JPS62224031A - Bariation-proof - Google Patents

Abstract

PURPOSE:To obtain a radiation proof semiconductor device in which sufficiently long life is provided even under a circumstance of radioactive rays in existence, by piling two or more kinds of insulating films in which electrons are different in mobility from holes. CONSTITUTION:Two or more kinds of insulating films 4 and 5, in which electrons are different in mobility from holes, are piled in a semiconductor device having an insulating film on at least one part of the semiconductor substrate 6 surface. The radiation resistance is further improve by making layer thickness of an insulating film 4 arranged at a semiconductor substrate 6 side particularly 50% or less of the whole insulating film thickness. When at least one of two kinds of the insulating films 4 and 5 is composed of an insulating film having 3X10<17>/cm<2> or more in electron-trap density, the radiation resistance is further improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a semiconductor device having an insulating film such as an oxide, such as an integrated circuit, and in particular a semiconductor device whose electrical characteristics change even when exposed to radiation. Concerning semiconductor devices with little fear.

[Conventional technology]

Conventionally, in order to reduce changes in the electrical characteristics of semiconductor devices under radiation irradiation, optimization of the process temperature for forming an oxide film, which is an insulating film, high-temperature annealing in a nitrogen atmosphere after oxide film formation, or Attempts have been made to improve semiconductor manufacturing processes such as ion implantation into
n and J,R,I3rews:,,MOS Ph
ys-ics and Technology, p.
807-808), and in Japanese Patent Application No. 60-74372, a film with a thickness of 10 nm to 40 nm is deposited on a silicon substrate by a thermal oxidation method.
A method for manufacturing bipolar transistors is described in which a silicon oxide film of 100 nm in thickness is formed, and then an insulating film is laminated on the silicon oxide film at a temperature of 600°C to 850"C using a chemical vapor deposition method. However, no consideration has been given to the combination of MM films based on IJ and charge accumulation in two types of insulating films to be laminated.

[Problem that the invention seeks to solve]

In the above conventional technology, when a semiconductor device is used under radiation irradiation, positive charges are accumulated in the oxide film, and the amount of accumulated positive charges increases as the absorbed dose of radiation increases, as shown in FIG. It continues to increase.

As a result, if the absorbed dose becomes 106 rad or more.

For example, MOS (Metal-Oxide-3emic
(onducfor) As shown in FIG. 3 in a field effect transistor.

The threshold voltage changes by 0.5 V or more, resulting in a large change in electrical characteristics, making the device unusable.

In addition, although radiation resistance performance is improved in bipolar transistors with a two-layer insulating film, the amount of positive charge accumulated in the insulating film increases as the absorbed dose of radiation increases, similar to the above-mentioned MOS transistor. As a result, performance deterioration such as a decrease in current amplification factor occurs.

Furthermore, Figure 2 shows the amount of accumulated positive charge under the conditions of a silicon oxide film thickness of 200 nm and a gate bias voltage of +2V.
The figure shows the case where the silicon oxide film thickness is 50 nm and the oxide film forming process temperature is 950°C. The solid line in FIG. 3 indicates the p-channel, and the broken line indicates the n-channel.

An object of the present invention is to provide a radiation-resistant semiconductor device that has excellent radiation resistance and can provide a sufficiently long life even in an environment where radiation is present.

[Means for solving problems]

The above purpose is to form two or more types of insulating films with different mobilities of electrons and holes as an insulating film on the surface of a semiconductor substrate, so that the ratio of the mobility of electrons or holes between the insulating films is 2.
The above is achieved by arranging an insulating film with high mobility on the semiconductor substrate side.

In particular, the total thickness of the insulating film layer placed on the semiconductor substrate side is JI4
! Radiation resistance is further improved by making it 50% or less of the I edge film thickness.

Moreover, by forming at least one of the two types of insulating films with an insulating film having an electron trap density of 3×10″/− or more, the radiation resistance is further improved.

[Effect]

The present invention was developed based on research results of analysis of electron/hole pair generation and capture behavior in a semiconductor oxide film under radiation irradiation. That is, when a semiconductor device is irradiated with radiation, pairs of electrons and holes are generated in an insulating film such as an oxide film in an amount corresponding to the absorbed dose of radiation. Some of these electrons and holes are captured by defects in the insulating film, but since the mobility of electrons is much greater than that of holes, many of the generated electrons are trapped outside the insulating film. run away.

On the other hand, many of the generated holes are captured by defects in the insulating film and become the main cause of positive charge accumulation. A portion of the trapped holes are annihilated by recombination of the trapped holes and free electrons. In this way, under radiation irradiation, positive charges gradually accumulate in the insulating film by repeating the process of generating electrons and holes, moving, partially capturing them in the insulating film, and extinguishing the captured charges by recombination.
The performance of the semiconductor device deteriorates.

The density of free electrons and holes in the insulating film and the density of trapped charges can be obtained by solving the following group of equations. As independent variables, consider the time t and the spatial locus 'W4' in the direction of the insulating film thickness (curvature #l measured from the insulating film surface to the semiconductor substrate side) x.

(1) Free electron, hole density equation % formula % (2) Electron flow, hole flow (:3) Electric field equation, aE q ” (p + pt n nt) ... (5
) at ε ox (4) Capture? Shigeko, captured hole density equation % formula % (6) 5) i'li child, hole capture speed Tn = tnn
(Nn nt) '...(8) Tp=t
pp (Np pt) ... (9) (
6) Recombination rate of electrons and holes Rn = r n n p t
°= (10) Rp= rpp n t
-(11) (7) Capture charge amount and flat band shift amount Qt” p t n t
-(12) where n is the free electron density, p is the hole density, n( is the trapped electron density, pt is the trapped hole density, Jn is the electron current, Jp is the hole current, Go is the free electron production rate, and Gp is the Hole generation rate (anl Gp is determined from the radiation absorption dose), Tn is the electron capture rate, Tp is the hole capture rate, R7 is the recombination rate of free electrons and trapped holes, Rp is the recombination rate of holes and trapped electrons, μ . is the electron mobility, μP is the hole mobility, E is the electric field, D is the electron diffusion coefficient (1) o = μn K T / q: k is Boltzmann's constant, T is temperature, q is the amount of violet charge), Dp is the hole diffusion coefficient (Dp=μp KT/q), EOX is the dielectric constant of the insulating film, Nn is the electron trap density, and Np is the hole trap density.

Q, is the total captured charge, ΔVpa is the flat band shift, xo, l is the insulation film thickness, tn HjP + f'n
+ rp is a constant.

Examples of the results obtained by solving equations (1) to (13) above are shown in the fifth section.
As shown in FIG. FIG. 5 shows the distribution of accumulated positive charges in the oxide film of a silicon MOS capacitor with an oxide film thickness of 1000, using the irradiation dose as a parameter. From this figure, it is predicted that at a high irradiation dose of 10' Rad or more, the captured positive charges will be concentrated and distributed near the interface of the oxide film, which is consistent with the experimental fact of measuring the charge distribution. Figure 6 shows the relationship between flat band shift and γ-ray irradiation amount for a silicon MOS capacitor with an oxide film thickness of 1000.
It can be seen that the analytical results and experimental data are in good agreement. These results demonstrated the validity of the calculation models of equations (1) to (13).

Equations (6), (7), (12) and (13
), in order to suppress the flat band shift (tl! representative example of temporal characteristic quantity) due to trapped charges in the insulating film, it is necessary to (i) suppress the trapping of electrons and holes, especially in the vicinity of the semiconductor substrate. It is effective to suppress the capture of , (it) promote the disappearance of captured charges by recombination, and (iii) cancel positive charges by capturing deuterons. Regarding (i) above, as seen from equation (9), it is effective to reduce the hole density near the semiconductor substrate. For this purpose,
As shown in Fig. 4, when the insulating film of a semiconductor device is divided into an insulating layer A near the substrate and the remaining insulating film layer B, the insulation film is insulated so that the hole mobility of the A layer is larger than that of the B layer. It is effective to form a film to suppress the flow of holes from BM to Am and to increase the moving speed of holes in the A layer. Regarding (ii), as seen from equation (1o), it is effective to promote the recombination of trapped holes and free electrons near the semiconductor substrate, and this rounding requires electron transfer in both layers A and B. It is desirable to minimize the amount of electrons escaping from the insulating film. In particular, it is effective to make the electron mobility of the B layer lower than that of the A layer to prevent electrons from flowing out from AM to B[. Regarding (in), it is effective to increase the electron trap density.

Based on the above idea, an example in which the trapped charge density was analyzed for a semiconductor device whose gate insulator is made of two layers, A layer and B layer, of a silicon semiconductor as shown in FIG.
As shown in the figure. In this example, the hole mobility of the insulating film A was set to five times that of the insulating film B, and the film thickness of the insulating film A was set to 15% of the total film thickness. As can be seen from the figure, when the insulating film is composed of a combination of insulating film Δ and insulating film B, insulating film A
The trapped charge density in the vicinity of the silicon substrate is reduced compared to the case where only the insulating film B is used or the insulating film B is used. As a result, the flat band shift becomes smaller.

In FIG. 8, the ratio of the hole mobility μ^ of the insulating film A to the hole mobility μB of the insulating film F3 is selected to various values, and the ratio of the hole mobility μ^ of the insulating film A layer J
The results of analyzing the change in flat band shift when changing the total insulating film thickness XOx are shown. μ^/μB
is 2 or more and X^/xox is 0.5 or less, the flat band shift will be lower than in the case of a single insulating film.

Therefore, by employing such a two-layer oxide film, performance deterioration due to radiation irradiation can be minimized and the life of the semiconductor device can be extended.

Even if μ^'/μB' is selected to be 2 or more for the electron mobility μΔ' of the insulating film A and the electron mobility μB' of the insulating film B, the flat band shift is almost the same as shown in Fig. 8. Can be reduced.

Figure 9 shows that the electron trap density of the B layer is selected to various values,
The graph shows the change in flat band shift when the thickness of the insulation film A layer/total insulation film thickness is changed. If the electron trap density of the B layer is set to 3X1017 or more.

Flat band shift can be reduced, and performance deterioration due to radiation irradiation can be kept to a minimum.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. 10. In this embodiment, a thermal oxide film is first formed on a silicon semiconductor by a thermal oxidation method at a high temperature to a thickness of 50% or less of the total film thickness. ,
A deposited oxide film is formed thereon by chemical vapor deposition. FIG. 11 shows experimental values of flat band shift in the case of only a deposited oxide film and only a thermal oxide film. Hole and electron mobilities and electron trap densities were determined to fit these values. Based on the results, the thermal oxide film thickness and the total oxide film thickness of the two-layer oxide film were changed and the changes in the flat band shift were analyzed, and the results are shown in the same figure. This figure shows that if the thermal oxide film thickness/deposited oxide film thickness is selected to be 0.5 or less, the flat band shift can be suppressed to be smaller than that of a single oxide film. Therefore, according to this embodiment, performance deterioration can be suppressed to a small level even when used under radiation irradiation.

Another embodiment of FIG. 12 is shown. In this example, a nitride film is first formed on a silicon substrate at high temperature, and then a deposited oxide film is formed thereon. In this case, as in Example 1, the nitride film thickness is set to 50% or less of the total 11 thicknesses.

Since the hole mobility of a nitride film is much higher than that of a deposited oxide film, even if such a semiconductor device is used under radiation irradiation, performance deterioration can be suppressed to a small level.

FIG. 13 shows another embodiment. In this example, a thermal oxide film is first formed on a silicon substrate at, for example, 1000° C. or higher, and then a thermal oxide film is formed at, for example, 900° C. or lower. At this time, the thickness of the oxide film formed at 1000° C. or higher is set to be 50% or more of the total oxide film thickness. By doing this, it is possible to relatively increase the hole mobility of the dense oxide film formed on the silicon substrate side at a relatively low temperature of 90°C (1°C), thereby creating a semiconductor device with little performance deterioration due to radiation irradiation. can be realized.

Note that a method may be adopted in which an element other than the main constituent elements is injected into the semiconductor insulating film in a small amount or added at the time of forming the insulating film, and the mobility of electrons or holes is adjusted for each position in the film thickness. The above method can also be applied to adjusting the electron trap density in the insulating film.

〔Effect of the invention〕

According to the present invention, changes in electrical characteristics can be suppressed to a small extent even when used under radiation irradiation, so that the life of the semiconductor device can be significantly extended.

[Brief explanation of drawings]

FIG. 1 is an example of the present invention, FIG. 2 is a relationship between the amount of positive charge captured in the oxide film and the absorbed dose of radiation, and FIG. 3 is a diagram of the relationship between the threshold voltage change of a MOS transistor and the absorbed dose of radiation.
Figure 4 is an explanatory diagram of a two-layer insulating film, Figure 5 is an example of analysis of the distribution of positive charges trapped in an oxide film, Figure 6 is an example of comparison between analytical and experimental values of flat band shift of a MOS capacitor, and Figure 7 8 and 9 are relationship diagrams of flat band shift and two-layer film thickness ratio, FIG. 10 is an example of the present invention, and FIG. 11 is an example of flat band shift and two-layer film thickness ratio. A relationship diagram of layer 1 thickness ratio, FIG. 12.13, is another embodiment of the present invention. 1... Insulating film A, 2... Insulating film B, 3... Semiconductor substrate. 4... Thermal oxide film, 5... Deposited oxide film, 6... Si
substrate. 7... Nitride film, 8... Thermal oxide film formed at 900°C or lower, 9... Thermal oxide film 1 formed at 1000°C or higher,
)

Claims (1)

[Claims] 1. A radiation-resistant semiconductor device having an insulating film on at least a portion of the surface of a semiconductor substrate, characterized in that two or more types of insulating films having different mobilities of electrons and holes are stacked and formed. Semiconductor equipment. 2. A radiation-resistant semiconductor device according to claim 1, characterized in that two types of insulating films having a mobility ratio of electrons or holes of 2 or more are formed by overlapping each other. 3. A radiation-resistant semiconductor device according to claim 2, characterized in that an insulating film having high hole or electron mobility is disposed on the semiconductor substrate side. 4. A radiation-resistant semiconductor device according to claim 3, characterized in that the thickness of the insulating film disposed on the semiconductor substrate side is 50% or less of the total insulating film thickness. 5. In claim 1, the electron trap density of at least one insulating film is 3×10^1^7/cm^
A radiation-resistant semiconductor device characterized by having a radiation resistance of 3 or more.