Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2002-02-28
2003-12-09
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S197000, C438S778000
Reexamination Certificate
active
06660572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates first of all to thin film semiconductor devices, and the fabrication methods for such devices, which are suitable for active matrix liquid crystal displays and the like. The present invention also relates to liquid crystal displays for which the thin film semiconductor devices are appropriate, the fabrication processes for such liquid crystal displays, electronic devices, and the fabrication processes for such electronic devices. Further, the present invention relates to thin film deposition processes using plasma-enhanced chemical vapor deposition (PECVD).
2. Description of Related Art
In recent years, along with increases in screen size and improvements in resolution, the driving methods for liquid crystal displays (LCDS) are moving from simple matrix methods to active matrix methods; and the displays are becoming capable of displaying large amounts of information. LCDs with more than several tens of thousands pixels are possible with active matrix methods which place a switching transistor at each pixel. Transparent insulating substrates such as fused quartz and glass which allow the fabrication of transparent displays are used as substrates for all types of LCDs. Although ordinarily semiconductor layers such as amorphous silicon or polycrystalline silicon are used as the active layer in thin film transistors (TFTs), the use of polycrystalline silicon which has higher operating speed is advantageous when producing monolithic displays which include integrated driving circuits. When polycrystalline silicon is used as the active layer, fused quartz is used as the substrate; and a so-called “high temperature” process in which the maximum processing temperature exceeds 1000° C. is used to fabricate the TFTs. In this case, the mobility of the polysilicon layer attains values from about 10 cm
2
×V
−1
×sec
−1
to 100 cm
2
×V
−1
sec
−1
.
On the other hand, for the case of an amorphous silicon active layer, a common glass substrate can be used since the maximum processing temperature is about 400° C. The mobility of an amorphous silicon layer attains values from about 0.1 cm
2
×V
−1
×sec
−1
to cm
2
×V
−1
×sec
−1
. For increases in LCD display size while maintaining low costs, the use of low-cost common glass substrates is indispensable. The previously mentioned amorphous silicon layers, however, have such problems as electrical characteristics far inferior to those of polysilicon layers and slow operating speed. Since the high temperature, process polysilicon TFTs use quartz substrates, however, there are problems with increasing display size and decreasing costs. Therefore, there is presently a strong need for technology which can fabricate a thin film semiconductor device employing a semiconductor layer such as polycrystalline silicon as the active layer upon a common glass substrate. But, when using large substrates which are well-suited to mass production, there is a severe restriction in that the substrates must be kept below a maximum processing temperature of about 400° C. in order to avoid substrate deformation. In other words, technology which can produce, under such restrictions, the active layer of thin film transistors capable of controlling a liquid crystal display and of thin film transistors which can operate driving circuits at high speed is desired. These devices are presently known as the low temperature poly-Si TFTS, and development of them is progressing.
The first type of prior art corresponding to existing low temperature poly-Si TFTs is shown on p. 387 of the SID (
Society for Information Display
) '93 Digest (1993). According to this description, 50 nm of amorphous silicon (a-Si) is first deposited at 550° C. by LPCVD using monosilane (SiH
4
) as the source gas and then converted from a-Si to poly-Si by laser irradiation. After patterning of the poly-Si layer, a gate insulator layer of SiO
2
is deposited by ECR-PECVD at a substrate temperature of 100° C. Following formation of the tantalum (Ta) gate electrode on top of the gate insulator layer, self-aligned transistor source and drain regions are formed in the silicon layer by ion implantation of donor or acceptor impurities while using the gate electrode as a mask. This ion implantation, known as “ion doping”, is accomplished by a non-mass separating ion implanter. Hydrogen-diluted phosphine (PH
3
), diborane (B
2
H
6
) or similar gas is used as a source gas for ion doping. Activation of the impurities is carried out at 300° C. Following deposition of an interlevel insulator layer, electrodes and interconnects such as indium tin oxide (ITO) and aluminum (Al) are deposited to complete the thin film semiconductor device. Therefore, the maximum processing temperature in this prior art is the 550° C. used for deposition of the a-Si by LPCVD (abbreviated hereafter as LPCVD a-Si).
The prior art exemplified in Japanese Unexamined Patent Application Heisei 6-163401 is representative of a second type of existing low temperature poly-Si TFT technology. The principle feature of this technology, in contrast to the use of LPCVD a-Si in the first type of prior art, is a process in which an a-Si film is deposited by plasma CVD (PECVD), the deposited film is given a so-called “dehydrogenation anneal” at a temperature between approximately 300° C. and 450° C., and then laser irradiated to form a poly-Si film. The following description is given in paragraph [0033] of the same document. “Using plasma CVD, a 200 nm thick silicon oxide passivation layer 2 and a 100 nm thick amorphous silicon thin film 3 were formed on top of glass substrate 1 (Coming 7059) at a substrate temperature of 200° C. After a 30 minute anneal at 300° C, amorphous silicon film 3 was polycrystallized using a 13 W argon ion laser, focused to approximately a 50 mm diameter spot size, with a scan speed of approximately 11 m/sec (scan speed of the beam spot diameter×220000/sec).”
As shown in this example, thermal annealing prior to the crystallization step is necessary when crystallizing an a-Si film deposited by PECVD (abbreviated hereafter as PECVD a-Si). If annealing is not done or if the temperature of the anneal is low, crystallization does not proceed at all when the irradiating laser energy intensity is weak. Conversely, if the energy intensity is strong, the Si film is damaged and the PECVD a-Si film again does not crystallize at all. This situation will be explained using the inventor's experimental results shown in FIG.
1
.
The sample is a glass substrate (Nippon Electric Glass Co., Ltd.'s OA-2) on top of which was deposited a 2500A thick a-Si film by a parallel plate PECVD reactor. The deposition conditions were a monosilane (SiH
4
) flow rate of 225 SCCM, a hydrogen (H
2
) flow rate of 1300 SCCM, RF power of 150 W, pressure of 1.3 Torr, electrode surface area of 1656 cm
2
(giving an applied power density of 0.091 W/cm
2
), electrode separation distance of 24.4 mm, a substrate temperature of 300° C., and a deposition rate of the PECVD a-Si film at this time of 547 A/mm. This film is a typical PECVD a-Si from the prior art and contains approximately 10.3 atomic % hydrogen and has a silicon atomic density of 4.38×10
22
cm
−3
. Additionally, the infrared absorption spectrum (IR) for this a-Si film is shown in
FIG. 5
; and Raman spectroscopy results are shown in FIG.
6
. The Si-H stretching mode can be seen around 2000 cm
−1
in the infrared absorption spectrum of FIG.
5
. On the other hand, the transverse optical component from amorphous silicon can be seen around 480 cm
−1
by Raman spectroscopy. Both results indicate that the amorphous film is a-Si in which hydrogen is bonded to silicon.
FIG. 1
shows the results of laser irradiation following annealing of the a-Si film in a nitrogen atmosphere at a fixed temperature for one hour. The horizontal axis of
FIG. 1
shows the annealing temperature, and the vertical axis sho
Garcia Joannie Adelle
Oliff & Berridg,e PLC
Seiko Epson Corporation
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