Temperature-stabilized Electronic Devices

Abstract

Vanadium oxide exhibits a substantial and relatively abrupt change in conductivity as the temperature of the material is varied through a particular characteristic temperature (about 68.degree. C.). In accordance with our invention, a vanadium oxide resistor is employed as a temperature-sensing element in the control circuit of a heater to stabilize the operating temperature of an electronic circuit at about 68.degree. C. over a wide range of ambient temperatures.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor devices; and, more particularly, to such devices which include provision for stabilizing their operating temperature for improved operating characteristics and improved reliability.

Most conventional semiconductor devices are inherently temperature sensitive; and, accordingly, these exhibit wide variations in certain of their operating characteristics in response to variations in their operating temperatures. Inasmuch as variations in ambient temperature produce corresponding changes in operating temperature if the device is not in some way temperature stabilized, the design of semiconductor circuits suitable for operation in a wide range of ambient temperatures is complicated by having to account for these variations.

In addition to causing variations in operating characteristics, temperature variations can cause other problems in semiconductor devices. For example, differences in thermal coefficient of expansion among the composite of different materials usually employed can cause intolerable strains and stresses when wide temperature variations are experienced. Further, at relatively low temperatures, e.g., room temperature and below, moisture can become absorbed on the surface of a device and can act as an electrolyte for interelectrode plating when operating voltages are applied to the device. This interelectrode plating is believed to be a major cause of device failure at the lower temperatures.

Heretofore, in the prior art, balanced bridge temperature control systems have appeared most promising from the standpoint of reliability and accuracy. See, e.g., U.S. Pat. No. 3,449,599, issued June 10, 1969, to J. J. Henry. However, these systems usually have been adapted for maximum flexibility and accuracy and not, apparently, for economy of either physical space or cost.

SUMMARY OF THE INVENTION

In view of the above and other considerations, it is an object of our invention to provide simple and inexpensive means for stabilizing the operating temperature of an electronic circuit at an elevated operating temperature.

It is a further object of our invention to reduce the size of a temperature control system.

To these and other ends, in accordance with our invention, there is provided a vanadium oxide (vo.sub.2) resistor as a temperature-sensing element in combination with electrically energizable heat-generating means for stabilizing the operating temperature of an electronic circuit over a wide range of ambient temperatures.

It is well known that certain crystalline compounds of the 3-d transition metals exhibit a substantial and relatively abrupt change in conductivity as the temperature of the material is varied through a particular characteristic temperature. See, e.g., U.S. Pat. No. 3,149,298, issued Sept. 15, 1964, to E. T. Handelman, and assigned to the assignee hereof. As is well known in the art, the 3-d transition elements are those elements numbered 21 through 28 in the Periodic Table of the Elements. In particular, monocrystalline samples of V0.sub.2 have exhibited changes in conductivity by a factor of greater than 10.sup.4 over a temperature interval of 2.degree. C. at about 68.degree. C. In deposited thin films of VO.sub.2 the conductivity transition usually occurs in a somewhat broader temperature interval (several degrees centigrade) centered at about 68.degree. C. and is not so substantial (10.sup.2 or greater). See, De.g., "Structural and Electrical Properties of Vanadium Oxide Thin Films," G. A. Rozgonyi et al., Journal of Vacuum Science and Technology, Vol. 5, No. 6, p.194, 1968. However, even using films having this less sharply defined conductivity transition, a temperature control system in accordance with our invention is capable of stabilizing the operating temperature of an electronic circuit to within several degrees centigrade; and this is more than sufficient for many applications.

More specifically, now, in accordance with a presently preferred embodiment of our invention, there is provided a vanadium oxide resistor in the bias circuit of a simple heater, De.g., a transistor, to maintain the controlled circuit at an elevated operating temperature. Both the heater and the vanadium oxide resistor advantageously are disposed in intimate thermal energy transfer relationship with the controlled circuit for optimum responsiveness. For example, both the vanadium oxide resistor and the heater advantageously are disposed in a monolithic semiconductor wafer along with the controlled circuitry.

In operation, as temperature increases through about 68.degree. C. the conductivity transition of V0.sub.2 causes the V0.sub.2 resistor to assume a relatively low resistance which causes the heater to be biased off. Conversely, as temperature decreases through about 68.degree. C. the V0.sub.2 resistor reverts to its high-resistivity state causing the heater to be biased on. This heating pushes the temperature back toward 68.degree. C.

It will be apparent that a temperature control system in accordance with our invention, as summarized hereinabove, is operable only for ambient temperatures below about 68.degree. C. since no cooling apparatus has been described. Further, it will also be apparent that cooling apparatus can be connected to our control system such that the cooling apparatus turns on when the V0.sub.2 switches to the low-resistance resistance state and so that the heater turns on when the v0.sub.2 switches to the high-resistance state.

However, cooling apparatus usually is complex, expensive, and bulky; and, therefore, is usually undesirable for use with miniaturized semiconductor devices to which our invention primarily is directed. Heating apparatus, on the other hand, can be virtually as simple, small and inexpensive as desired. Further, for many applications, the ambient temperature is not expected to exceed 68.degree. C. (about 155.degree. F.); and so for these applications cooling apparatus would be superfluous.

BRIEF DESCRIPTION OF THE DRAWING

The aforementioned and other objects, features, and advantages of our invention will be more readily understood from the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a graph of conductivity versus temperature for a typical material exhibiting the conductivity transition utilized by our invention;

FIG. 2 shows a schematic diagram of a simple control circuit in accordance with a presently preferred form of our invention;

FIG. 3 shows a cross section of one small portion of a semiconductor wafer including one possible integrated circuit embodiment of the control circuit of FIG. 2; and

FIG. 4 shows a generalized cross section of a temperature-stabilized integrated-circuit device in accordance with a presently preferred form of our invention.

It will be appreciated that for simplicity and clarity of explanation the figures have not necessarily been drawn to scale.

DETAILED DESCRIPTION

More particularly now, with reference to FIG. 1, a portion 11 of the graph shows a substantially constant and relatively low conductivity at temperatures below the characteristic temperature T.sub.c of transition. At temperatures above T.sub.c, portion 12 of the curve shows a substantially constant and relatively high conductivity. At T.sub.c, the transition temperature, conductivity ideally increases (or decreases) abruptly, as indicated by portion 13 of the curve. In imperfect crystals and thin films, however, the conductivity transition usually occurs over a temperature interval (several degrees centigrade), as shown by portion 14 of the curve. Particularly in thin films, there is usually some hysteresis in the conductivity versus temperature curve, as shown by portion 15 of FIG. 1. This hysteresis is at least partially due to built-in stresses in the films.

Referring now to FIG. 2, there is shown a schematic diagram of a simple control system in accordance with a presently preferred form of our invention. A transistor 21 is shown with the common node of a series pair of bias resistors 22 and 23 connected to its base terminal. Resistor 22, connected between the collector and base of transistor 21, is selected from the well-known resistors which maintain a fairly constant resistance over the temperatures to be experienced, De.g., a carbon resistor or any of a variety of impedance devices commonly used in integrated circuits. Resistor 23 includes a material which undergoes a conductivity transition such as described with reference to FIG. 1. Vanadium oxide (V0.sub.2) is presently preferred for resistor 23, for reasons discussed hereinbelow.

In operation, a voltage (+V) is applied to the collector of transistor 21 and the emitter is grounded. The resistor values are adjusted so that transistor 21 is biased on at temperatures below T.sub.c (conductivity transition temperature). The power dissipated in transistor 21 provides heat energy which tends to raise the temperature of both resistors. The resistor values are also adjusted so that when the temperature of resistor 23 increases through T.sub.c the resistance of resistor 23 decreases to a value small enough to cause transistor 21 to be biased substantially off.

Thus, for ambient temperatures less than T.sub.c, the operating temperature of any circuits in intimate thermal relationship with transistor 21 and resistor 23 will oscillate about T.sub.c. The amplitude of this thermal oscillation will depend upon the abruptness of the conductivity transition of resistor 23 and upon the amount of thermal hysteresis in that resistor.

In a particular embodiment which was constructed and tested, resistor 22 was about 7,000 ohms; resistor 23 varied from about 30,000 ohms at a temperature below T.sub.c to about 50 ohms at a temperature above T.sub.c ; and +V was about 7.5 volts. The operating temperature of transistor 21 was stabilized to within a few degrees centigrade at about 68.degree. C. over an ambient from room temperature to about 70.degree. C.

It will be appreciated that it would be desirable for some applications to provide a higher stabilized operating temperature, De.g., up to about 200.degree. C. so that a greater range of ambient temperatures could thereby be accommodated. However, no practical thin film materials are presently known to exhibit such a conductivity transition within the range of temperatures from about 70.degree. C. to 200.degree. C. One material, silver sulfide (Ag.sub.2 S), is known to exhibit a conductivity transition at about 170.degree. C.; however, this material exhibits ionic conductivity due to mobile silver ions under applied electric fields. Because of the instability resulting from this ionic conductivity, silver sulfide will generally be undesirable for most uses in accordance with our invention. Of course, if a suitable material is discovered, it may be substituted for resistor 23, as desired, within the scope of our invention.

With reference now to FIG. 3, there is shown a portion of a semiconductor wafer 31 including one of the many possible integrated circuit embodiments of the control circuit of FIG. 2. A semiconductive bulk portion 32, e.g., of P-type silicon, includes a pattern of localized conductivity type zones within the bulk and a composite of layered patterns formed thereover. N-type zone 33 provides the collector; P-type zone 34 provides the base; and N-type zone 35 provides the emitter for transistor 21. P-type zone 36 provides fixed resistor 22. N.sup.+ -type zone 37 provides a low-resistance path, within the bulk, around the aforementioned zones.

A pattern of dielectric material, e.g., silicon oxide, provides electrical isolation between electrodes on the surface of the semiconductor; and, additionally, provides a substrate upon which a pattern of vanadium oxide 39 can be formed to provide resistor 23.

An electrode 40 provides electrical connection to N.sup..sup.+ -type 37 and thus to the collector of the transistor. Electrode 41 provides electrical connection to the emitter of the transistor. Electrode 42 provides a common electrical connection to: the base of the transistor (zone 34); one end of resistor 22 (zone 36); and one end of resistor 23 (pattern 39). Electrode 43 provides electrical connection to the end of resistor 23; and electrode 44 provides electrical connection between the other end of resistor 22 and the collector of transistor 21 via semiconductive zone 37.

A plurality of methods for fabricating semiconductor integrated circuits similar to that shown in FIG. 3 are well known in the art. However, a brief description of some possible techniques for forming the vanadium oxide layers may aid the practitioner inasmuch as the particular technique used should depend upon the particular circuit in which the vanadium oxide resistor is used and upon the desired accuracy of temperature control.

It is known in the art that thin films of vanadium oxide formed on amorphous substrates, e.g., silicon oxide, usually do not exhibit a sharply defined conductivity transition. For example, as reported in the aforementioned publication by Rozgonyi et al. a thin film of vanadium oxide formed directly on glass by reactive sputtering exhibited a conductivity transition by a factor of only 77 with an hysteresis of 7.degree. C. As will be appreciated, this type of characteristic is suitable for those applications in which the operating temperature need not be so precisely controlled.

However, if more precise control is desired, a technique, such as disclosed in the copending U.S. application Ser. No. 776,732, filed Nov. 18, 1968, now U.S. Pat. No. 3,491,000, issued Jan. 20, 1970, and assigned to the assignee hereof, may be advantageous. As disclosed therein, if a thin layer of tantalum oxide (T.sub.2 0.sub.5) is formed over the amorphous substrate prior to forming the vanadium oxide layer, the conductivity transition of the vanadium oxide is more sharply defined.

A variety of techniques for forming the vanadium oxide also are well known in the art. For example, one can deposit a thin layer of vanadium and then convert this layer to its oxide, as disclosed by R. J. Powell in Stanford Electronics Laboratories Report No. 5220-1, May, 1967. Alternatively, reactive sputtering may be used to deposit directly the vanadium oxide layer, as disclosed in the aforementioned publication by Rozgonyi et al. Still another technique includes reactive evaporation of vanadium in an oxygen atmosphere followed by an annealing process, as disclosed in Philips Res. Rept., Vol. 22, p.170 (1967) by K. van Steensel et al.

With particular relation now to integrated circuits, FIG. 4 shows a generalized cross section of a presently common type of integrated circuit technology. A semiconductor wafer 51 having conductive beam-lead connections 52 and 53 is registered with and attached to corresponding, preformed conductive portions 54 and 55 on an insulating substrate 56, e.g., a circuit board, as disclosed, for example, in U.S. Pat. No. 3,426,252, issued Feb. 4, 1969, to M. P. Lepselter and assigned to the assignee hereof.

As presently contemplated, a control circuit embodiment, such as shown in FIG. 3, can be included as a small portion of wafer 51 in FIG. 4. There may be a plurality of wafers 51, each including its own control circuit, attached to a single insulating substrate. In this manner, the good thermal conduction properties of each semiconductor wafer enable close thermal relationship between the control circuit and the controlled circuits in each wafer.

Alternatively, there may be provided a separate wafer 51 providing the temperature control circuit mounted on a single insulating substrate along with other wafers which do not contain their own control circuits. In this case, the electrode connections (52-55) and the insulating substrate (56) are advantageously selected to provide close thermal relationship between the wafer(s) which contain the control circuit (s) and those wafers which do not.

Claims

What is claimed is:

1. Apparatus for maintaining an electronic device at a stabilized elevated operating temperature comprising:

A body of material including the device whose temperature is to be stabilized;

Electrically energizable heat-generating means mounted in intimate thermal energy transfer relationship with the body for heating the device; and

control means including temperature sensitive means connected to the heat-generating means for controlling the rate of heat generation in the heat-generating means; said temperature sensitive means comprising a resistor including material selected from the group consisting of the 3-d transition materials which exhibit a relatively abrupt change in conductivity at a transition temperature corresponding essentially to the desired elevated operating temperature, and being located to have its recited in claim 1 wherein the temperature means includes a resistor, controlled by the temperature of the body.

2. A device recited in claim 1 wherein the temperature sensitive means comprises vanadium oxide.

3. A device as recited in claim 1 wherein the temperature means includes a resistor, the operative part of which consists essentially of vanadium oxide.

4. An electronic device adapted for operation at a stabilized elevated temperature comprising:

a body of material including the device whose temperature is to be stabilized;

electrically energizable heat-generating means mounted in close thermal energy transfer relationship with said body for heating said body;

a vanadium oxide resistor in close thermal energy transfer relationship with said body for sensing the temperature of said body; and

circuit means to which said vanadium oxide resistor is connected for controlling the rate of heat generation in the heat-generating means.

5. A device as recited in claim 4 wherein the vanadium oxide resistor consists essentially of vanadium dioxide.

6. A temperature stabilized semiconductor integrated circuit device comprising in combination:

a body of silicon including at least one device whose temperature is to be stabilized; and

means for stabilizing the operating temperature of the device comprising electrically energizable heat-generating means in combination with a temperature sensitive means which includes a resistor consisting essentially of vanadium oxide.

7. A device as recited in claim 6 wherein the vanadium oxide resistor comprises a relatively thin layer of vanadium oxide disposed over the surface of the body.

8. Means for stabilizing the temperature of a body of material at an elevated temperature comprising in combination:

a transistor disposed in close thermal energy transfer relationship with the body for heating the body; and

a vanadium oxide resistor connected in shunt with the base emitter junction of the transistor for controlling the rate of heat generation in the transistor; said vanadium oxide resistor also disposed in close thermal energy relationship with the body so that its temperature is controlled by the temperature of the body.

9. Apparatus as recited in claim 8 further comprising means for biasing the transistor and the resistor by an amount sufficient to elevate the temperature of the body to the vanadium oxide transition temperature.

10. An electronic device adapted for operation at a stabilized elevated temperature comprising:

A body of material including the device whose operating temperature is to be stabilized;

Electrically energizable heat-generating means mounted in close thermal energy transfer relationship with said body for heating said body;

temperature sensitive means connected to the heat-generating means and mounted in close thermal energy transfer relationship with the body for controlling the rate of heat generation in the heat-generating means, the temperature sensitive means including a resistor, an operative part of which is a material which exhibits a conductivity transition at a predictable transition temperature; and

means for biasing the heat-generating means and the resistor by an amount sufficient to elevate the temperature of the body, the heat-generating means, and the resistor to the transition temperature.

11. Apparatus as recited in claim 10 wherein the resistor is coupled to the heat-generating means in such a way that heat is generated when the temperature of the resistor is substantially below the transition temperature and heat is not generated when the temperature of the resistor is substantially above the transition temperature so that the operating temperature is stabilized about the transition temperature.

12. Apparatus as recited in claim 11 wherein the conductivity transition material is a film of vanadium oxide having a transition temperature at about 68.degree. C.