpose of explana
corresponding to the threshold value or higher were to tjon
be irradiated, there occur two problems. One is a prob- ^ thermal sealing of the amorphous semiconlem which involves abrupt evolution of a considerable ductor m vacuum or m an mactive gas atmosphere at a amount of hydrogen from the surface of an amorphous 40 temperature not higher than the crystallization tempersilicon film upon irradiating the laser beam. Such a ature of said amorphous semiconductor is conducted phenomenon greatly impairs the smoothness of the film for tjje purpose of driving hydrogen out of the amorsurface; the resulting film cannot provide a favorable phous semiconductor. If this step of thermal annealing interface level when an insulator film is established on were t0 ^s conducted at a temperature not lower than the surface of the thus crystallized silicon film, because 45 trle crystallization temperature of the amorphous semia level develops at the interface between the silicon film conductor, crystallization would occur on the semiconand the insulator film. The other problem is the hydro- ductor, thereby making the subsequent crystallization gens present in a large amount in the amorphous silicon by iaser irradiation insufficient. Accordingly, it is an film; they not only evolve out of the surface upon irradi- important point that the thermal annealing is conducted ation of a high energy laser beam having an energy 50 at a temperature not higher than the crystallization density not lower than the threshold value, but also temperature of the semiconductor, move inside the melting silicon film with a large kinetic The thermal annealing step should be effected in energy to impede the crystallization of the silicon itself. vacuum or in an inactive gas atmosphere to avoid forAccordingly, a conventional laser annealing pro- mation of an undesirable thin film, such as an oxide film, cesses involve a so-called pre-laser annealing step 55 on the surface of the amorphous semiconductor, which comprises irradiating a low energy density laser Hydrogen can be uniformly and thoroughly driven beam to sufficiently drive out hydrogen atoms having out of the film by annealing the amorphous semiconducincorporated inside the film, followed by the irradiation tor at a temperature not higher than the crystallization of a laser beam having a satisfactorily high energy den- temperature. The semiconductor films thus obtained sity to effect crystallization of the film. In this manner, 60 have improved uniformity for both the intra-planar the influence of the hydrogen inside the film on the film distribution of crystallinity and the size of the constitutcrystallization can be eliminated. ing crystal grains. Such semiconductor films enable The conventional laser annealing processes, how- fabrication of polycrystalline silicon (abbreviated someever, suffer disadvantages as follows. times as "p-Si", hereinafter) TFTs having uniform charFirstly, the laser annealing process should be con- 65 acteristics over a large-area substrate, ducted in two steps. Such a process is not suitable for The crystallization of the amorphous semiconductor processing large-area substrates. Moreover, it suffers in vacuum or in an inactive gas atmosphere by irradiatpoor efficiency. ing a laser beam thereto is conducted to prevent the