JP3000688B2 - Radiation hardening protection device and artificial satellite for electronic circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for controlling the deterioration of semiconductor device characteristics caused by irradiation of radiation and extending the life of an electronic circuit.

[0002]

2. Description of the Related Art It is known that semiconductor elements mounted on artificial satellites and the like are exposed to cosmic rays and deteriorate or even break down. Among electronic circuits mounted on satellites, components whose characteristics are most likely to change due to radiation are semiconductor elements such as ICs. CMOS that operates with low power among ICs
Circuit is promising for space use, but MOSF with IC mounted
ET (Metal Oxide Semiconductor Field Effect Transis
tor) (hereinafter abbreviated as FET) tends to deteriorate in characteristics due to radiation.

[0003] Normally, maintenance of artificial satellites such as communication satellites after launch is impossible, so that it is necessary to reduce the deterioration of semiconductor elements due to cosmic rays as much as possible and to extend the life of electronic circuits. For this reason, the electronic circuits in the satellite are housed in aluminum shields that also function as heat sinks, and are designed to minimize the exposure dose. However, due to the weight of the satellite, the shield cannot be made unlimitedly thick. Therefore, irradiation experiments of the components of the electronic circuit are performed on the ground, and in consideration of the results and the life of the satellite, an allowable dose rate is determined and the thickness of the shield is determined.

However, in the universe, it is impossible to easily perform maintenance even if the equipment breaks down. Therefore, it is important to prepare a means for extending the life of the electronic circuit as long as possible even with the same thickness of the shield. is there.

For this reason, the following method has been considered as a means for recovering deterioration of characteristics of the FET due to radiation.

[0006] The main characteristic deterioration of the FET due to radiation irradiation is a change in threshold voltage and a decrease in channel mobility. In particular, the former threshold voltage change amount (hereinafter referred to as threshold voltage shift) is large, which causes a malfunction of the FET. It is known that the threshold voltage shift is recovered (annealed) depending on the temperature. As a means for recovering the deterioration of the FET due to radiation, this is used to store the FET in a high temperature environment (100 ° C.).
It has been conceived to recover the threshold voltage shift of the deteriorated FET by leaving it to the extent of about).

Conventionally, degradation of an FET due to an integrated dose has been evaluated by a dose rate of cosmic rays to which the FET is actually exposed, about 1 Gy /
h or less, a very high dose rate of 10 ² to 10 ⁴ Gy /
h, and it has been believed that the results of the ground tests are more reliable since the degradation of the device at higher dose rates is faster than at lower dose rates.

However, in recent years, it has been found that in a radiation environment, a recovery (annealing) phenomenon occurs simultaneously with damage to the device, so that the amount of deterioration of the device varies depending on the dose rate even with the same integrated dose. Therefore, evaluation based only on the results of irradiation experiments at high dose rates does not always lead to correct evaluation at low dose rates. That is, the threshold voltage shift of the FET shifts in the negative direction at a high dose rate, and shifts in the positive direction at a low dose rate.
"Total Dose Radiation and Annealing Study", IEE Transaction on Nuclear Science, Vol-3
3, No. 6, pp. 1343-1351 (1986)
(PSWinokur, et.al .: Total-Dose Radiation and Ann
ealingStudies, IEEE Trans. on NS, Vol. NS-
33, No. 6, PP. 1343-1351 (198)
6)).

The cause is considered as follows. The threshold voltage shift is a threshold voltage shift in the negative direction due to accumulation of fixed positive charges in the Si oxide film of the FET gate,
Recovery of the threshold voltage by recombination neutralization of fixed positive charges and electrons and shift in the positive direction due to an increase in the interface state at the Si / Si oxide film interface. And are approximately proportional to dose and approximately proportional to the logarithm of time. It is also known that the higher the temperature, the higher the recovery speed.

When the dose rate is low, that is, in the outer space, since the negative shift caused by the light is naturally recovered by the annealing phenomenon, the FET is deteriorated mainly by the positive shift. Therefore, it is considered that the conventional method of recovering the threshold voltage shift by annealing the device while maintaining the device at a high temperature is less effective in this case.

Annealing the interface state at a high temperature of 200 ° C. or more can be performed by P. Bookman, “Total Dose Hardness Assurance for Micro.
Circuit for Space Environment ”,
IEE Transaction on Nuclear Science, Vol. NS-33, No.
6, pp. 1352-1358 (1986) (P. Buchma
n,: Total Dose hardnessAssurance for Microcircuits
for Space Environment, IEEE Trans.on NS, Vo
l. NS-33, NO. 6, PP. 1352-1358
(1986)). However, 125 ° C. is the upper limit temperature for use of ordinary electronic parts, and annealing at a high temperature of 200 ° C. or more impairs the reliability of the electronic parts and is not practical.

In addition, during high-temperature annealing, it is necessary to stop the operation of the electronic circuit, and parts having no problem with radiation resistance, such as resistors and capacitors in the circuit, are simultaneously exposed to high temperatures. There is a problem that the property may be reduced.

As described above, the prior art described above does not consider the main factors of deterioration of the semiconductor element in outer space where the dose rate is low and does not consider the reliability of components other than the semiconductor element of the electronic circuit. There has been a problem that it is impossible to recover the degradation characteristics of the device, and there is a possibility that the reliability of components other than the semiconductor device may be reduced.

[0014]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method and an apparatus capable of reducing the deterioration of a semiconductor element due to radiation such as cosmic rays and achieving a longer life of an electronic circuit or the like. I do.

It is another object of the present invention to provide a method and an apparatus for preventing a decrease in reliability of electronic circuit components other than a semiconductor element.

[0016]

According to the present invention, in order to achieve the above object, a control means for controlling the amount of fixed positive charge existing in a semiconductor device is provided in an electronic circuit.

The control means controls the dose rate of the cosmic rays incident on the electronic circuit, so as to change the thickness of the shield, or control the temperature of the electronic circuit so as to lower it, or these means. Consists of combinations.

[0018]

The control means, that is, the means for controlling the thickness of the shield, reduces the thickness of the shield when the threshold voltage shift of the semiconductor element approaches the upper limit of the allowable range. As a result, the dose rate of cosmic rays is increased, the generation rate of fixed positive charges in the semiconductor device is increased, and the threshold voltage is intentionally shifted in the negative direction. Is compensated for in the positive direction. Thereby, the threshold voltage shift of the semiconductor element is reduced, the life of the electronic circuit is extended, and the radiation resistance is enhanced.

The latter control means, that is, means for lowering the temperature of the electronic circuit, controls the temperature of the electronic circuit to be lower than before. As a result, the annealing speed of the fixed positive charge in the semiconductor device is reduced, and the recovery of the negative shift component of the threshold voltage is delayed, thereby compensating for the positive shift component of the threshold voltage due to the interface state in the semiconductor device. Therefore,
The threshold voltage shift of the semiconductor device is reduced, the life of the electronic circuit is extended, and the radiation resistance is enhanced.

[0020]

Embodiments of the present invention will be described below. First, an embodiment in which a means for controlling the thickness of a shield to reduce the thickness of a shield in order to increase the dose rate of cosmic rays incident on the electronic circuit will be described with reference to FIGS. 1 to 3.

First, a description will be given of a degradation model of a FET element due to radiation. Although no FET degradation model has been established, it is considered approximately as follows. FE
The deterioration of T can be mainly expressed as a shift amount of the threshold voltage. The threshold voltage shift ΔV _T is obtained as the sum of the shift ΔV _ox due to the fixed positive charge and the shift ΔV _it due to the interface state.

[0022]

(Equation 1)

The unit dose γ-ray impulse γδ (t) is F
Assuming that the ET is irradiated, the shift Δ due to the fixed positive charge
V _ox and the shift ΔV _it due to the interface state are given by the following equations, respectively.

[0024]

(Equation 2)

A: Voltage shift due to fixed positive charge generated per unit dose (V / Gy) B: Annealing coefficient (V / Gy · h)

[0026]

(Equation 3)

C: Voltage shift due to interface level generated per unit dose (V / Gy) Therefore, the impulse response of the threshold voltage shift of the FET is as follows.

[0028]

(Equation 4)

Therefore, the threshold voltage shift ΔV _T at the time of irradiation at the dose rate γ (t) is represented by the impulse response ΔV _T0 (t) and γ
It is given as the convolution integral of (t).

[0030]

(Equation 5)

When irradiating at a constant dose rate γ, from Equations 4 and 5,

[0032]

(Equation 6)

[0033]

(Equation 7)

Here, Equation 6 represents ΔV _T during irradiation at a constant dose rate γ, and Equation 7 represents ΔV _T after irradiation.

Therefore, by using the above model,
Parameters A depending on the manufacturing process and structure of the FET,
If B and C are determined by experiments, a threshold voltage shift ΔV _T at an arbitrary dose rate can be obtained from Expression 5.

In this embodiment, the following parameter values relating to an n-channel FET obtained by an irradiation experiment of a commercial IC are used.

[0037]

(Equation 8)

[0038]

(Equation 9)

FIG. 1 shows a configuration diagram of this embodiment. The electronic circuit 4 comprises a fixed shield 5 and a movable shield 6 arranged on the outer periphery thereof.
Is shielded from radiation. The electronic circuit 4 is provided with a control circuit 7 for controlling the movable shield 6, and the control circuit 7 controls a driving device 9 of the movable shield 6 via a control line 8.

The thickness of the shield is determined as follows. FIG. 3 is a diagram showing the relationship between the dose rate received by the electronic circuit 4 and the aluminum shield thickness in a geosynchronous orbit of a satellite. In this embodiment, the dose rate of the cosmic rays received by the electronic circuit is designed to be 0.01 Gy / h, and the thickness of the shield is 3.5 cm in total. Conventionally, this thickness is the thickness of the shield. In this embodiment, the total thickness of the fixed shield 5, the movable shield 6, and the housing of the satellite is 3.5 cm. The thickness of the movable shield 6 is set to about 1.5 cm so that the dose rate received by the electronic circuit 4 when the movable shield 6 is removed is 0.1 Gy / h.
At this time, the thickness of the fixed shield is about 2 cm.

FIG. 2 is a diagram showing the relationship between the threshold voltage shift of the FET in the electronic circuit 4 and the elapsed time in the geosynchronous orbit.
Curve 1 shows the change in the threshold voltage shift when the dose rate is 0.1 Gy / h, and curve 2 shows the change in the threshold voltage shift when the dose rate is 0.01 Gy / h. These calculations were performed using the model and model parameters described above. Book F
The allowable range of the threshold voltage shift of ET is determined by circuit design conditions, and is within ± 0.1 V here. Curve 1 deviated from the above-mentioned allowable range below 1 × 10 ⁴ h, and curve 2
In this case, about 1 × 10 ⁵ h (t _{1 in} this figure, about 12 years) is out of the allowable range. Curve 3 shown by a solid line is the case of the present embodiment. In the present embodiment, about 1.4 × 10 ⁵ h (t in FIG.
₂ , about 15.7 years), the threshold voltage shift is within an acceptable range. Therefore, in the present embodiment, the electronic circuit 4 is the conventional one (curve 2).
In other words, the life was extended by 3.7 years.

The operation of the present embodiment will be described below with reference to FIGS. The control circuit 7 counts the time since the artificial satellite was launched and entered a geosynchronous orbit. When the time reaches a time t ₁ (FIG. 2) previously calculated using the model and the on-orbit dose rate, the control circuit 7 controls the driving device 9 via the control line 8 and To move.
In the present embodiment, the driving device 9 winds the movable shield 6 to reduce the total thickness of the shield to 1.5 cm of the thickness of the movable shield 6.
Make it thinner. At this time, only the fixed shield 5 having a thickness of about 2 cm is shielded.

As a result, the cosmic ray dose rate received by the electronic circuit 4 increases from 0.01 Gy / h to 0.1 Gy / h.
As the dose rate increases, the generation rate of fixed positive charges increases, so that the threshold voltage shift temporarily shifts in the negative direction. Hence, t ₁ since the threshold voltage shift to vary in the same trend as the curve 1, does not exceed the allowable range until t _2. Thus, the time during which the time remains within the allowable range (t ₁
−t ₂ ). Electric components such as resistors and capacitors in the electronic circuit 4 also receive radiation, but the radiation resistance of these components is one to two orders of magnitude better than that of FETs, which adversely affects the reliability of these components. Never.

In this embodiment, the time t _{1 at} which the movable shield 6 starts to move is calculated in advance.
A circuit for actually measuring the threshold voltage shift of the FET is added to the movable shield 6 when the measured value exceeds the allowable range.
May be moved.

Further, a sensor for measuring the cosmic ray dose rate is provided in the control circuit 7, and the time during which the threshold voltage shift exceeds the allowable range is calculated using the measured dose rate data.
The movable shield 6 may be moved. In the above embodiment, the dose rate is increased by changing the thickness of the shield, but alternatively, the dose rate may be increased by bringing the radiation source closer to the electronic circuit.

Next, another embodiment in which the radiation resistance is enhanced by controlling the temperature of the electronic circuit will be described. FIG. 4 shows the configuration of this embodiment. An electronic circuit 11 is installed in a shield 10 which also functions as a heat radiator, and a control circuit 13 takes in a signal of a temperature sensor 12 for measuring the temperature of the electronic circuit 11, and a heating / cooling device via a control line 14. 15 is controlled.

The operation principle of this embodiment will be described below. The FET used in the electronic circuit 11 is the same as the embodiment in FIG. Therefore, the model and its parameters are the same as above. The dose rate received by the electronic circuit 11 is 0.0.
1 Gy / h. The annealing constant of fixed positive charge (model parameter B) decreases as the temperature decreases. FIG. 5 shows the relationship between the threshold voltage shift of the FET and the elapsed time at a dose rate of 0.01 Gy / h. Curve 16 is normal temperature (20 ° C)
2 shows the threshold voltage shift in FIG. This curve is equivalent to curve 2 in FIG. Curves 17, 18, 19 as the temperature decreases
Thus, the threshold voltage shift changes. Curve 17 is when the value of parameter B is 0.95 times that at 20 ° C.
Curve 18 is at 0.9 times and curve 19 is at 0.8 times. The time during which the threshold voltage shift exceeds the allowable range ± 0.1 V is as long as about 1 × 10 ⁵ h for the curve 16, 1.4 × 10 ⁵ h for the curve 17, and 2 × 10 ⁵ h for the curve 18. That is, by reducing the annealing speed and delaying the recovery of the negative shift component of the threshold voltage, the positive shift component of the threshold voltage due to the interface state in the semiconductor device is compensated. However, when the temperature is too low, the negative shift component due to the fixed positive charge increases, and the time required to exceed the allowable range as shown by the curve 19 decreases.

Therefore, the dose rate received by the electronic circuit 11 and the FE
By controlling the temperature of the electronic circuit 11 in consideration of the model parameter of T, the fluctuation of the threshold voltage can be suppressed, and the radiation resistance of the electronic circuit can be improved.

Here, the control value of the temperature, depending on the type of FET, is several degrees to several tens of degrees, depending on the type of FET, compared to the high temperature of 100 to 200 ° C. required for annealing the fixed positive charge and the interface state. Range. Therefore, there is no adverse effect on the resistors and capacitors on the electronic circuit.

Of course, the embodiment of FIG. 1 and the embodiment of FIG. 4 can be used in combination. Further, the description has been made in an environment where the dose rate is constant. However, even in an environment where the dose rate varies depending on time, the above-described model can predict the time change of the threshold voltage shift, and thus can be similarly implemented.

FIG. 6 shows a configuration diagram when the embodiment of FIG. 4 is mounted on an artificial satellite. The shield 10 is fixed to the housing 23 of the artificial satellite by the stands 21 and 22. The heat generated by the electronic circuit 11 is transferred to the gantry 20 connected to the shield 10,
The heat is radiated to the outer space through the mounts 21 and 22 and the housing 23. Since heat radiation is performed only by heat conduction and heat radiation unlike the earth, the amount of heat generated by the electronic circuit 11 and the material, shape, and artificialness of the structures (the gantry 20, the shield 10, the gantry 21, 22 and the housing 23) It can be easily calculated by the amount of sunlight radiated to the satellite. Therefore, it is possible to design the structure such that the temperature of the electronic circuit 11 is brought as close as possible to the temperature corresponding to the curve 18 in FIG. 5 in advance.
The power consumption of the electronic circuit 11 at the time of design, that is, the amount of generated heat uses an average amount. However, the power consumption of an electronic circuit generally varies depending on the operating conditions thereof. Therefore, in actual use, the temperature is preferably adjusted by the heater / cooler 15 to keep the temperature constant. When the power consumption of the electronic circuit 11 is larger than the average value, the heater / cooler 15 operates as a cooler, and when the power consumption is smaller than the average value, the heater / cooler 15 operates as a heater. As aforementioned,
If the structure is designed in advance so that the electronic circuit 11 has a constant temperature on average in advance, the heating / cooling device 15 needs only a small ability to control the temperature change due to the change in the operation state of the electronic circuit 11. It is easy to apply the present invention to artificial satellites having a limitation. Further, the temperature control may not be performed on the entire electronic circuit 11, but may be performed only on the IC including the FET. Then, operation is possible with low power.

In the above embodiment, the case where the n-channel FET is used as the element has been described. FET has p
Although there is a channel element, the same can be applied to this case. FIG. 7 shows the relationship between the drain current and the gate-source voltage of the n-channel FET and the p-channel FET. The drain current characteristic of the n-channel FET before irradiation is indicated by a curve 24, and the characteristic of the p-channel FET before irradiation is indicated by a curve 25. The threshold voltage is defined as a gate-source voltage at which the drain current starts flowing, and is positive for an n-channel FET and negative for a p-channel FET.

When the characteristics of the device are deteriorated by irradiation with radiation, the drain current characteristic of the n-channel FET is a curve 26, and the drain current characteristic of the p-channel FET is a curve if the threshold voltage shift component due to the fixed positive charge is dominant. 27
And the threshold voltage moves in the negative voltage direction.
, The threshold voltage shift is negative regardless of the type. Conversely, when the threshold voltage shift component due to the interface state is dominant, the drain current characteristic of the n-channel FET becomes a curve 28, and the drain current characteristic of the p-channel FET becomes a curve 29, regardless of the type of FET. Becomes positive.

As is clear from the above, if the characteristic deterioration of the FET is evaluated based on the threshold voltage shift, all the embodiments of the n-channel device can be applied to the case of the p-channel.

[0055]

Since the present invention is constructed as described above, deterioration of the semiconductor element due to radiation such as cosmic rays can be reduced as much as possible, and the life of the electronic circuit can be prolonged. It is possible to provide a radiation-resistant electronic circuit that does not cause a decrease in the reliability of electronic circuit components other than the above.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of the present invention.

FIG. 2 is a graph showing a change with time of a threshold voltage shift of a semiconductor in the embodiment.

FIG. 3 is a graph showing a relationship between a dose rate and a shield thickness.

FIG. 4 is a schematic sectional view showing another embodiment of the present invention.

FIG. 5 is a graph showing a time change of a threshold voltage shift in the embodiment shown in FIG. 4;

FIG. 6 is a sectional view showing a schematic configuration when the embodiment of FIG. 4 is mounted on an artificial satellite.

FIG. 7 is a graph showing a relationship between a drain current and a gate-source voltage of an n-channel FET and a p-channel FET.

[Explanation of symbols]

1. Threshold voltage shift curve at a dose rate of 0.1 Gy / h, 2. Threshold voltage shift curve at a dose rate of 0.01 Gy / h, 3. Threshold voltage shift curve of one embodiment of the present invention,
4, 11 ... electronic circuit, 5 ... fixed shield, 6 ... movable shield, 7, 13 ... control circuit, 8, 14 ... control line, 9 ... drive device, 10 ... shield, 12 ... temperature sensor, 15 ... heating·
Cooler, 16: threshold voltage shift curve at room temperature at a dose rate of 0.01 Gy / h, 17: dose rate of 0.01 Gy / h
, The threshold voltage shift curve at a temperature lower than 16, 18 ...
Threshold voltage shift curve at a temperature lower than 17 when the dose rate is 0.01 Gy / h, 19 ... Threshold voltage shift curve at a temperature lower than 18 at a dose rate of 0.01 Gy / h, 20, 21,
22: mount, 23: housing, 24: drain current curve before irradiation of n-channel FET, 25: drain current curve before irradiation of p-channel FET.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H01L 29/78 (56) References JP-A-1-283873 (JP, A) JP-A-62-32641 (JP, A) ( 58) Surveyed fields (Int.Cl. ⁷ , DB name) H01L 21/8238 H01L 23/02 H01L 27/092

Claims (6)

(57) [Claims] [Claim 1] In order to suppress the variation in the positive direction of the threshold voltage caused by radiation of the interface state of the semiconductor element, the semiconductor
A radiation- resistant protection device for an electronic circuit, comprising: means for controlling the thickness of a shield for shielding a body element from radiation to control the fixed positive charge amount of the semiconductor element. 2. The semiconductor device according to claim 1, wherein said semiconductor device is provided with a semiconductor device.
A radiation protection apparatus for an electronic circuit, comprising: means for controlling a distance between a body element and a radiation source to control a fixed positive charge amount of the semiconductor element. 3. The semiconductor device is caused by radiation at an interface state of a semiconductor device.
In order to suppress the threshold voltage from fluctuating in the positive direction, the semiconductor
Control to lower the temperature of the semiconductor element,
A radiation-resistant protection device for an electronic circuit, comprising: means for controlling a fixed positive charge amount . 4. Due to the radiation of the interface state of the semiconductor device.
When the threshold voltage fluctuates in the positive direction and reaches a predetermined tolerance
Means for detecting when the semiconductor device has reached the allowable range.
Controlling the fixed positive charge amount of the body element to shift the threshold voltage in the negative direction.
And means for returning to within the permissible range.
A radiation-resistant protective device for electronic circuits , characterized by the following . 5. The semiconductor device according to claim 1, wherein said fixed positive charge amount control means includes a semiconductor element.
To reduce the thickness of the shield that shields the child from radiation
The radiation-proof protection device for an electronic circuit according to claim 4, wherein the device is a control unit. 6. Attributable to radiation of an interface state of a semiconductor element.
Cooling the semiconductor device to compensate for threshold voltage variations
Means for controlling the fixed positive charge amount of the semiconductor element.
Electronic circuit having a cooling source for the cooling
An artificial satellite characterized by the following .