This invention relates to a semiconductor device employing a polycrystalline semiconductor film on a substrate.

In recent years, development of an elongate active device array and large area active device array of large area has been sought. These active devices are intended to be used as a photosensor for reading a picture in a facsimile transmitter, and for the driver circuit of a liquid crystal display panel or electroluminescence display panel which serves as a flat display panel to replace a cathode-ray tube.

Ordinary semiconductor devices employing a single crystal of silicon are limited in size by the size of the single crystal silicon wafer serving as a parent material. On the other hand, thin film transistors etc. have been attempted, but none has yet attained satisfactory characteristics.

Examples of polycrystalline silicon devices are described in the following references. None of these devices, however, can achieve satisfactory characteristics if formed as a film at a low temperature. No semiconductor device having satisfactory characteristics has been realized on a glass substrate or the like.

• (1) THIN SOLID FILMS, vol. 35, June 1976, no. 2, pp. 149-153,
• (2) ELECTRICAL DESIGN NEWS, vol. 18, no. 13, July 1973, pp. 30-31
• (3) IBM TECHNICAL DISCLOSURE BULLETIN, vol. 14 no. 10, March 1972, pp. 2900-2901,
• (₄) Applied Physics Letters, vol. 35, 15th July 1979, no. 2, pp. 173-175,
• (5) IBM TECHNICAL DISCLOSURE BULLETIN, vol. 17 no. 8, January 1975, pp. 2455-2456
• (6) SOLID STATE ELECTRONICS, vol. 15, no. 10 October 1972, pp. 1103-1106

An object of this invention is to provide a thin film semiconductor device which can be produced even under low temperature conditions, such as a deposition temperature of, for example, below 700°C, and which exhibits good operating characteristics.

The present invention is defined in the claims.

With the present invention, a semiconductor device can be produced on a substrate made of glass or the like having a comparatively low softening point, and an evaporation process can be used, allowing large area or elongate semiconductor devices to be fabricated readily.

The polycrystalline semiconductor film may be formed by vacuum evaporation under a high vacuum of below 1 x 10^-8 Torr pressure. In the evaporation a deposition temperature of 400 - 700°C may suffice.

An embodiment of this invention will now be described by way of example, with reference to the accompanying drawings, in which:-

• Figure 1 is a graph showing the relationship between the thickness and grain size of an evaporated film,
• Figures 2a - 2h are sectional views showing a process for producing a MOSFET (metal-oxide-semiconductor field effect transistor) using a polycrystalline semiconductor film, and
• Figure 3 is a graph showing the drain current - versus - drain voltage characteristic of a MOSFET embodying this invention.

In our patent application no. we describe a technique for-producing a semiconductor device using a silicon film produced by evaporation under ultra-high vacuum.

We have made further investigations of the crystallinity of a polycrystalline silicon film formed by the evaporation in the ultra-high vacuum, and information described below has been obtained. It has also been discovered that improvement of operating characteristics can be achieved. The formation of a thin film in ultra-high vacuum is the best process among those presently available for forming thin films, but as regards essential points, the results obtained apply similarly to the thin films produced by the other processes. Likewise, although the results referred to concern silicon film the same applies to different semiconductor films, such as germanium etc.

In the initial stage of the growth of a film, the portion which has initially adhered as an amorphous layer is subject to crystal growth and turn^s polycrystalline with the subsequent growth of the film. When the film has been formed to a sufficient thickness, it crystallizes and a polycrystalline layer grows from the beginning. From this, it can be understood that a factor hindering crystallization exists in the vicinity of the interface between a substrate and the silicon film. It is accordingly concluded that, even when in the vicinity of the interface owing to the subsequent growth of the film crystallization has occurred so that the whole layer becomes polycrystalline, the crystallinity in the vicinity of the interface is worse than that of the surface layer of the film. In order to produce a good thin-film semiconductor device, it is thus desirable to arrange a channel on the surface side having better crystallinity.

If the silicon film is grown by evaporation at a substrate temperature of 400°C - 700°C in ultra-high vacuum, crystal grains whose {100} faces are substantially parallel to the substrate plane and crystal grains whose {110} faces are substantially parallel to the substrate plane coexist in the surface layer if the film thickness is at least 0.5 µm. When the thickness of the film exceeds 1 µm, the growth can be controlled so that the crystal grains whose {100} faces are substantially parallel to the substrate plane becomes overwhelmingly more numerous within the surface layer. In this specification, the state in which the crystal grains with {100} faces substantially parallel to the substrate plane are predominant shall be termed the " <100> preferred orientation" (sometimes expressed as the {100} texture).

A semiconductor device using a polycrystalline semiconductor film whose major surface is a surface having the <100> preferred orientation has good characteristics. In addition, the variations of the characteristics can be reduced. Although a polycrystalline semiconductor film of the <100> preferred orientation is most preferable, the same effect can be achieved even with a polycrystalline semiconductor film whose major surface is a surface having the <110> preferred orientation. In addition, a polycrystalline semiconductor film having the mixed orientation of <100> and <110> may be useful. As previously stated, it is favourable that a region in which carriers move (for example, the channel in the FET) is disposed on the surface side of the polycrystalline film. The structure of the film can be fully confirmed by electron diffraction, or X-ray diffraction. Silicon, germanium etc. which are semiconductor materials having the diamond structure exhibit the same tendency.

Figure 1 shows the dependency of grain size in the surface layer on the-thickness of the film. Since, in the temperature range mentioned above, the substrate temperature-dependency of the grain size may be considered almost nil, a case where the substrate temperature is 600°C is typically illustrated in Figure 1. The illustrated curve corresponds to results obtained by measuring the grain size in the thickness direction of an evaporated film 1.4 µm thick.

The foregoing study of the structure and grain size of a polycrystalline film shows that the grain size can be increased and the structure can be made more favourable by increasing the thickness of the film. A film thickness of at least 0.5 µm may be used for semiconductor devices. A film thickness of at least 1µm is more preferred. The polycrystalline semiconductor film thus obtained can have a carrier mobility of at least 1 cm²/V·sec.

Needless to say, an impurity can be present in the film as may be needed.

The substrate can be an amorphous or polycrystalline substrate such as glass or A1₂0₃ or the like ceramics. A glass substrate is most useful from the point of view of price. A light- transmitting substrate can also be used, e.g. as the substrate of a photodiode or the like in which light enters the substrate side.

One method of producing the polycrystalline semiconductor film of a device according to this invention is as follows.

A vacuum evaporator which can achieve ultra-high vacuum may be a conventional evaporator having an ultra-high vacuum device.

The degree of vacuum during evaporation is made as high as less than 1 x 10^-8 Torr pressure. 0₂ especially in the gas remaining during evaporation affects the characteristics of the product adversely, and hence the partial pressure of oxygen is preferably made less than 1 x 10^-9 Torr.

The deposition rate is preferably in the range 1,000 A/hour to 10,000 A/hour.

In order to fabricate a semiconductor device by working the polycrystalline silicon film several steps must be performed. Heat-treatment temperatures in these steps have been kept lower than 820°C which is the softening point of very hard glass. This is because the present invention is most useful for producing a semiconductor device on. a glass substrate or the like. If employing a glass substrate of low softening point, it is also possible to keep the temperatures still lower, for example to be below 550°C.

A MOS field effect transistor will now be described as an example of a semiconductor device of the invention.

To form a gate insulator, thermal oxidation of a silicon substrate is generally relied on. Since, however, thermal oxidation requires a temperature of at least 1,000°C, it cannot be used for the present purpose. Therefore, an Si0₂ film is formed by chemical vapor deposition by reacting SiH₄ and 0₂ at a temperature of at least 300°C and at most 500°C or by reacting SiH₄ and NO₂ at a temperature of at least 400°C and at most 800°C and the Si0₂ film thus formed is employed as the gate insulator. Si0₂ films formed by chemical vapor deposition have in the past been used for passivation, and no example has been employed as the gate insulator.

p impurity-diffused layers or n impurity-diffused layers have previously been made by thermal diffusion in order to form a source region and a drain region. Since, however; this requires a heat treatment at about 1,150°C it cannot be used for the present purpose. The present invention can employ a method in which p⁺ impurity layers or n⁺ impurity layers are formed by ion-implantation instead of the thermal diffusion. After ion implantation, a heat treatment is conducted for electrical activation, the heat-treatment temperature being kept below the softening point of the substrate used. For instance, there can be implanted ions, e.g. BF₂⁺ which can be highly activated by a heat treatment at a low temperature of approximately 550°C and B⁺ ions, and thereafter heat treatment can be performed at a temperature of approximately 500°C - 600°C immediately before the reverse anneal phenomenon occurs. While P⁺ ions, As ions etc. are not so good as B⁺ ions from the point of view of the reverse anneal phenomenon, they can be sufficiently activated by heat treatment at approximately 500°C - 600°C. Accordingly, both the p impurity layers and the n⁺ impurity layers can be formed with a low-temperature step of 500°C - 600°C or so. Of course, the heat treatment may well be carried out at a temperature of 800°C if a substrate, such as very hard glass substrate, with a softening point higher than 800°C is used.

With this method of manufacture, it is possible to obtain a semiconductor device of large area or great length in which the mobility of carriers is at least 1 cm²/V·sec.

The embodiment of this invention shown in Figures 2a - 2h, will now be described. In this case, an n-channel MOS field effect transistor is fabricated by forming a polycrystalline silicon film on a glass substrate and providing channel in the surface layer of the film.

First, the substrate is installed in a vacuum evaporator which can achieve ultra-high vacuum. The evaporator may be a conventional one. A silicon film 2 is deposited to a thickness of 1µm on the substrate 1 of hard glass (Corning 7059 Glass) by performing vacuum evaporation under the conditions of a substrate temperature of 600°C, a degree of vacuum during evaporation of 8 x 10^-9 Torr, a partial pressure of oxygen during evaporation of 1 x 10^-10 Torr, and a deposition rate of 5,000 A/hour (figure 2a). The silicon film 2 formed is of p-type polycrystalline silicon slightly doped with boron, and has a carrier mobility of about 2 cm²/V·sec. When the crystalline structure in the surface layer of this film was investigated, it was found that the polycrystalline silicon film had a strong <100> preferred orientation and that the <110> preferred orientation coexisted weakly. In other words, it was found that, in the surface layer, predominantly the crystal grains constituting the polycrystalline film had their {100} faces or faces close thereto located parallel to the substrate plane, while some crystal grains whose {110} faces or faces close thereto were parallel to the substrate plane coexisted. It was conjectured that at least half of the grains had their {100} faces located parallel to the major surface of the substrate.

Subsequently, an Si0₂ film 3 is deposited to a thickness of 5,000 A at a substrate temperature of 400°C by chemical vapor deposition (Figure 2b). Then, as shown in Figure 2c, the Si0₂ film 3 is provided with windows for source and drain regions. The size of each window was 300 x 200 (µm) and the gate length was 20 µm. Subsequently p ions at an energy level of 100 keV are implanted at a-dosage of 1 x 10¹⁶/cm², and the resultant substrate is heat-treated at 600°C in an N₂ atmosphere for 30 minutes, to form n⁺ layers 4 in the source and drain regions (Figure 2d). The depth of each n⁺ layer was 2,000 A.

In the next step, the Si0₂ is removed to leave a field oxide film 5 as shown in Figure 2e. An Si0₂ film 6 is deposited for a gate oxide film to a thickness of 7,500 A by chemical vapor deposition (Figure 2f). Holes for electrode contacts (each having the dimensions of 100 x 80 (µm)) are provided as shown in Figure 2g by photo-etching. After evaporating Al onto the whole surface of the resultant substrate, the Al is worked by photoetching to form a source electrode 7, a drain electrode 8 and a gate electrode 9 (Figure 2h). Thereafter, heat treatment at 400°C for 30 minutes is carried out in an H₂ atmosphere.

By these steps, a thin-film KOS field effect transistor of the structure in which the channel is provided in the surface layer of the polycrystalline silicon film is produced. This semiconductor device exhibits good and stable characteristics as a transistor.

Figure 3 shows a characteristic at room temperature of a MOSFET which was manufactured experimentally. The characteristic is the drain current I_D - versus - drain voltage V_DS characteristic with the gate voltage V_G being a parameter.

MOSFETs were produced using polycrystalline Si films having various structures. The threshold voltages of these MOSFETs were compared, and the results are listed in the following table.

Although above examples describe MOSFETs, the invention is applicable to other semiconductor devices, for example, bipolar transistors, diodes, and ICs (integrated circuits) or LSIs (large-scale integrated circuits) and such devices may be present in combination.

Although p-type silicon has been described n-type silicon can be produced by doping the silicon film with an appropriate impurity.