JP2713979B2 - Method of forming insulating film - Google Patents

Description

The present invention relates to a method for forming an insulating film on a semiconductor crystal, and more particularly to a compound semiconductor crystal containing a phosphorus element as a constituent element, particularly indium. The present invention relates to a method for forming an insulating film in an insulator gate field effect transistor (MISFET) using phosphorus (InP).

(Prior art) Currently dominant in materials for microwave semiconductor devices
InP has attracted attention as a semiconductor element material that is expected to operate at a higher frequency than GaAs because of its characteristics such as high electron saturation velocity and high thermal conductivity, as compared to GaAs.

InP is difficult to form a good Schottky junction with small reverse leakage current like GaAs.
MISFETs using an insulating film-semiconductor junction as a gate have been mainly developed. One of the biggest problems in putting InP MISFETs to practical use is that a so-called current drift occurs in which the drain current varies with time. At present, there are many unclear points about the cause of the current drift, but it is mainly that the operating channel is modulated with time by the charge and discharge of electrons to the interface state existing at the insulating film / InP interface (MIS interface). This is considered to be one of the causes. Therefore MI
Reducing the interface state density of the S interface as much as possible is a necessary condition for reducing the current drift. However, since a group III-V compound semiconductor such as InP generally evaporates a group V element easily, when an insulating film is formed at a high temperature, deterioration of a semiconductor surface due to dissociation of phosphorus (P) which is a group V element is caused. As a result, defects are generated on the InP surface (MIS interface), which leads to generation of high-density interface states. For this reason, it is common practice to form an insulating film at a low temperature of, for example, 200 to 300 ° C. or lower. However, even when the insulating film is deposited at a low temperature, for example, in a method in which the insulating film is deposited in plasma, a surface defect occurs due to plasma damage, and an interface level also occurs.

For this purpose, various conventional methods for forming an insulating film, such as a thermal oxidation method, an anodic oxidation method, a chemical vapor deposition (CVD) method, and a light (C
Attempts have been made to form various insulating films, for example, In ₂ O ₃ films, anodic oxide films, SiO ₂ films, Al ₂ O ₃ films, Si ₃ N ₄ films, etc. by the VD) method or the like. However, an insulating film that can reduce the interface state density to a satisfactory level has not yet been found.
Therefore, in reality, an InP MISFET in which no current drift occurs has not been realized.

(Problems to be Solved by the Invention) As described above, in a compound semiconductor such as InP,
An insulating film capable of forming an MIS interface having good characteristics with a low interface state density and a deposition method thereof have not been found. For this reason, for example, the problem of current drift cannot be solved in the InP MISFET, which has been a major obstacle in putting the InP MISFET into practical use.

The present invention has been made to explain the above problem, and has a low interface state density, a compound semiconductor crystal having P as a constituent element thereof, and particularly an insulating film suitable for use as a gate insulating film of a MISFET using InP. It is an object to provide a forming method.

[Configuration of the invention]

(Means for Solving the Problems) According to a method for forming an insulating film according to the present invention, a compound semiconductor crystal substrate containing a phosphorus element as a constituent element is partially held in a reaction vessel, and nitrogen is contained in a chamber different from the reaction vessel. Nitrogen plasma generated by exciting the glass is introduced into the reaction vessel together with phosphine to form an insulating film containing at least phosphorus on the compound semiconductor crystal substrate. Further, the compound semiconductor crystal substrate in the above is one of indium phosphide and gallium phosphide. Further, the insulating film in the above is a phosphor silicate glass.

(Operation) The present invention prevents the semiconductor crystal substrate from being directly exposed to the plasma by generating nitrogen plasma in a chamber different from the reaction vessel for depositing the insulating film, and also generates the nitrogen plasma. By introducing nitrogen plasma into the reaction vessel together with PH ₃ before the insulating film deposition step, the semiconductor surface was stabilized, and the interface state density when the MIS interface was formed was found to be significantly reduced. It was done. Further, in the present invention, an insulating film containing phosphorus is formed after the above step, thereby preventing the dissociation of phosphorus from the surface of the semiconductor containing phosphorus.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view schematically showing an insulating film forming apparatus according to one embodiment. In FIG. 1, 1 is a microwave-excited plasma generator, 2 is a reaction vessel, 3 is an InP substrate, 4 is a susceptor for holding the InP substrate, and a heater is embedded in the susceptor. The InP substrate 3 can be heated to a desired temperature.

The plasma generator 1 is provided with two gas introduction pipes 5 for nitrogen and 6 for oxygen, and can be switched by a switching cock 7. The generated plasma is introduced into the reaction vessel 2 through the tube 8. The reaction vessel 2 is provided with introduction pipes 9 and 10 for introducing silane (SiH ₄ ) and phosphine (PH ₃ ), respectively. Further, a gas discharge port 11 is provided below the reaction vessel 2, and the gas discharge port 11 is connected to an exhaust pump (not shown) so that the pressure inside the reaction vessel 2 can be reduced to about 10 ⁻³ Torr. It has become.

A case where a phosphosilicate glass film (hereinafter, abbreviated as a PSG film) is formed on an InP substrate by using an apparatus having such a configuration will be described below. First, the InP substrate 3 is placed on the susceptor 4, and the inside of the reaction vessel 2 is evacuated to about 10 ⁻³ Torr from the outlet 11. Next, the heater embedded in the susceptor 4 is energized to heat the InP substrate 3 to a predetermined temperature, for example, 300 ° C.
At the time of heating, a predetermined amount of PH ₃ is allowed to flow into the reaction vessel 2 from the introduction pipe 10. When the substrate temperature reaches a steady value, a predetermined amount of nitrogen gas is introduced from the introduction pipe 5. After flowing the nitrogen gas, the plasma generator 1 is operated to generate nitrogen plasma, and is introduced into the reaction vessel 2 through the pipe 8. Note that PH ₃ is kept flowing from the introduction pipe 10. After maintaining this state for a predetermined time, the gas introduced into the plasma generator 1 is switched from nitrogen to a predetermined amount of oxygen by the switching cock 7. At this time, the plasma generator 1 may be kept operating or temporarily stopped. In this embodiment, the operating state is changed to oxygen gas. The introduction amount of PH ₃ was adjusted to the composition of the next PSG film simultaneously, SiH ₄ and subsequently
Flows into the reaction vessel 2 through the introduction pipe 9 to deposit a PSG film 12 having a desired thickness.

The specific resistance and dielectric breakdown electric field strength of the PSG film thus formed were 1 × 10 ¹³ Ω-c when the film thickness was 600 °.
m, 6 × 10 ⁶ V / cm. This can be sufficiently used practically as a gate insulating film for MISFET. Further, the metal electrode area having the MIS interface formed by the above-described method is 2 × 10
^-3 cm ² MIS diode was fabricated and C-
Measure V characteristics (bias is + 10 → 0 → −10 → 0 → + 10V
As shown in FIG. 2, characteristics with almost no hysteresis were obtained. The InP shown in FIG.
Is an n-type with a carrier concentration of 5 × 10 ¹⁵ cm ⁻³ and a PSG film thickness of about 600
It is about the case of Å. This indicates that the method of the present invention can form an MIS interface having a low interface level.

Further, a depletion-type InP MISFET using the film formed by the method according to the present invention as a gate insulating film was prepared, and the drain current drift was measured. As shown by the solid line in FIG. 3, the drift amount was extremely less than 1 mA. It was small. In FIG. 3, 5 V is applied between the source and the drain, and the gate bias voltage is set at time (t) = 0.
It shows the time change of the drain current when changing stepwise from 0V to -4V.In order to clarify the effect of the present invention, SiO formed by the normal pressure CVD method formed in the same shape is shown. The results of the InP MISFET using the _two films as gate insulating films are also shown by broken lines in the figure. This shows that the drift characteristics have been greatly improved.

In the above embodiment, the case where the insulating film is a PSG film has been described, but in the method of the present invention, prior to the formation of the insulating film, the nitrogen plasma generated in a plasma generation chamber different from the reaction vessel is used. a step (referred to as step a) introducing into the reaction vessel together with PH ₃ has played an important role, therefore, the insulating film, is not limited PSG film, or a P-doped silicon nitride film, phosphorous nitride film And so on.
Further, the step A and the subsequent step of forming an insulating film need not necessarily be performed continuously in the same container.
After that, an insulating film may be formed by another appropriate method. Further, in the above embodiment, the degree of vacuum in the reaction vessel is
Although the case where the temperature for forming a 10 ^-3 Torr film is 300 ° C. has been described, the present invention is not limited to these deposition conditions.

〔The invention's effect〕

As described above, the present invention has the following remarkable advantages. That is, when the interface state density at the MIS interface is significantly reduced and the MIS interface is applied to the gate insulating film for the MISFET, the drift amount of the drain current can be significantly reduced as compared with the related art.

In the above embodiment, the case where the semiconductor crystal substrate is InP has been described. However, excellent effects can be obtained by applying the present invention to a semiconductor containing phosphorus such as GaP.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an outline of an apparatus for forming an insulating film according to the present invention, FIG. 2 is a diagram showing CV characteristics of an MIS diode using an insulating film formed by the method of the present invention, FIG. 3 shows the time change of the drain current of the MISFET.
FIG. 4 is a diagram showing a result of comparison between an insulating film according to a method of the present invention and an insulating film according to a conventional method. 1 Plasma generator 2 Reactor 3 InP substrate 12 PSG film

Claims (3)

(57) [Claims] 1. A compound semiconductor crystal substrate containing a phosphorus element as a constituent element is partially held in a reaction vessel, and nitrogen gas generated by exciting nitrogen gas in a chamber different from the reaction vessel is produced together with phosphine. A method for forming an insulating film, comprising introducing an insulating film containing at least phosphorus on the compound semiconductor crystal substrate by introducing the insulating film into a container. 2. The method for forming an insulating film according to claim 1, wherein the compound semiconductor crystal substrate is one of indium phosphide and gallium phosphide. 3. The method according to claim 1, wherein the insulating film is a phosphor silicate glass.