Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-05-03
2002-12-17
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S098000, C345S100000
Reexamination Certificate
active
06496171
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to semiconductor display devices with thin film transistors. More particularly, the invention relates to a technology to manufacture a plurality of thin film transistors free of characteristic variations through the use of a linear laser beam.
2. Description of the prior art
In recent years studies have been eagerly made in order to decrease the process temperature in the manufacture of thin film transistors (hereinafter referred to as TFTS). The major reason for this is due to a necessity that a semiconductor device is to be formed on an insulator substrate of such as a glass that is low in cost but high in processability. The temperature decrease in the process to manufacture a semiconductor device is also demanded from a viewpoint of put forward with device scale down and multilevel structure.
The manufacture of a high performance semiconductor device requires a process to crystallize an amorphous ingredient or amorphous semiconductor material contained in a semiconductor material. Meanwhile, there might require a process to restore deteriorated crystallinity in a semiconductor material whose property is crystalline in nature but deteriorated by ion irradiation, or a process to improve crystallinity furthermore. Conventionally, thermal anneal has been utilized for such purposes. Where silicon has been employed as a semiconductor material, anneal has been conducted at temperatures of 600° C. to 1100° C. for 1 to 48 hours or longer in order to cause amorphous crystallization, crystallinity restoration, crystallinity improvement and so on.
In the thermal anneal for the above purposes, the high the process temperature the shorter the required process time becomes. However, there is almost no preferred effect at a temperatures of 500° C. or below. From a viewpoint of temperature decrease, it has been conventionally considered that the process using thermal anneal has to be substituted by another means. Particularly it has been considered that, where a glass is used as a substrate, the glass substrate has a heat resisting temperature of around 600° C. and accordingly requires such a means to be effected at a temperature below this that corresponds to the conventional thermal anneal.
In recent, attentions have been drawn to techniques attempted to irradiate laser light onto a semiconductor material in order to perform various types of anneal. The thermal anneal with laser light irradiation has an advantage that there is no necessity to exposing the entire substrate to a high temperature because of its capability to give high energy equivalent to that of thermal anneal to a desired limited point.
There are, roughly, two proposals as a method to irradiate laser light.
The first method uses a continuous oscillation laser such as an argon ion laser, in order to irradiate a spot-formed beam to a semiconductor material. This method utilizes a difference of in-beam energy distribution and beam movement to cause fusion in a semiconductor material and then moderate solidification thereby crystallizing the semiconductor material.
The second method uses a pulse oscillation laser alike an excimer laser to irradiate a great energy laser pulse to a semiconductor material, wherein upon laser irradiation the semiconductor material instantaneously fuses and solidifies thus utilizing crystal growth proceeding.
The first method involves a problem of taking a long time to perform the process. This is because the continuous oscillation laser is limited in its maximum energy and the beam spot size is at most on a order of a millimeter square. In contrast to this, the second method the laser has a great maximum energy by which a spot as large as several centimeter square or greater can be used to enhance mass productivity.
However, where using a beam in a usual square or rectangular form, there is a necessity of moving the beam in vertical and horizontal directions in order to process an entire substrate with a wide area. Thus a problem has still been left in respect of productivity (throughput).
To cope with this, the throughput can be largely improved by adopting a method wherein the beam shape is changed into a linear form having a beam width greater than that of a substrate to be processed in order to implement scanning the beam over the substrate relative thereto. The scan herein refers to linear laser irradiation with slightly shifting while overlapping.
However, where applying the above technique using linear pulse laser irradiation with overlap while slight shifting, linear fringes naturally occur on a surface of a laser-irradiated semiconductor material. These fringes has a great adverse effect upon characteristics of a device having been formed or to be formed on the surface of the semiconductor material. In particular, a serious problem will be encountered when a plurality of devices are to be formed on the substrate with an even characteristic on a one-by-one device basis. In such a case, the fringe pattern has variation in characteristic occurring between the fringes despite each fringe is homogeneous in characteristic.
In also the anneal method using a linear laser light, a problem rises in respect of evenness by the effect of irradiation. High evenness herein refers to the ability to provide an even device characteristic regardless of a device forming position on the substrate. The improvement in evenness means to make homogeneous the crystallinity of a semiconductor material. The following attempts have being made in order to raise the evenness.
It is known that the evenness is improved by preparatorily irradiating (hereinafter referred to as preparatory irradiation) a pulse laser light with a weaker intensity prior to irradiating a stronger pulse laser light (hereinafter referred to as main irradiation) in order to relax unevenness due to laser irradiation effects. This is extremely effective and improve a semiconductor device circuit characteristic to a significant extent with variation suppressed.
The reason why the preparatory irradiation effective for film homogeneousness is that a semiconductor material film containing an amorphous portion as stated before has such a property of laser energy absorption ratio that is significantly different from that of a polysilicon film or single crystal film. That is, two stage irradiation acts to crystallize, in a first irradiation, amorphous portions remained in the film and, at a second irradiation, accelerates entire crystallization. The moderate crystallization as this serves to suppress to a certain extent fringes from occurring on the semiconductor material due to linear laser irradiation. This attempt considerably improves the laser light irradiation effect and the fringes as observed become comparatively modest.
However, in the case that a multiplicity (on the order of several millions to several tens of millions) of thin film transistors are required to form on a glass substrate as in an active matrix semiconductor display device, e.g., a liquid crystal display device, even the laser irradiation method with two stage irradiation is unsatisfactory in respect of its evenness effect.
Here, a schematic configuration diagram of a conventional active matrix liquid crystal display is shown in FIG.
8
. In
FIG. 8
,
801
is a shift register on a source signal line side,
802
and
803
are buffers (inverters),
804
is an analog switches,
805
is a video signal line,
806
is a source signal line,
807
is a shift register on a gate signal line side,
808
is a buffer (inverter),
809
is a gate signal line,
810
is a pixel TFTs and
811
is a liquid crystal. Also, FIGS.
9
(
a
) and
9
(
b
) demonstrate circuit diagrams for the buffers (inverters)
802
,
803
and
808
and the analog switch
804
.
In the buffer of FIG.
9
(
a
), IN represents that a timing signal is inputted from the shift register while OUT denotes outputting an inverted signal thereof. Also, Vdd is a constant power voltage. In the analog switch of FIG.
9
(
b
), IN represents inputting o
Costellia Jeffrey L.
Hjerpe Richard
Nguyen Kimnhung
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
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