JPH03265142A - Thin film transistor and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a sufficiently large crystal to make it possible to drive a thin film transistor with a high operating frequency and to enable application, the drive circuit part of a facsimile or a liquid crystal display, requiring a high-speed drive, by a method wherein the concentration of at least one of oxygen, nitrogen or a diluent gas element in the vicinity of the interface between a substrate and a polycrystalline silicon thin film is specified. CONSTITUTION:When a thin film transistor having an active layer consisting of a polycrystalline Si thin film is formed on a substrate, whose surface is insulated, the concentration of at least one of oxygen, nitrogen or a diluent gas element in the vicinity of an interface between the thin film and the substrate is set at 1X10<20>/cm<3> or higher. For example, an oxygen-doped amorphous Si thin film 3 and an oxygen- nondoped amorphous Si thin film 5 are laminated on a quartz substrate 1 by a low pressure plasma CVD method, but SiH4 gas and N2O gas are used as reaction gas, which is used at the time of this lamination, and the dose of the oxygen in the film 3 is set by adjusting the flow rate of the N2O gas in such a way that the oxygen concentration in the film 3 is 1X10<20>/cm<3>. After that, the films 5 and 3 are annealed in an N2 atmosphere, are solid-phase grown, are turned into a polycrystalline silicon thin film 7 and parts of this film 7 are respectively provided with a source region 13a and a drain region 13b.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a thin film transistor used in a drive circuit section of a reading device or a display device, and particularly relates to a thin film transistor capable of high-speed response and a method for manufacturing the same. Regarding.

(Prior Art) Conventionally, in a reading device, a display device, or the like, a document reading section or a display section and a drive circuit section are formed on separate substrates and connected by wire bonding or flexible wiring to form the device.

However, such a method is inferior in productivity or reliability of the device, and it has been impossible to provide inexpensive products to the market.

Therefore, attempts have been made to form a document reading section or a display section and a drive circuit section on the same substrate.

In order to make such a configuration possible, it is necessary to enable the elements constituting the drive circuit section to respond faster than the document reading section or the display section. For example, JP-A-60-2
In the solid-state image sensor described in Japanese Patent No. 2881, it is described that the document reading section uses amorphous silicon, which can be manufactured at low cost, and the drive circuit section uses polycrystalline silicon, which can respond at high speed. The polycrystalline silicon used for the drive circuit section is formed to a thickness of 2000 to 3000 angstroms by low-pressure CVD at 570°C, and after patterning, it is thermally oxidized at 1100 to 1150°C in an oxygen atmosphere to a thickness of 1500 angstroms. It is stated that good characteristics can be obtained by simultaneously forming an angstrom gate insulating film, growing the crystal grain size of the first layer of silicon thin film to form polysilicon, and further performing hydrogen plasma treatment.About the characteristics of the solid-state image sensor mentioned above. , “Materials from the 23rd workshop of the 147th Committee on Amorphous Materials, Japan Society for the Promotion of Science (HIJ
, 23) J states that the maximum reading speed for A4 size 8 pel/w is 2.6111 s/1 in.

With these characteristics, facsimile standard 63 (A
4th edition 8pel/mm maximum reading speed 10i s/1
ine), but it is impossible to satisfy the G4 standard (maximum reading speed of 2 ms/1 ine for A33 version 1 pel/mm), for which demand is expected to increase in the future.

A major factor that determines the reading speed is the operating speed of the drive circuit section, and in particular, it is thought that it depends on the thin film transistors that make up the drive circuit section.

In addition to reading devices, there is also a demand for a drive circuit section with an operating frequency of 10 Mz or higher for display devices such as liquid crystal televisions, but this is not possible with the thin film transistors described above.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has an object to provide a thin film transistor capable of high-speed response required particularly for facsimile machines or display devices, and a method for manufacturing the same.

[Structure of the Invention (Means for Solving the Problem) The thin film transistor of the present invention is a thin film transistor having an active layer made of a polycrystalline silicon thin film on a substrate whose surface is at least insulated, the thin film transistor comprising a polycrystalline silicon thin film substrate. Oxygen near the interface with.

The concentration of nitrogen or at least one of the rare gas elements is 1×
It is characterized by being 1020cIn-3 or more.

Further, the method for manufacturing a thin film transistor of the present invention is a method for manufacturing a thin film transistor that forms a thin film transistor having an active layer made of a polycrystalline silicon thin film on a substrate having at least an insulated surface, the method comprising: a step of increasing at least a concentration of oxygen, nitrogen, or a rare gas element near the interface of the silicon thin film with the substrate; and a step of annealing the silicon thin film to form a polycrystalline silicon thin film. It is characterized by having the following.

(Function) It is known that the improvement in the operating frequency of the drive circuit section can be achieved by increasing the electron mobility of the thin film transistors forming the drive circuit section.

Additionally, experiments have shown that increasing the electron mobility of thin film transistors can be achieved by increasing the crystal grain size of polycrystalline silicon. Figure 2 shows the results of an experiment conducted by the present inventors on the relationship between the electron mobility of a thin film transistor and the crystal grain size of polycrystalline silicon, with the vertical axis representing the electron mobility and the horizontal axis representing the crystal grain size. This shows the relationship between the two. In the figure, the curve (,a) shows a P-type MOS transistor, and the curve (b) shows an N-type MOS transistor.
It can be seen from these curves (a) and (b) that high electron mobility can be achieved by increasing the crystal grain size of polycrystalline silicon.

Therefore, in order to make a thin film transistor optimal for configuring the drive circuit section of a facsimile machine or a display device, it is necessary to use polycrystalline silicon with an electron mobility of at least 50cj/V・S. Then 1.
It is necessary to form a good polycrystalline silicon thin film having a grain size of 5 microns or more.

Therefore, the present inventors have determined that polycrystalline silicon formed on conventional insulating substrates has a thickness of about 0.5 microns (for example, the Institute of Electronics and Communication Engineers Silicon Materials and Devices Study Group SDM89-122).
When we investigated the reason why only particles with a particle size of (1989) ) could be obtained, we found that it was due to the following reasons. In other words, crystal nuclei occur more frequently near the interface between the substrate and the amorphous silicon thin film than in the amorphous silicon thin film, and the growth of the crystal nuclei is mutually suppressed. This is probably because polycrystalline silicon could not be obtained.

Furthermore, it is known that by doping an impurity such as oxygen, nitrogen, or a rare gas element near the interface, the growth of crystal nuclei near the interface can be exponentially reduced. (J
our own of Applied Ph1sics
, Vol. No. 10,0ctober 1977)
Therefore, the present inventor conducted various studies to obtain polycrystalline silicon with a crystal grain size of 1.5 microns or more so as to ensure an electron mobility of 50 cJ/V-8 or more. Or the concentration of rare gas is especially 1×1020
It has been found that by setting cIn-3 or more, it becomes possible to grow crystals with a large grain size of 1.5 microns or more.

This is because by setting the concentration of oxygen, nitrogen, or rare gas near the interface to I x 1020 cm -3 or more, the growth of many crystal nuclei generated near the interface doped with impurities can be sufficiently suppressed. This is thought to be because the growth of crystal nuclei in the film not doped with impurities is not affected.

Among these, it is preferable to set the oxygen concentration to 1×10 2 C11-3 or higher from an industrial standpoint and from the viewpoint of device characteristics.

When the concentration of at least one of oxygen, nitrogen, or rare gas near the interface of the silicon thin film with the insulating substrate is lower than IX1020C1l+-3, the growth of many crystal nuclei generated near the interface can be sufficiently suppressed. Since this is impossible, it suppresses the growth of crystal nuclei generated in a film not doped with impurities, and does not lead to the growth of large-grain crystals that provide the above-mentioned characteristics of a thin film transistor.

In the present invention, methods for increasing the concentration of at least one of oxygen, nitrogen, or rare gas elements near the interface of the insulating substrate of the silicon thin film include a method of ion implantation near the interface, and a method of depositing a silicon thin film containing the above impurities near the interface. Various methods can be used, such as a method in which the impurity is diffused into the vicinity of the interface during annealing using an insulating substrate containing the impurity in the vicinity of the interface.

Furthermore, the annealing temperature during the formation of the polycrystalline silicon of the present invention is preferably a relatively low temperature that can particularly suppress the generation of crystal nuclei, and is particularly preferably 570 to 600''C. By doing so,
Generation of crystal nuclei is suppressed in both films doped with impurities and films not doped with impurities, and large crystal nuclei can be grown in solid phase.

(Example) Hereinafter, a thin film transistor and a manufacturing method thereof according to an example of the present invention will be described with reference to the drawings.

FIG. 1 shows the manufacturing method of the thin film transistor of this example. First, as shown in (a) in the figure, an amorphous silicon thin film (3) doped with oxygen by low pressure plasma CVD method is placed on a quartz substrate (1). ) is deposited, and then an amorphous silicon thin film (5) not doped with oxygen is deposited. Monosilane (SiH) is used as a reactive gas when depositing the oxygen-doped amorphous silicon thin film (3). and - dinitrogen oxide (N20).Also, an amorphous silicon thin film (3
), the oxygen concentration in the amorphous silicon thin film (3) is at least 4×1020 CIIl.
The flow rate of dinitrogen monoxide (N20) was adjusted so as to be -3.

In this example, the oxygen-doped amorphous silicon thin film (3) and the non-oxygen-doped amorphous silicon thin film (5) were discontinuously formed.
) By gradually lowering the flow rate of dinitrogen monoxide (N20) during deposition, an amorphous silicon thin film (3) in which the oxygen concentration continuously decreases from the interface with the quartz substrate (1) is deposited. Also good. By doing so, it is possible to obtain an amorphous silicon thin film (3) in which good nucleus growth can be obtained.

As described above, there are two layers of amorphous silicon thin film (3).

After depositing (5), as shown in (b) in the figure, nitrogen (
N2) Annealing is performed at a temperature of 600° C. in an atmosphere for solid phase growth to form a polycrystalline silicon thin film (7).

Two layers of amorphous silicon thin film (3) as described above.

(5), the growth of a large number of crystal nuclei generated at the interface between the quartz substrate (1) and the oxygen-doped amorphous silicon thin film (3) during the annealing process is suppressed from the growth of the amorphous silicon thin film that is not doped with oxygen. (5) It is suppressed to a very low level compared to the crystal nuclei generated within. For example, if the oxygen doping amount is 4X1
The growth rate of crystal nuclei in the amorphous silicon thin film (3) with a temperature of 0''° is about 1740 times the growth rate of crystal nuclei in the amorphous silicon thin film (5) not doped with oxygen. Therefore, the crystal nuclei generated at a relatively low probability in the amorphous silicon thin film (5) that is not doped with oxygen are not hindered from growing by crystal nuclei generated at the interface, and the growth rate is 1.5. It is possible to grow to a sufficiently large crystal grain size of ~2.0 microns.

After this, a commonly used thin film transistor manufacturing process may be used.

For example, as shown in FIG. 1(c), after buttering the polycrystalline silicon thin film (7) obtained in the above step,
The surface is heat-treated to obtain a gate insulating film (9).

Then, if necessary, impurities are doped to form a channel. This channel-forming impurity doping may be performed, for example, when depositing an amorphous silicon thin film that is not doped with oxygen, but since controlling the growth of crystal nuclei in the annealing process becomes complicated, 7> Preferably carried out after formation.

Then, as shown in FIG. 1(d), the gate electrode (11
) is formed, and then, using the gate electrode (11), ions are implanted into the regions where the source and drain are to be formed to form a source region (13a) and a drain region (13b). For example, boron may be used to obtain a P-type MOS transistor, and phosphorus or arsenic may be used for an N-type MOS transistor.

Further, as shown in FIG. 1(e), after depositing an insulating film (21) of silicon oxide or silicon nitride, etc., contact holes (15a) and (15b) are opened, and further AI, Al-5i, A metal thin film (17a) for wiring is formed by depositing and patterning Al-5i-Cu or the like.

By obtaining (17b), the thin film transistor is completed.

In the thin film transistor thus formed, the crystal grain size of the polycrystalline silicon thin film forming the active layer is 1.5.
Since it could be made sufficiently large, such as ~2.0 microns, it was possible to secure an electron mobility of 50 cj/V·S or more. Therefore, the thin film transistor of this embodiment can be driven at a high operating frequency, and is particularly suitable for facsimiles or liquid crystal display drive circuits that require high-speed driving.

Here, monosilane (S iH4) and dinitrogen oxide (N
20) was used to form an oxygen-doped amorphous silicon thin film (3) with an oxygen concentration higher than I x 1020 an-'', but instead of dinitrogen oxide (N20), for example, ammonia (NH4) was used. Alternatively, a nitrogen-doped amorphous silicon thin film (3) may be used.

In addition, oxygen or the like may be doped at the interface between the substrate and the amorphous silicon thin film using an ion implantation method. The accelerating voltage or doping amount at this time may be appropriately selected in consideration of the thickness of the amorphous silicon thin film, etc. For example, when the thickness of the amorphous silicon thin film is 1500 angstroms, the accelerating voltage is set to 6° keV, Dope amount 5X10
It is best to dope oxygen to about -2 degree. In this way, the peak value of the oxygen concentration at the interface between the substrate and the amorphous silicon thin film is 4×10 20 cIn −3 , and the growth rate of crystal nuclei near the interface can be suppressed to a low level. Therefore, a polycrystalline silicon thin film with sufficiently large crystals can be formed.

Furthermore, in addition to doping the amorphous silicon thin film with oxygen, etc. using the ion implantation method as described above, the same effect can also be obtained by doping the substrate surface with oxygen, etc. using the ion implantation method. . In other words, even if an attempt is made to form a polycrystalline silicon thin film by depositing and annealing an amorphous silicon thin film after ion implantation in this way, impurities such as oxygen, which are present in high concentrations in the substrate near the interface, will be deposited during annealing. Since the impurity concentration is diffused near the interface of the amorphous silicon thin film, the growth rate of many crystal nuclei generated near the interface is sufficiently suppressed, and the growth of crystal nuclei within the film is not hindered. It's for a reason.

Even when such a method of implanting ions into the substrate surface is used, oxygen, nitrogen, or a rare gas can be used as in the embodiments described above.

In the above-described example, impurities such as oxygen are directly added to the substrate or silicon thin film near the interface, but it is also possible to use a substrate that contains impurities such as oxygen at a high concentration in advance. When silicon ions are implanted at the interface after a silicon thin film is deposited on the substrate using such an insulating substrate, the area near the interface becomes amorphous because it contains impurities such as oxygen at a high concentration, and large numbers of silicon ions are generated near the interface. The growth of crystal nuclei can be sufficiently suppressed. Therefore, a polycrystalline silicon thin film having large grain size crystals suitable for the present invention can be formed.

[Effects of the Invention] As described above, the thin film transistor of the present invention achieves a thin film transistor of the present invention by setting the concentration of oxygen, nitrogen, or a rare gas element near the interface with the polycrystalline silicon thin film of the substrate to 1×102°Cl11−3 or more. , a sufficiently large crystal can be obtained,
Driving at a high operating frequency becomes possible. Therefore, the thin film transistor of the present invention is particularly suitable for use in facsimiles or liquid crystal display drive circuits that require high-speed drive.

Furthermore, by using the thin film transistor manufacturing method of the present invention, a high performance thin film transistor that can operate at a high operating frequency can adjust the concentration of oxygen, nitrogen, or rare gas near the interface with the substrate within a predetermined range. It was easy to manufacture.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing a method for manufacturing a thin film transistor according to an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between average crystal grain size and electron mobility. (1)...Quartz substrate (3)...Doped amorphous silicon thin film (5)...Undoped amorphous silicon thin film (7>...Polycrystalline silicon thin film)

Claims (2)

[Claims] (1) In a thin film transistor having an active layer made of a polycrystalline silicon thin film on a substrate whose surface is at least insulated, at least one of oxygen, nitrogen, or a rare gas element near the interface between the polycrystalline silicon thin film and the substrate. concentration is 1
A thin film transistor characterized in that it is 10^2^0cm^-^3 or more. (2) A method for manufacturing a thin film transistor in which a thin film transistor having an active layer made of a polycrystalline silicon thin film is formed on a substrate with at least an insulated surface, the step of depositing a silicon thin film on the substrate with at least an insulated surface; The method further comprises: increasing the concentration of at least one of oxygen, nitrogen, or a rare gas element near the interface between the silicon thin film and the substrate; and annealing the silicon thin film to form a polycrystalline silicon thin film. Characteristic method for manufacturing thin film transistors.