Method of crystallizing amorphous silicon using a mask

Details Method of crystallizing amorphous silicon using a mask Method of crystallizing amorphous silicon using a mask

2002-05-22

2004-06-29

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from solid or gel state

C117S008000, C117S041000, C117S043000

Reexamination Certificate

active

06755909

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2001-29913, filed in Korea on May 30, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of crystallizing amorphous silicon. More particularly, the present invention relates to a sequential lateral solidification (SLS) crystallizing method suitable for forming polycrystalline silicon having uniform grains.
2. Discussion of the Related Art
Due to a rapid development in information technology, display devices have evolved into instruments that can process and display a great deal of information. While cathode ray tubes (CRT) have served as mainstream display devices, to meet current needs flat panel display devices that are small, light weight, and consume low power, such as liquid crystal displays (LCDs), are becoming increasing important.
LCD devices are typically comprised of two substrates and a liquid crystal layer that is interposed between those substrates. LCD devices produce an image by controlling light transmissivity by varying the arrangement of liquid crystal molecules that are arranged by an electric field.
One LCD substrate includes thin film transistors (TFTs) that act as switching devices. Those TFTs are often formed using an amorphous silicon active layer. On reason for this is that amorphous silicon can be formed on a large, low cost substrate such as glass.
LCD devices also include drive integrated circuits (drive ICs) that control the TFTs. Unfortunately, amorphous silicon does not form a suitable active layer for drive ICs, which are usually CMOS (complementary metal-oxidesemiconductor) devices that require crystalline silicon active layers. Because of this, drive ICs are usually connected to a TFT substrate using a TAB (tape automated bonding) system. This adds significant cost to LCD devices.
Because of limitations of amorphous silicon, LCD devices that incorporate polycrystalline TFT active layers are undergoing research and development. Polycrystalline silicon is highly beneficial because it better suited for use in drive IC devices than amorphous silicon. Polycrystalline silicon thus has the advantage that the number of fabrication steps could be reduced because thin film transistors and drive IC could be formed on the same substrate, eliminating the need for TAB bonding. Furthermore, the field effect mobility of polycrystalline silicon is 100 to 200 times greater than that of amorphous silicon. Polycrystalline silicon is also optically and thermally stable.
Polycrystalline silicon can be formed by depositing amorphous silicon on a substrate, such as by plasma enhanced chemical vapor deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD), and then crystallizing that amorphous silicon into polycrystalline silicon. There are a number of different methods of crystallizing amorphous silicon into polycrystalline silicon, including solid crystallization (SPC), metal induced crystallization (MIC), and laser annealing.
In SPC, a buffer layer is formed on a quartz substrate. Then, amorphous silicon is deposited on the buffer layer. The amorphous silicon is then heated at a high temperature, over 600 degrees Celsius, for a relatively long time. The buffer layer prevents impurities from diffusing into the amorphous silicon. The high temperature causes the amorphous silicon to crystallize. However, the SPC method results in irregular grain growth and non-uniform grain size. Therefore, gate insulators grow irregularly on SPC-formed polycrystalline. This decreases the breakdown voltage of the resulting TFTs. Moreover, the electric properties of the TFTs are reduced because of the irregular grain sizes. Additionally, quartz substrates are expensive.
In MIC, a metal deposited on amorphous silicon induces crystallization at a relatively low temperature. This has the advantage that lower cost glass substrates can be used. However, the deposited metals remain in the silicon layer act as detrimental impurities.
In laser annealing, an excimer laser irradiates an amorphous silicon layer on a substrate for several tens to several hundreds of nanoseconds. This causes the amorphous silicon layer to melt. The melted silicon subsequently solidifies into polycrystalline silicon. In the laser annealing method, crystallization can be accomplished at less than 400 degrees Celsius. Unfortunately, crystallization is relatively poor, particularly if the silicon layer is crystallized using a single laser shot. In practice, re-crystallization is usually performed by irradiating the laser beam about 10 times or so to increase the grain size. Therefore, laser annealing suffers from low productivity. Furthermore, laser irradiation can heat the silicon layer to about 1400 degrees Celsius. Because such temperatures would readily oxidize the silicon layer to produce silicon dioxide (SiO
2
), laser annealing is usually performed under a high vacuum of 10
−7
to 10
−6
torr.
Recently, a new method of crystallization, often referred to as sequential lateral solidification (SLS), has become of interest. The SLS method takes advantage of the fact that silicon grains grow laterally from the boundary between liquid silicon and solid phase silicon. The SLS method can increase the size of the silicon grains that grow by controlling the energy intensity of a laser beam and the irradiation range of the laser beam (reference, Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956~957, 1997). This enables TFTs having channel areas of single crystalline silicon.
A conventional SLS method will be described in detail with reference to the attached drawings.
FIG. 1
illustrates a conventional SLS apparatus. In
FIG. 1
, the conventional SLS apparatus includes a light source
1
, an attenuator
2
, a focusing lens
5
, a mask
6
, an imaging lens
7
, and a translation stage
10
, on which a sample
9
having an amorphous silicon layer (element
20
of
FIG. 2A
) is situated. The SLS apparatus also includes reflective mirrors
3
,
4
, and
8
to change the direction of the light. The reflective mirrors
3
and
4
are disposed between the attenuator
2
and the focusing lens
5
, and the reflective mirror
8
is disposed between the imaging lens
7
and the translation stage
10
.
The light source
1
is beneficially a XeCl excimer laser having a wavelength of 308 nm, or a KrF laser having a wavelength of 248 nm. The attenuator
2
controls the energy of the laser beam through the system. The focusing lens
5
and the imaging lens
7
condense the laser beam, while the focusing lens
5
makes the intensity of the laser beam more uniform The mask
6
forms the laser beam into a predetermined shape.
Therefore, the laser beam from the light source
1
is transmitted through the attenuator
2
and is reflected by the reflective mirrors
3
and
4
. The laser beam is then condensed by the focusing lens
5
, shaped by the mask
6
, and passed through the imaging lens
7
. Next, the laser beam is reflected by the reflective mirror
8
onto the sample
9
. The translation stage
10
then moves the sample
9
and irradiation is repeated.
FIGS. 2A
to
2
C illustrate a process of crystallizing an amorphous silicon film using the SLS apparatus of FIG.
1
.
FIG. 2A
illustrates an initial step of crystallizing the silicon film wherein a first laser beam irradiation is carried out at a region “A” of the amorphous silicon film
20
. As stated above, because the grains of silicon grow laterally from the boundary between liquid phase silicon and solid phase silicon, grains
22
a
and
22
b
of the region “A” grow from both sides of the “A” region. Growth of the grains
22
a
and
22
b
stops at the line “IIa” where the grains
22
a
and
22
b
meet.
FIG. 2B
illustrate crystallizing the silicon film when a second laser beam irradiation is carried out at a region “B” of the amorphous silicon film
20
. The region “B” includes part of the region “A.” The gr

Affiliated with

Also associated with

Rating

Method of crystallizing amorphous silicon using a mask does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method of crystallizing amorphous silicon using a mask, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of crystallizing amorphous silicon using a mask will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3358544