JP2899052B2 - Thin film semiconductor device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin-film semiconductor device including an optical sensor for converting light into an electric signal by photoelectric conversion, and more particularly, to an image reading document such as a facsimile or an image reader. The present invention relates to a thin-film semiconductor device including an optical sensor used for a photoelectric conversion unit of a reading device.

[Conventional technology]

2. Description of the Related Art Conventionally, two types of optical sensors, a photoconductive type and a photovoltaic type, are known.

Above all, the photoconductive type has a gain in photoelectric conversion of 1 or more (generally about 100), so that the output can be increased, and it has been used for an image reading device such as a facsimile or an image reader.

As a photoconductive optical sensor, one having a coplanar structure as shown in FIG. 1 is known. In FIG. 1, 11 is a support such as glass, 12 is a gate electrode, 13 is a gate insulating film, 14 is a thin film semiconductor layer such as amorphous silicon, and 15 is an n + layer in which amorphous silicon contains N-type impurities. Ohmic contact layer, 16
Is a source / drain layer which is an upper electrode, and 17 is a protective layer.

FIG. 8 is a schematic sectional view when the optical sensor shown in FIG. 1 is used in an image reading device.

In FIG. 8, 81 is a support such as glass, 82 is a gate electrode, 83 is a gate insulating film, 84 is a thin film semiconductor layer such as amorphous silicon, and 85 is amorphous silicon.
Ohmic contact layer which is an n + layer containing an impurity of the type, 86 is a source and drain layer which is an upper electrode, 87 is a protective layer, 88 is a photoelectric conversion section, 89 is an adhesive layer, 90 is a wear-resistant layer,
Reference numeral 91 denotes a document, 92 denotes a light source, and 93 denotes an optical sensor.

The light emitted from the light source 92 is reflected by the original 91, enters the photoelectric conversion unit, and is converted into an electric signal.

As shown in FIGS. 1 and 8, the coplanar optical sensor removes the ohmic contact layers of the photoelectric conversion units 18 and 88 to expose the thin film semiconductor layers 14 and 84.

Therefore, to protect the thin-film semiconductor layer, a protective layer
It is common to use with 17, 87.

Conventionally, an organic film made of an organic material such as polyimide, a silicon nitride (hereinafter abbreviated as SiN) film, an inorganic film made of an inorganic material such as a silicon oxide film has been proposed as the protective layer 87.

From the viewpoint of water resistance (moisture resistance), an inorganic film is superior, but a thin film semiconductor layer is formed on an amorphous silicon sensor as described in JP-A-63-92052. It is necessary to consider that the amorphous silicon is damaged when forming the protective layer. Practically, polyimide, which is an organic material, is used as a protective layer.

However, with a protective layer made of an organic material, it is impossible to absolutely prevent the intrusion of moisture, and the film thickness or the range in which the protective layer is provided is increased by increasing the thickness of the protective layer or the edge of the protective layer. In reality, even if there is intrusion of water, the time until the water reaches the sensor unit is extended and used.

That is, in practice, the width of the support of the sensor array used in the image reading apparatus is increased to increase the water resistance (moisture resistance).

[Problems to be solved by the invention]

However, the protective layer made of the organic material described above poses a major obstacle to reducing the width of the substrate (support) of the sensor array of the image reading device in response to the demand for further miniaturization, cost reduction, and increase in production volume. became.

Therefore, it was considered difficult to realize a narrow sensor array due to damage to the amorphous silicon sensor, but it was essential to use a SiN film having excellent water resistance as a protective layer.

In addition, as the performance of thin-film semiconductor devices including sensors and the like has increased, the demand for environmental stability has further increased. For this reason, there has been a demand for the use of a SiN film having excellent water resistance (moisture resistance) as a protective layer. Increased.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin film semiconductor device including an optical sensor and the like that can meet the demand for further miniaturization and cost reduction and that can reduce the substrate width of a support.

Another object of the present invention is to provide a thin-film semiconductor device including an optical sensor or the like that has excellent moisture resistance and can maintain stable characteristics at all times even in a high-temperature environment for a long period of time. And

Still another object of the present invention is to provide a thin film semiconductor device including an optical sensor and the like protected by an optimum material as a protective layer.

The present invention also includes a substrate, a thin-film semiconductor layer, a pair of electrodes formed on the thin-film semiconductor layer via an ohmic contact layer, and a protective layer provided on the thin-film semiconductor layer. But the ratio of nitrogen to silicon is from 0.5
It is an object of the present invention to provide a thin-film semiconductor device including an optical sensor or the like which is a silicon nitride film having a thickness of 0.9.

[Means and actions for solving the problems]

To solve the above problems, a thin film semiconductor device including an optical sensor and the like of the present invention includes a substrate, a thin film semiconductor layer, a pair of electrodes formed on the thin film semiconductor layer via an ohmic contact layer, And a protective layer provided on the
The protective layer is a silicon nitride film having a nitrogen to silicon ratio of 0.5 to 0.9.

That is, according to the present invention, the composition of the SiN film serving as the protective layer of the thin film semiconductor device including the optical sensor and the like is not considered to be a good quality film in terms of stoichiometry (1.3), and the N / Si ratio is 0.5 to 0.9. Accordingly, an object of the present invention is to provide a thin-film semiconductor device including an optical sensor or the like having extremely good moisture resistance without sacrificing optical sensor characteristics such as optical response.

Further, according to the present invention, for a coplanar-type optical sensor made of amorphous silicon, which has been considered difficult to realize, SiN under optimal conditions for realizing an optical sensor having a SiN film as a protective layer. The protective layer can be obtained by setting the ratio of the silicon element (hereinafter abbreviated as Si) and the nitrogen element (hereinafter simply abbreviated as N) of the SiN film to N / Si = 0.5 to 0.9.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings.

Example 1 A preferred example of the optical sensor of the present invention is the same as that shown in FIG.

In the present invention, the protective layer is made of a material as described above.

 The manufacturing procedure will be specifically described.

First, on a glass or the like insulating substrate 11, a C _r deposited 1000Å in sputter method, followed by patterning into a desired shape, and the gate electrode 12.

Thereafter, a gate insulating film 13, a semiconductor layer 14, and an ohmic contact layer 15 are deposited by a plasma CVD method.

 The film forming conditions for each layer are as shown in Table 1.

Thereafter, a metal, for example, Al, which is a conductive material to be the source and drain electrodes, is deposited by a 5000 ° sputter method and patterned into a desired shape to form the source and drain electrodes 16.

After that, an unnecessary ohmic contact layer is removed by etching, and a channel portion (photoelectric conversion portion) 18 is formed.
The removal of the ohmic contact layer was performed by reactive ion etching.

Thereafter, unnecessary portions between the optical sensors were removed to separate the optical sensors.

Thereafter, a plasma SiN film 17 was deposited on the entire surface as a protective layer by a plasma CVD method.

When forming the protective layer, if the substrate temperature is excessively increased, hydrogen may escape from the semiconductor layer 14, or a problem may occur that interdiffusion occurs between Al of the source / drain metal 16 and the ohmic contact 15, so that this problem occurs. It is preferable that the substrate temperature at this time is not higher than the substrate temperature at the time of forming the gate insulating film, the semiconductor layer, and the ohmic contact layer.

That is, since the substrate temperature at the time of deposition of the sensor using amorphous silicon is 150 ° C. to 250 ° C. (the gate insulating film can be higher than that), it is preferable that the substrate temperature at the time of forming the protective layer is also 150 ° C. or less. .

It is generally said that the higher the substrate temperature, the higher the quality of the SiN film can be formed. Here, the SiN film protective layer 17 was formed at a substrate temperature of 150 ° C.

In FIG. 2, the substrate temperature was set to 150 ° C., and SiH ₄ = ₄ SCCM, N ₂ = 2
The composition ratio N / Si of a SiN film when a film is formed at a pressure of 0.2 _Torr using a gas of 00SCCM (SiH ₄ : N ₂ = 1: 50) is shown.

In FIG. 2, the horizontal axis represents the PF power and the power density divided by the size of the electrode. The graph shows that the ratio of SiH ₄ to N ₂ is
1:50, but the ratio of SiH ₄ to N ₂ is 1: 100 and even at 100 W, N / Si =
It was 0.75, similar to 1:50 (this is probably due to the power-limiting composition during deposition). The Ip and response at this time were the same as those of the SiN film optical sensor at 1:50.

FIG. 3A shows the characteristics of the optical response of the optical sensor at this time.

As shown in the figure, it can be seen that the responsiveness, particularly the T _off when the light is off, suddenly deteriorates from around the N / Si ratio in the protective layer exceeding 0.9, and the S / N ratio deteriorates. .

Fig. 4 (A) shows the N / Si ratio and optical band gap (Eg).
Is shown.

When used as a protective layer of an optical sensor, light transmittance becomes a problem. In the image reading apparatus which is the application field of the present invention, the light source has an LED or Xe tube having a center wavelength of 550 to 570 nm,
A fluorescent lamp or the like is used. Therefore, as the protective layer, it is necessary to pass light of the wavelength emitted from these light sources without loss, and Eg is required to have a value larger than 2.2 eV corresponding to 550 nm. At this time, the N / Si ratio of the SiN film is 0.5. Therefore, in terms of optical response, the response becomes poor when the N / Si ratio is 0.5 or less.
Actually, when a photocurrent is measured for a TFT type (with an auxiliary electrode) optical sensor as shown in FIG. 1, the result is as shown in FIG. 3 (B). Bias V _SD of the optical sensor at this time 12V, the potential V _G of the auxiliary electrode is the same V _S = V _G = 0V and the potential V _S of the source electrode.

Optimal photoresponse at N / Si ratio of 0.5 to 0.9, photocurrent I _p
The clear reason why the N / Si decreases sharply at 0.5 or less has not been elucidated, but is considered to be due to the following reasons.

In the SiN film, when N is small and silicon is excessive, dangling bonds of silicon increase (so-called dangling bonds). This dangling bond forms a level at the interface between the SiN film and the semiconductor (i) layer, traps a carrier generated by light, and reduces the photocurrent.
Further, after the light is turned off, trapped electrons are emitted to the interface, which becomes a dark current when the light is turned off, and deteriorates the light response. Of course, because of the small Egopt, the amount of decrease in the amount of transmitted light (E
gopt = 2.0 and several percent reduction of _Ip ).

When N / Si exceeds 0.9, the number of dangling bonds due to N increases, and it is considered that the optical response is deteriorated for the same reason as described above.
Alternatively, when increasing the N / Si ratio, the discharge power during film formation is large, so that the plasma damage to the underlying semiconductor layer is large,
It is conceivable that many levels due to defects are generated in the semiconductor layer.

 FIG. 4B shows the state of the band and the interface of the SiN film.

In particular, when the N / Si ratio is 0.9 or more (Eg is 4 eV or more), the interface characteristics deteriorate, as reported by Hiranaka and others (Dentsu Technical Report ED85-78) and the like. It is thought that a defect level due to mismatch occurs at the interface with the SiN film (FIG. 4B).
(C). In a reverse stagger type TFT, N
Even if a film with / Si of 0.9 or more (Eg of 4 eV or more) is used, the channel is formed on the gate side opposite to the protective layer.
There is little effect on the TFT characteristics such as current. However, as in the present invention, the vicinity of the surface of the a-Si film in contact with the SiN film is used as a photocurrent path. In the case of a coplanar optical sensor, particularly a TFT optical sensor, it is considered that the level near the interface with the SiN film serving as a protective layer greatly affects sensor characteristics such as response. Therefore, the inventor has set the composition ratio N / Si of the SiN film as the optimal protective layer as an optical sensor to 0.9.
It was found that this was the case for smaller values.

Conversely, if the N / Si ratio is too small and the band gap is too small, as in (a) of FIG. 4 (B), it is considered that the insulating film is incomplete, the band barrier is small, and electron injection occurs. .

From the above, the inventors have found that Si used as a protective film
It is said that the N film usually has poor film quality (it is generally said that the film quality is poor unless N / Si is 1.1 or more. For example, Toshiba et al.
3p-ZD-6), it was found that N / Si of 0.5 to 0.9 had better characteristics.

In the embodiment has been described SiN film formed by N ₂ and SiH _4, ammonia (NH ₃₎ N ₂ and SiH when N / Si is the same of the SiN film also SiN film formed by SiH _{4 It} has been confirmed that it has the same electrical and optical characteristics as those formed in Example ₄ , and that the present invention is not limited to the effect of only the SiN film formed from N ₂ and SiH ₄ .

FIG. 9 shows the water resistance of the SiN film of this embodiment having an N / Si of 0.7. Conventional polyimide protective layer optical sensor (b) is several hundred
While the dark current increases with time, the optical sensor (a) with the SiN film protective layer of this example did not show an increase in dark current even after 1,000 hours.

The effect of suppressing the increase in dark current did not change when the N / Si ratio was between 0.5 and 0.9.

Further, as this embodiment, FIG. 1 shows a photoconductive photosensor with an auxiliary electrode, but a photoconductive photosensor without a normal auxiliary electrode has exactly the same effect.

Embodiment 2 As a second embodiment of the present invention, FIG. 5 is a side sectional view when applied to an image reading apparatus such as a facsimile. In FIG. 5, 50 is a light source, 51 is a support such as glass, 52 is a gate electrode, 53 is a gate insulating film, 54 is a thin-film semiconductor layer such as amorphous silicon, and 55 is amorphous silicon containing N-type impurities. An ohmic contact layer as an n + layer, 56 is a source and drain layer as an upper electrode, 57 is a protective layer, 58 is a photoelectric conversion unit, 59 is a manuscript, 60
Is a wear-resistant layer, and 61 is an optical sensor.

The incident light from the light source 50 is reflected by the original 59 and is photoelectrically converted by the optical sensor created in the process shown in FIG.

FIG. 6 shows an example of a plan view of a circuit of a complete contact type sensor constituted by a thin film transistor type optical sensor and a thin film transistor of the present invention.

In the figure, 101 is a wiring portion formed in a matrix, 102 is an optical sensor portion using a thin film transistor type optical sensor according to the present invention, 103 is a charge storage portion, 104a is a transfer switch using a thin film transistor according to the present invention, 104b Reference numeral 105 denotes a discharge switch using a thin film transistor according to the present invention for resetting the charge of the charge storage unit 103, and reference numeral 105 denotes a lead connecting the signal output of the transfer switch to a signal processing IC. In this embodiment, the optical sensor unit 21 and the transfer switch 104a
A-Si: H film is used as a photoconductive semiconductor layer constituting the discharge switch 104b, and plasma CVD is used as an insulating layer.
Is used.

In FIG. 6, for the sake of simplicity, only upper and lower electrode wirings are shown, and the photoconductive semiconductor layer and the insulating layer are not shown. Further, an n + layer is formed at the interface between the upper electrode wiring and the semiconductor layer, and an ohmic junction is established.

FIG. 7 shows an equivalent circuit of a thin-film transistor type photosensor of the present invention and a circuit of a complete contact type sensor constituted by a thin-film transistor. In the figure, S _{i, 1} , S _{i, 2} , S
_{i, 3} ,... S _{i, n} are optical sensors constituting the optical sensor unit 102 in FIG. 6, where i is a block number and 1 to n are the number of bits in the block. (Hereinafter referred to as _{Si, n} ). In the same figure, C _{i, n} is a capacitor of the charge storage unit 22 and stores respective photocurrents corresponding to the optical sensors S _{i, n} . The transistor ST _i of the transfer switch 23a for transferring storage capacitor C _i, the charge of the _n load capacitor CX _{n, n,} transistor SR _i of the discharge switch 23b to reset the _{charge, n} also correspond similarly ing.

The optical sensor S _{i, n} , the storage capacitor C _{i, n} , the transfer switch transistor ST _{i, n} , and the discharge switch transistor SR _{i, n} are respectively arranged in a row in an array, and the number of n is one. A block is formed, and is divided into m blocks as a whole. For example, if the number of sensors is 1728, n = 32 and m = 54. Transfer switches S provided in an array
The gate electrode of T _{i, n} and the discharge switch SR _{i, n} is connected to the gate wiring section. The gate electrode of the transfer switch ST _{i, n} is commonly connected in the first block, and the gate electrode of the discharge switch SR _{i, n} is connected to the gate electrode of the transfer switch of the next order block.

The common lines (gate drive lines G ₁ , G ₂ , G
_3, ... G _m) is driven by the gate driver 246. On the other hand, the signal output is a lead line in a matrix configuration.
230 (signal output lines D ₁ , D ₂ , D ₃ ,... D _n )
7 Connected (in blocks). Also, the optical sensor S _{i, n}
Are connected to the drive unit 250, and a negative bias is applied.

In this configuration, the gate driving line _{G 1, G 2, G 3} , ... are sequentially selected pulses from the gate driver 246 to _{G m (VG 1, VG 2} , VG 3,
... VG _m ). First, when G ₁ is selected gate driving line, transfer switches ST _1,1 ~ST _{1, n} is turned ON,
Storage capacitor C _{1, 1} -C 1, the charge accumulated in the _n is transferred to the load capacitor CX ₁ ~CX _n. Next, the gate drive line G ₂
Is selected, the transfer switches ST _{2,1 to} ST _{2, n} are turned on, and the charges stored in the storage capacitors C _{2,1 to} C _{2, n} are transferred to the load capacitors CX _{1 to} CX _n. are simultaneously charges of the discharge switch SR _{1, 1} to SR _1, storage of _n capacitors C _{1, 1} -C 1, _n is reset. Similarly, the gate drive line
G ₃ , G ₄ , G ₅ ,... G _m are also selected and the reading operation is performed. These operations are performed for each block, the signal output VX ₁ of each _{block, VX 2, VX 3, ...} VX n signal processing unit 247
Input D _1, D _2, D _3, and is sent to the ... D _n, is output after being converted into a serial signal.

Further, as shown in FIG. 5, the one-dimensional contact sensor array of the present invention has a lensless contact for reading a document 59 by forming a wear-resistant layer 60 on the upper part of the optical sensor and illuminating the light source 50 from the back of the sensor. Can be used for sensor arrays. Further, the present invention can also be used for a contact sensor array using an equal-magnification imaging lens (for example, a self-occurring lens of Nippon Sheet Glass).

[Effects of the Invention] As described above in detail, the present invention further reduces the size,
It is possible to provide a thin-film semiconductor device including an optical sensor or the like that can reduce the width of a substrate of a support member in response to a demand for cost reduction.

Further, the present invention provides a thin-film semiconductor device including an optical sensor or the like which has excellent moisture resistance and can always maintain stable characteristics even when placed in a high-humidity environment for a long period of time. it can.

Further, the present invention can provide a thin-film semiconductor device including an optical sensor or the like protected by an optimum material as a protective layer.

Further, the present invention does not consider that the composition of the SiN film used as the protective layer of the optical sensor is a high-quality film in terms of stoichiometric ratio (1.3).
By setting the i ratio to 0.5 to 0.9, it is possible to provide an optical sensor having extremely good moisture resistance without sacrificing optical sensor characteristics such as optical response.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of an optical sensor, and FIG.
FIG. 3 (A) is a graph showing the dependence of the / Si ratio and the deposition power, and FIG.
Light response characteristics of the optical sensor when the film is used as a protective layer,
FIG. 3 (B) is a characteristic diagram of the photocurrent and dark current of the optical sensor when the SiN film is used as a protective layer, and FIG. 4 (A) is the N / Si ratio and the optical band gap in the SiN film. Figure (B) shows the Si
FIG. 5 is a diagram for explaining the state of the band and the interface of the N film, FIG. 5 is a schematic cross-sectional view of an image reading apparatus using an optical sensor employing the SiN film protective layer of the present invention, and FIG. FIG. 7 is a schematic plan view of an image reading device using the optical sensor, FIG. 7 is an equivalent circuit of the image reading device using the optical sensor of the present invention, FIG. 8 is a schematic sectional view of the image reading device, and FIG. It is a figure which shows the change of the dark current at the time of high temperature and high humidity leaving of the optical sensor. 11,51,81… Substrate (base) 12,52,82… Gate electrode 13,53,83… Gate insulating film 14,54,84… Semiconductor layer 15,55,85… Ohmic contact layer 16,56,86 Source / drain electrodes 17,57,87 Protective layer 18,58,88 Photoelectric converter 59,91 Original 60,90 Thin glass 61,93 Optical sensor 50,92 …… Light source

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takayuki Ishii 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Naotoku Tsuda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (72) Inventor Masato Yamanobe 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-2-150060 (JP, A) JP-A-63-124002 (JP) JP-A-1-226181 (JP, A) JP-A-64-50535 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 21/318-21/32 H01L 27/14-27/148 H01L 29/786

Claims (4)

(57) [Claims] A substrate, a thin film semiconductor layer, a pair of electrodes formed on the thin film semiconductor layer via an ohmic contact layer,
And a protective layer provided on the thin film semiconductor layer, wherein the protective layer is a silicon nitride film having a nitrogen to silicon ratio of 0.5 to 0.9. 2. The thin film semiconductor device according to claim 1, wherein said optical sensor has auxiliary electrodes provided corresponding to said pair of electrodes. 3. The thin film semiconductor device according to claim 1, wherein said thin film semiconductor layer is made of amorphous silicon. 4. The energy band gap Eg of the protective layer.
2. The thin film semiconductor device according to claim 1, wherein is 2.2 to 4.0 eV.