Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-08-15
2003-05-06
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S486000, C438S487000, C438S166000, C438S158000, C438S149000, C438S125000, C117S044000, C117S092000, C219S121690, C355S067000, C355S053000, C359S622000, C372S057000, C372S025000, C427S008000
Reexamination Certificate
active
06558991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to annealing of a semiconductor thin film irradiation with laser light.
2. Description of the Related Art
Extensive studies have been made of laser annealing techniques in which an amorphous or crystalline semiconductor thin film formed on a substrate is illuminated with laser light to crystallize or improve the crystallinity of the semiconductor thin film.
In particular, by using a laser light having a line-shaped cross section (linear laser light, hereinafter), annealing can be performed uniformly with high productivity.
A description will be made of linear laser light. Usually, laser light as output from a laser light source has a spot-like cross-section of a several centimeter square. This laser light is expanded and made uniform by a homogenizer or a bean expander, and then converged by a cylindrical lens. Thus, linear laser light is obtained which has a size of several millimeters by several tens of centimeters on an irradiation surface.
By inserting a slit between the cylindrical lens and the irradiation surface, the energy density profile of the linear laser light in its line-width direction on the irradiation surface is rendered rectangular, to enable more uniform annealing.
The laser light source is a pulsed laser having large output power, such as an excimer laser. After a crystalline silicon film is annealed with laser light of a low energy density, annealing is again performed with laser light of a high energy density.
This procedure can not only provide larger crystal grain diameters but also reduce the degree of roughening of a film as compared to the annealing only with laser light of a high energy density.
That is, in crystallizing or improving the crystallinity of a thin film of a semiconductor such as silicon by using laser light, it is important in terms of reduction in the degree of film roughening to irradiate the thin film first with laser light of a low energy density and then with laser light of a higher energy density.
However, such multiple-stage laser irradiation has a problem that the annealing process takes loner time than in single-stage laser irradiation.
One method for solving this problem is to make the energy density profile of linear laser light in its line-width direction trapezoidal or Gaussian.
A trapezoidal energy density profile can be obtained by controlling the distance between the final-stage cylindrical lens (focusing lens) and the irradiation surface in an optical system for forming laser light having a rectangular energy density profile which system has a slit.
A Gaussian energy density profile can be obtained by removing a slit or widening the slit in the line-width direction of linear laser light in an optical system for forming laser light having a trapezoidal energy density profile.
If the irradiation surface is irradiated with linear laser light having such a trapezoidal energy density profile while being scanned with it, laser light of a low energy density is applied to the irradiation surface first, and thereafter laser light of an increasingly high energy density is applied.
Therefore, a coating is not irradiated suddenly with laser light of a high energy density unlike the case of using laser light having a rectangular energy density profile. Thus, the coating can be crystallized satisfactorily by single scanning on the entire irradiation surface. Almost the same results are obtained also in the case of using laser light having a Gaussian energy density profile.
When laser annealing is performed by using laser light having a trapezoidal or Gaussian energy density profile, the inventors of the present invention discovered that an annealed semiconductor film is occasionally roughened in a strip-like manner; that is, a plurality of stripes are formed in the longitudinal direction of the linear laser light.
One reason of this stripe-like film roughening is that depending on the energy density profile of laser light, different locations on the irradiation surface may not be irradiated with laser light of the same energy density. This will be explained below with reference to
FIGS. 6A-6D
, with respect to a case of crystallizing an amorphous silicon film by laser annealing.
FIGS. 6A-6D
show a scanning process with linear laser light having a trapezoidal energy density profile. More specifically,
FIGS. 6A-6D
show a case where linear laser light having a trapezoidal energy density profile which is emitted from a pulsed laser light source such as an excimer laser is moved in the scanning direction by a pitch D for each shot of irradiation. In
FIGS. 6A-6D
, characters &agr;′ and &bgr;′ indicate positions on the irradiation surface and E
1
and E
2
represent energy density levels (E
1
<E
2
).
In a first shot, as shown in
FIG. 6A
, a laser light having a trapezoidal energy density profile is applied to a portion near position &agr;′.
Next, in a second shot, the laser light that is moved by the pitch D is applied as shown in FIG.
6
B. The energy density of laser light at position &agr;′ is E
1
. As a result, an amorphous silicon film in the vicinity of position &agr;′ is crystallized well.
It is assumed that an amorphous silicon film is crystallized well at the energy density E
1
, and that the crystallinity of a silicon film obtained by irradiation at the energy density E
2
is improved properly.
In the second shot, on the other hand, almost no laser light is applied to a portion in the vicinity of position &bgr;′. (Although actually there is a portion in the vicinity of position &bgr;′ which is irradiated at a low energy density, no change in film quality occurs there because the energy density is much lower than E
1
.)
In a third shot, laser light that is further moved by the pitch D is applied as shown in FIG.
6
C. The energy density of laser light at position &agr;′ is E
2
. As a result, the crystallinity of a crystalline silicon film in the vicinity of position &agr;′ is improved.
However, a portion in the vicinity of position &bgr;′ is suddenly irradiated at an energy density higher than E
1
. As a result, an amorphous silicon film in the vicinity of position &bgr;′ is crystallized but roughened to a large extent.
In a fourth shot, as shown in
FIG. 6D
, The energy density of laser light at position &agr;′ is again E
2
.
In laser crystallization of a silicon film, the film quality obtained at a certain position of the silicon film is greatly influenced by laser light that is applied there first and has a sufficiently high energy density to change the film quality.
In other words, second application onward to a certain position of laser light whose energy density is approximately the same as or lower than in first application to the same position is not important to the resulting film quality at that position.
Therefore, second application to position &agr;′ of laser light having the energy density E
2
does not much affect the film quality at that position.
On the other hand, at the time of the fourth shot, the film is already roughened at position &bgr;′. Although the crystallinity is improved by the irradiation at the high energy density E
2
, the resulting film quality at this position becomes different than at other positions.
Thus, the intrasurface uniformity in film quality is impaired, resulting in a problem that a plurality of semiconductor devices formed on the same substrate by using the above silicon film have different characteristics.
In addition, there is a problem of output power variation of a laser light source. A pulsed laser light source (oscillator) produces laser beams of a given energy density at a predetermined frequency (i.e., pulse interval). However, there may occur an event that the output power of the laser light source suddenly decreases. Usually, reduction in output power occurs at a rate of several shots per tens to hundreds of shots. It is rare that output power reduction occurs consecutively.
FIGS. 7A-7D
show a scann
Kusumoto Naoto
Tanaka Koichiro
Yamazaki Shunpei
Elms Richard
Fish & Richardson P.C.
Luhrs Michael K.
Semiconductor Energy Laboratory Co,. Ltd.
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