Method and system for manufacturing semiconductor device

A method and system for manufacturing a semiconductor device having a semiconductor layer using a pulsed laser includes the steps of generating a laser beam using a solid laser source, generating a multi-harmonic wave from the laser beam using a multi-harmonic oscillator, filtering the multi-harmonic wave, and irradiating the filtered wave onto the semiconductor layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of manufacturing a semiconductor 
device, and more particularly, to a method of manufacturing a 
semiconductor device through crystallization and activation. 
2. Discussion of the Related Art 
Forming a low temperature polycrystalline silicon layer of good quality is 
a key technology in manufacturing a thin film transistor-liquid crystal 
device of high resolution and low cost. One current method uses a pulsed 
ultraviolet beam of an excimer laser irradiated onto an amorphous silicon 
thin film, thereby instantaneously melting and crystallizing the amorphous 
silicon thin film. 
In a method of manufacturing a polycrystalline silicon liquid crystal 
display device by using a polycrystalline silicon or a method of 
manufacturing a dynamic random access memory (DRAM), after implanting ions 
into a semiconductor layer, the crystal structure of the semiconductor 
layer surface may be damaged due to the ion-implantation. The damaged 
portion of such a crystal structure deteriorates the contact 
characteristics to an upper wiring, thereby causing the deterioration of 
the device characteristics. Accordingly, a method has been proposed where 
a laser irradiates the damaged portion, thereby restoring the crystal 
defect. In a method of manufacturing a DRAM, a high temperature heat 
treatment under a nitrogen gas atmosphere, a rapid thermal annealing 
(RTP), or the equivalent is primarily used. However, in a method of 
manufacturing the liquid crystal display device, it is difficult to use a 
high temperature heat treatment because the semiconductor device is formed 
on a glass substrate. Therefore, the method of crystallizing and 
activating the semiconductor layer has a great significance. 
In general, a laser used in a method of crystallizing and activating a 
semiconductor layer must irradiate a target with a pulsed laser beam 
having a short wavelength and a low permeability in order to transmit a 
high energy only to the thin semiconductor layer and to prevent heat 
transmission to a substrate located thereunder. As a result, the excimer 
laser of short wavelength is primarily used among the gas lasers. 
In the method of crystallizing and activating the semiconductor layer by 
using such a laser, it is possible to manufacture a high performance 
polycrystalline silicon thin film transistor on a conventional glass 
substrate with a large area at a low temperature without an expensive 
substrate, such as quartz. Accordingly, a drive circuit incorporating a 
thin film transistor-liquid crystal display device can be manufactured at 
a low cost. 
As shown in FIG. 1, a device used in the crystallization and activation 
using the laser is composed of a laser radiation portion 11, an optical 
system 12, and a substrate 13. A laser beam is generated from laser 
radiation portion 11, and the optical system 12 is composed of a 
reflecting mirror 12-1 and a homogenizer 12-2 having a lens unit. Through 
a homogenizer 12-2 controlling the strength, size, shape and spatial 
uniformity of the laser beam, the laser beam irradiates a semiconductor 
layer 14 formed on substrate 13 to crystallize and activate the 
semiconductor layer 14. 
Laser radiation portion 11 is an excimer laser and generates a short 
wavelength laser beam of approximately 196.about.308 nm, using a pulsed 
gas laser using a gas such as KrF, ArF, XeCl, XeF or the like. In a short 
wavelength laser beam, most of the energy is absorbed at a depth of 
hundreds of .ANG. from the semiconductor layer, and particularly, from the 
surface of the silicon thin film. 
At this time, the pulse width of the laser beam is several tens of 
nanoseconds. The silicon thin film instantaneously melts during this time 
and then solidifies so that the interior of the silicon thin film 
crystallizes, thereby obtaining the polycrystalline silicon thin film. 
Furthermore, in order to manufacture the polycrystalline silicon thin film 
having a wide area, a method has been adopted where the laser beam is 
overlapped, part by part, as it scans. As shown in FIG. 2, unit laser beam 
21 of equal size is located such that the boundary thereof overlaps each 
other. Then, the scan irradiation is performed over the entire surface of 
an amorphous silicon thin film 20. 
When such a crystallization process is performed, the uniformity of the 
thin film is determined by factors such as laser power, degree of overlap, 
film thickness, and substrate temperature. In other words, the grain size 
of the inside of the thin film and the mobility obtained when forming the 
device are greatly changed according to a minute variation of such 
factors. 
Moreover, since the method of overlapping and scanning the laser beam is 
used in order to manufacture the polycrystalline silicon thin film having 
a wide area, each pulse of the laser beam has to be uniform in order to 
obtain a thin film of good quality having uniform characteristics over the 
entire area. 
However, since a gas laser is used in the conventional method of 
crystallizing and activating the semiconductor layer, it is difficult to 
maintain a pulse to pulse power stability of the gas laser. Therefore, 
using such a laser beam results in non-uniform characteristics of the 
polycrystalline silicon thin film. The uniformity of the characteristics 
of the device formed with the thin film transistor having non-uniform 
characteristics is also deteriorated. Furthermore, in terms of 
productivity, the production operating time of the equipment is reduced, 
thereby deteriorating the yield. Therefore, research in maintaining the 
pulse to pulse power stability of the laser is necessary. 
In another conventional method of crystallizing and activating the 
semiconductor layer, a solid laser (e.g., a rubidium laser) having a 
wavelength in the visible region is used, instead of the excimer laser. 
In the case of using a visible wavelength solid laser, such as a rubidium 
laser, when the polycrystalline silicon thin film is manufactured, the 
absorption coefficient of the amorphous silicon sample is about 0.6 
.mu.m.sup.-1 in the visible region. Therefore, the length of the 
absorption depth becomes about 1 .mu.m in the visible ray region. 
Accordingly, if a solid laser is used for crystallizing and activating the 
thin film having a thickness of about 1000 .ANG., only about 10% of the 
incident energy is absorbed and the rest 90% is transmitted, thereby 
causing a large energy loss. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to a method of manufacturing 
a semiconductor device that substantially obviates one or more of the 
problems due to limitations and disadvantages of the related art. 
In order to solve the aforementioned problems, it is an object of the 
present invention to provide a method of manufacturing a semiconductor 
device which is suitable to obtain a semiconductor layer having uniform 
characteristics. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the present invention, as embodied and broadly described, a method of 
manufacturing a semiconductor device including a semiconductor layer 
includes the steps of generating a laser beam using a solid laser source; 
generating a multi-harmonic wave from the laser beam using a 
multi-harmonic oscillator; filtering the multi-harmonic wave; and 
irradiating the filtered wave onto the semiconductor layer. The method 
crystallizes and activates the semiconductor layer. 
In another aspect, a system for manufacturing a semiconductor device having 
a semiconductor layer including a solid laser source for producing a laser 
beam; an oscillator for generating a multi-harmonic wave in response to 
the laser beam; a filter for producing a wave having a wavelength 
corresponding to an absorption rate of the semiconductor layer; and an 
optical system for directing the filtered wave onto the semiconductor 
layer. 
In a further aspect, in a method of crystallizing a semiconductor layer by 
using a pulse laser for an excellent pulse to pulse stability and easy 
maintenance, a method is provided for manufacturing a semiconductor device 
including the steps of generating a laser beam from a laser radiation part 
using a solid laser, generating a multi-harmonic wave from the laser beam 
using a multi-harmonic oscillator generation part using a non-linear 
crystal, and filtering the multi-harmonic wave and irradiating it to the 
semiconductor layer. 
In another aspect, a method of activating a semiconductor layer having a 
damaged crystal structure caused by an ion-implanation or an ion-doping, 
using a pulse laser includes a method of manufacturing a semiconductor 
device including the steps of generating a laser beam from a laser 
radiation part using a solid laser, generating multi-harmonic wave from 
the laser beam using a multi-harmonic oscillator generation part using a 
non-linear crystal, and filtering the multi-harmonic wave and irradiating 
it to the semiconductor layer. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
The crystallization and activation device used for crystallizing and 
activating a semiconductor layer of the present invention is shown in FIG. 
3. 
Referring to FIG. 3, the crystallization and activation device includes a 
laser radiation portion 31 which uses an Nd-based solid laser. A 
multi-harmonic oscillator generation portion 32 receives the laser beam 
generated from laser radiation portion 31 and generates a beam which is a 
composite of several harmonic waves whose oscillator frequencies are 
multiples of the oscillator frequency of an incident laser beam. A filter 
portion 33 extracts and passes a laser beam of a short wavelength whose 
absorptivity is the greatest among the beams of the composite harmonic 
waves of several oscillator frequencies considering the type and thickness 
of the semiconductor layer of the irradiation target. 
Furthermore, an optical system 34 includes a reflecting mirror 34-1 that 
directs the light path and a homogenizer 34-2 having a lens unit with 
multiple lenses that controls the size and spatial uniformity of the beam. 
A semiconductor layer 36 is formed on a substrate 35. 
FIGS. 4A to 4C show several ways of crystallizing an amorphous silicon thin 
film formed on a glass substrate in accordance with the present invention. 
In FIG. 4A, the laser beam is directly irradiated onto an amorphous 
silicon thin film 41 formed on a substrate 40. In FIG. 4B, the laser beam 
is irradiated onto a thin capping layer 42, such as a nitride film or an 
oxide film, that has been deposited over the amorphous silicon thin film 
41. In FIG. 4C, the amorphous silicon thin film 41 is irradiated by the 
laser beam through the back side of substrate 40, thereby crystallizing 
amorphous silicon thin film 41. 
After the ion-implantation or ion-doping of the polycrystalline silicon 
layer or the amorphous silicon layer, an activation is performed in order 
to restore a crystal defect on the silicon surface due to the 
ion-implantation or ion-doping. As shown in FIG. 5A, the laser beam may be 
directly irradiated onto a semiconductor layer 51 having a damaged crystal 
structure. As shown in FIG. 5B, the laser beam may be irradiated onto a 
structure in which a capping layer 52, such as a nitride film or an oxide 
film, is formed on semiconductor layer S1 having the damaged crystal 
structure, thereby activating the semiconductor layer. 
When a polycrystalline silicon thin film is manufactured by irradiating the 
laser beam onto an amorphous silicon layer, a solid laser is used as the 
source of the laser beam. Thus, the pulse to pulse power stability is 
improved and the grain size of the polycrystalline silicon thin film and 
its electrical properties become uniform. Furthermore, when a device such 
as a thin film transistor is manufactured, mobility and other 
characteristics of the device become uniform. Therefore, a circuit having 
a stable operation can be manufactured and a crystal defect at the silicon 
surface that is damaged due to ion-implantation or ion-doping can be 
removed. In addition, because maintenance is easier relative to a gas 
laser, the production operating time is improved to improve productivity. 
For the laser radiation portion, a solid laser is used where an active 
medium is a solid. Particularly, a neodymium (Nd)based laser such as 
Neodymium doped Yttrium Aluminum Garnet (Nd:YAG) laser, Neodymium doped 
GLASS (Nd:GLASS) laser or the like is used. Moreover, the wavelength of 
the laser beam generated from the Nd-based solid laser is in the infrared 
region of about 1 .mu.m. Since the short wavelength of the ultraviolet ray 
region is primarily necessary in the crystallization and activation of the 
silicon thin film, the Nd-based laser cannot be used as is. Therefore, in 
the present invention, multi-harmonic waves, i.e., secondary, cubic, etc., 
are generated using a multi-harmonic oscillator generation portion. Among 
these, with respect to the silicon layer of the wavelength having a high 
absorptivity to the semiconductor layer, i.e., 1000 .ANG., ultraviolet 
emissions with short wavelengths within 300.about.400 nm are used. The 
multi-harmonic oscillator generation portion is composed of a non-linear 
crystal. Particularly, any one of KTiOPO.sub.4 (KTP), KDP, or other types 
of non-linear crystals can be used. 
The principle of the multi-harmonic wave generation using such non-linear 
crystals is as follows. 
In general, when an electromagnetic wave of strong intensity passes through 
a non-linear crystal, the electrical polarization P is not proportional to 
the electric field E (P.noteq..epsilon..sub.0 .chi.E), but is expanded to 
a higher degree. This is expressed in the following equation: 
EQU P=.epsilon..sub.0 (.chi.E+.chi..sub.2 E.sup.2 +.chi..sub.3 E.sup.3 +. . . ) 
(1) 
Here, .chi. is a susceptibility where the linear coefficient .chi. has the 
largest value, and the value gradually decreases from .chi..sub.2 to 
.chi..sub.3. 
If the harmonic wave of E=E.sub.0 sin .omega.t is incident upon such 
medium, the polarization P is expressed as follows. 
##EQU1## 
Therefore, as shown in equation (2), when an electromagnetic wave of 
oscillator frequency .omega. passes through the non-linear crystal medium, 
depending on the electrical polarization of the medium, multi-harmonic 
waves are generated where the harmonic wave has an oscillator frequency 
.omega. the same as that of the incident wave and composite harmonic waves 
of oscillator frequencies of 2 .omega., 3 .omega., . . . , n .omega., . . 
. (where, .omega. is an oscillator frequency of the incident wave). 
Accordingly, in the present invention, in filtering and using 
multi-harmonic waves in which an oscillator frequency of the harmonic wave 
is a multiple (two, three, etc.) to that of the incident wave generated 
through the non-linear crystal medium of the multi-harmonic oscillator 
generation portion, an optical and electromagnetic filter is used to 
extract a desired beam of oscillator frequency and applied in the 
crystallization and activation of the semiconductor layer. Optionally, if 
the filter is not used, a desired beam of oscillator frequency is 
extracted using a transmission angle characteristic of the wave according 
to the oscillator frequency, and is used in the crystallization and 
activation of the semiconductor layer. 
Moreover, in accordance with the present invention, the grain size and the 
mobility of the TFT can be increased by varying the pulse width of a laser 
beam. During silicon crystallization, the rate of solidification of a 
silicon that was melted through laser energy significantly affects the 
grain size and mobility of the TFT. In particular, reducing the rate of 
solidification increases both the grain size and the TFT mobility. For 
excimer lasers, the pulse width of a laser beam is fixed in accordance 
with the laser oscillator geometry and the characteristic of the medium. 
For solid lasers, however, the pulse width may be varied through 
Q-switching, which changes the power and temporal characteristic of the 
beam obtained from a laser oscillator by enhancing the storage and dumping 
of electronic energy in and out of the lasing medium, respectively. 
For Q-switching, the quality factor "Q" is defined as follows: 
##EQU2## 
where "cavity" is the lasing medium. When the stored energy of the cavity 
increases through optical pumping, cavity loss is also increased to lower 
the quality factor Q and prevent laser oscillation. During that time, when 
population inversion is increased to be considerably higher than the 
threshold of the normal lasing action and if Q is again increased, the 
stored energy of the cavity can be released in a short period of time (see 
Walter Koechner, "Solid State Laser Engineering," Springer-Verlag, 1988 
for more details on Q-switching). 
Thus, using a solid laser, the pulse width may be varied to change the 
solidification rate for large grain sizes to produce high performance 
polysilicon films. As the pulse width is increased for the laser beam of 
the solid laser, the rate of solidification is reduced (while within the 
scope of parameters where the substrate is not overheated and damaged). 
The pulse width of solid lasers using Q-switching may be varied by many 
hundreds of nanoseconds (or more) while the pulse width of excimer lasers 
is only about 20-50 ns. Accordingly, the grain size and the mobility of 
the TFT can be improved by using a solid laser with increased pulse width. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the method of manufacturing a semiconductor 
device of the present invention without departing from the spirit or scope 
of the invention. Thus, it is intended that the present invention cover 
the modifications and variations of this invention provided they come 
within the scope of the appended claims and their equivalents.