Thin-film semiconductor device fabrication method

Details Thin-film semiconductor device fabrication method Thin-film semiconductor device fabrication method

2001-06-08

2003-02-18

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

C438S150000, C438S151000, C438S669000

Reexamination Certificate

active

06521492

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns technology for forming, at a relatively low temperature of about 600° C., polycrystalline semiconductor layers having outstanding crystalline properties. This technology is related particularly to a fabrication step capable of markedly improving the performance of thin-film semiconductor devices typified by polysilicon thin-film transistors.
2. Description of Related Art
Conventionally, a fabrication scheme such as that described below has been used when fabricating thin-film semiconductor devices such as polysilicon thin-film transistors (p-Si TFT) at low temperatures of approximately 600° C. or less, where general-purpose glass substrates can be used. First, an amorphous silicon layer serving as a semiconductive layer is deposited on the substrate to a thickness of approximately 50 nm by low-pressure chemical vapor deposition (LPCVD). This amorphous layer is then irradiated with a XeCl excimer laser (wavelength: 308 nm) to form a polysilicon film (p-Si film). Since the absorption coefficient of the XeCl excimer laser in the amorphous silicon and polysilicon is large (0.139 nm
−1
and 0.149 nm−
1
, respectively), 90% of the laser light impinging on the semiconductor films is absorbed at a depth of 15 nm or less from the surface. In addition, the absorption coefficient of amorphous silicon is approximately 7% smaller than the absorption coefficient of polysilicon. Next, a silicon oxide layer serving as a gate dielectric layer is formed by chemical vapor deposition (CVD) or by physical vapor deposition (PVD). Gate electrodes are then created using a material such as tantalum to form MOSFETs—field effect transistors consisting of a metal (gate), an oxide layer (gate dielectric layer) and a semiconductor (polysilicon layer). Finally, an interlevel dielectric layer is deposited on top of these layers; and, after contact holes are opened, a thin-film of metal interconnects is patterned, completing the thin-film semiconductor device.
However, controlling the energy density of the excimer laser light used in the conventional method of fabricating these thin-film semiconductor devices was difficult, and even slight fluctuations in the energy density caused the semiconductor layer to exhibit significant nonuniformity, even within the same substrate. Moreover, if the irradiated energy density was even slightly higher than the threshold value determined by film thickness and hydrogen content, the semiconductor layer incurred extensive damage, inviting marked deterioration of semiconductor characteristics and product yield. Therefore, the energy density of the laser light had to be set considerably lower than the optimum value to obtain a uniform polycrystalline semiconductor layer. For this reason, obtaining high quality polycrystalline thin films meant that an insufficient energy density could not be avoided. Furthermore, enlarging the grains that comprise the polycrystalline layer was difficult even if the laser was radiated at the optimum energy density; and a large number of defects were left in the layer. Therefore, to consistently fabricate thin-film semiconductor devices such as p-Si TFTs using the conventional fabrication method, the electrical characteristics of the finished thin-film semiconductor devices had to be sacrificed.
SUMMARY OF THE INVENTION
In view of the aforesaid situation, the purpose of the present invention is to provide a method for consistently fabricating extremely high quality thin-film semiconductor devices using a low temperature step of 600° C. or less.
Following an overview of the present invention, the effects of the present invention and its fundamental principles will be described in detail.
The present invention includes a step for fabricating thin-film semiconductor devices having as the active layer a crystalline semiconductor film comprised mainly of silicon (Si) formed on a substrate; a semiconductor layer formation step in which a silicon oxide layer that serves as an underlevel protection layer is formed on the substrate if necessary and an amorphous semiconductor layer comprised mainly of silicon (Si) is deposited on top of the aforesaid underlevel protection layer or on the substrate; a solid phase crystallization step that crystallizes the amorphous semiconductor layer in a solid state and obtains a solid phase crystallization film; and a light irradiating step in which light from a pulsed laser is irradiated on the solid phase crystallization film thus obtained to obtain a crystalline semiconductor film; and is characterized by the fact that the wavelength of the pulsed laser beam used is greater than about 370 nm and less than about 710 nm. The absorption coefficient of said light in polysilicon is greater than the absorption coefficient in amorphous silicon. Moreover, the present invention is also characterized by the fact that the wavelength of the pulsed laser beam is greater than about 450 nm and less than about 650 nm. Accordingly, the absorption coefficient &mgr;
pSi
of a pulsed laser beam in polysilicon is from approximately 10
−2
nm
−1
to 10
−3
nm
−1
. In this case, if the film thickness of the semiconductor layer is taken to be d, it is desirable for the film thickness d(nm) and the absorption coefficient of the pulsed laser beam in polysilicon &mgr;
pSi
(nm
−1
) to satisfy the following relationship:
0.105·&mgr;
pSi
−1
<d<
0.693·&mgr;
pSi
−1
.
Ideally, the following relationship is satisfied:
0.405·&mgr;
pSi
−1
<d<
0.693·&mgr;
pSi
−1
.
In order for the present invention to be applicable to devices such as liquid crystal displays, it is desirable for the substrate to be transparent to visible light. Further, regardless of the application, it is also desirable for the substrate to be essentially transparent to the pulsed laser beam. “Essentially transparent” means that the absorption coefficient of the pulsed laser beam in the substrate is approximately one tenth the absorption coefficient in polycrystalline silicon or lower. Specifically, the absorption coefficient of the substrate &mgr;
Sub
should be approximately 10
−4
nm
−1
or lower. Normally, formation of the amorphous semiconductor layer mentioned above would include a deposition step by chemical vapor deposition (CVD). Within the chemical vapor deposition process category, low pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition are particularly applicable for deposition of amorphous semiconductor thin films; and it can be said further that amorphous semiconductor layer deposition in a high-vacuum low pressure chemical vapor deposition chamber or in a high-vacuum plasma-enhanced chemical vapor deposition chamber is ideal. A high-vacuum low pressure chemical vapor deposition chamber is one in which the background pressure immediately prior to semiconductor layer deposition is typically 5×10
−7
Torr or less, and that can achieve an atomic oxygen concentration within the amorphous semiconductor layer of approximately 2×10
16
cm
−3
or less even when the amorphous semiconductor layer is formed at a slow deposition rate of approximately 1.5 nm/min or less. Similarly, “high-vacuum plasma-enhanced chemical vapor deposition chamber” refers to a deposition system in which the background pressure immediately before semiconductor layer deposition is typically 1×10
−6
Torr or less, and that can achieve an atomic oxygen concentration within the deposited amorphous semiconductor layer of approximately 2×10
16
cm
−3
or less even when the deposition rate of the amorphous semiconductor layer is approximately 1 nm/sec or less. When a YAG 2&ohgr; laser beam impinges on a semiconductor layer comprised mainly of silicon, it is preferable for the thickness of the semiconductor layer to be approximately 25 nm or more and approximately 165 nm or less, and, ideally, approximately 25 nm or more and approximately 95 nm or less.
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