US3381255A - Thin film resistor - Google Patents

Description

p 1968 A. P. YOUMANS 3,381,255

THIN FILM RESISTOR Filed April 12, 1965 INVENTOR.

Albert P Youmans Attorneys United States Patent 3,381,255 THIN FILM RESISTOR Albert P. Youmans, Cupertino, Calif-L, assignor to Signetics Corporation, Sunnyvale, Calif., a corporation of California Filed Apr. 12, 1965, Ser. No. 447,206 6 Claims. (Cl. 338-308) ABSTRACT OF THE DISCLOSURE Thin film resistor formed of an alloy of chrome and silicon in which the silicon ranges from 55 to 73 percent.

This invention relates to a thin film resistor and material therefor particularly adapted for use in integrated circuitry.

Thin film resistors heretofore developed have, for example, utilized Nichrome, an alloy of nickel and chrome, and tantalum as resistive materials. It has been found that such thin film resistors have a distinct disadvantage in that their use is usually limited to a low ohms per square value as, for example, a maximum of 400 parts per million. Utilizing tantalum films at a substantially higher ohms per square as, for example, 1,000 to 2,000 ohms per square gives an undesirably large negative temperature coetficient of resistance (TCR). With the nickelchromium alloy, the resistive film is very thin even at 400 ohms per square and, therefore, it is difiicult to obtain much higher ohms per square values before the resistor becomes very thin and, therefore, unstable. There is, therefore, a need for a new and improved thin film resistor and material for making the same.

In general, it is an object of the present invention to provide a thin film resistor and material therefor which overcomes the above named disadvantages.

Another object of the invention is to provide a thin film resistor of the above character which has a relatively high ohms per square.

Another object of the invention is to provide a thin film resistor of the above character which is relatively stable through a range of temperatures.

Another object of the invention is to provide a thin film resistor of the above character which can be manufactured with substantially conventional techniques.

Another object of the invention is to .provide a thin film resistor of the above character which is particularly adapted for use in integrated circuitry. I

Another object of the invention is to provide a thin film resistor which makes it possible to obtain a substantially zero TCR.

Another object of the invention is to provide a thin film resistor of the above character which can be very thin and still be stable.

Another object of the invention is to provide a thin film resistor of the above character having excellent power handling capabilities.

Another object of the invention is to provide a thin film resistor of the above character which need not be encapsulated.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

Referring to the drawings:

FIGURE 1 is a top plan view showing an integrated circuit utilizing thin film resistors incorporating the present invention.

FIGURE 2 is an enlarged cross-sectional view taken along the line 22 of FIGURE 1, showing a thin film resistor.

In general, the thin film resistor consists of a suitable body of insulating material having a surface. A thin layer of an alloy of silicon and chromium is disposed on the surface to form a resistor. Leads are secured to the thin film to make electrical contact therewith.

As shown more particularly in the drawings, the thin film resistor incorporating the present invention is particularly adapted for use in integrated circuitry as shown in FIGURE 1. The integrated circuit shown in FIGURE 1 consists of a body 11 of a suitable semiconductor material such as silicon formed with a surface 12 which, as shown in FIGURE 2, may be planar. All of the body 11, or a substantial portion of the body 11, can be doped either during crystal growth or by diffusing either an n-type or a p-type dopant through one or more surfaces of the body 11 to provide a region 13 of one conductivity as, for example, an n-type conductivity as shown in FIGURE 2.

A silicon oxide layer 16 is formed at an elevated temperature in an oxidizing atmosphere on the surface 12 of the body 11. After the oxide layer has been formed, semiconductor devices are formed in the region 13 of the body 11 by conventional techniques. Thus, by way of example, openings may be formed in the oxide layer 16 and thereafter regions of opposite conductivity are formed in the region 13 diffusing a p-type dopant through the exposed area of the surface 12 into the region 13 to form a p-type layer or region. The oxide layer can then be regrown over the opening. When only a diode is desired, holes can be formed in the oxide layer to make electrical contact with the regions of opposite polarity. When transistors are desired, an opening is also formed in the oxide layer which has been regrown and an n-type dopant is diffused through the opening into the p-type region to form an ntype within the p-type region.Thereafter, the oxide layer is regrown and holes are formed therein so that contact can be made with the different regions to provide a transistor. Thus, as shown in FIGURE 1, there is shown a Zener diode 18 and a plurality of transistors 19 made in a conventional manner.

Leads 22 and contact pads 23 are also provided to make connections to various portions of the integrated circuit.

The structure thus far described is conventional and has only been described because it forms a part of the integrated circuit in which the thin film resistors are provided.

As also shown in FIGURE 1, the integrated circuit includes thin film resistors 26 embodying the present invention. As shown particularly in FIGURE 2, the thin film resistors are formed on a suitable body of insulating material which, in the embodiment shown in FIGURE 2, consists of a layer 16 of silicon dioxide which serves as a good insulator mounted on the body 11.

The :thin film resistors 26 are formed by depositing a thin layer of an alloy of chromium and silicon on the exposed surface of the silicon dioxide layer 16 in a predetermined pattern and thickness to provide a resistor of the desired characteristics. Thus, as can be seen in FIGURE 1, the resistors 26 are various sizes and have varying lengths with a plurality of convolutions or loops 26a. Additional thin film resistors 27 and 28 are provided. Means is provided for forming electrical contact with the resistive material which can be either in the form of the leads 22 or portions of other devices as, for example, transistors 19.

The thin film forming the resistors is deposited in a conventional manner as by heating the material or bombarding the same with electrons, both of which are conventional practices.

In particular, one method is to utilize vacuum evaporation with an electron beam. A chunk of the chromiumsilicon alloy is placed in a water-cooled copper crucible 3 4 and the electron beam is utilized to evaporate the chrowords, a high sheet resistivity. Secondly, they have a high mium-silicon alloy in the vacuum chamber so that it is temperature stability which includes a storage stability deposited upon the substrate in the desired predetermined and a long lead life under high temperature. The third pattern. Alternatively, vapor deposition can be utilized important advantage is the substantially zero TCR which which is very similar to that utilized for epitaxial growth. 5 can be obtained.

In this process, hydrogen reduction of silicon tetrachlo- Another di i advantage f th present thi film ride in a reaction chamber is utilized. The chromium gas resistor i h it h a l ti l hi h ow handling of P VaPOF of a chromium P P is capability. By way of example, it has been found that tfoqllced the sy so that chromium Is also under load life tests, approximately 500 milliwatts of posited with the silicon to obtain the desired ratio of power can be dissipated ova long Pgriods f time with. c miu and S'1 C out adversely aifecting the characteristics of the resistor.

By way of example, very satisfactory thin film re- In addition, it has been found that under such load life sistors formed of an alloy of chromium silicon have been tests, the TCR changes less than one-half of 1 percent manufactured using electron bombardment having a during the first 1,000 hours, and less than .1 of 1 perminimum resistance of 1,000 ohms per square and having 1 cent during the second 1,000 hours of the load life test. a TOR of 01-50 parts per million per degree Centigrade, It is believed that the resistors have particularly good the :50 parts per million corresponds to 005% per depower handling capabilities because they are deposited gree C. These examples are as follows: upon silicon dioxide layer 16 which has good contact Substrate Percent Percent Treatment Tempegature, Evaporation Rate Density Silicon Chromium at 550 C.

1 600 3 Angstroms per 4. 20 60. 1 39. 9 Vacuum.

second.

2 600 .do 3. 80 68. 6 31.4 All.

3 do 3.80 68.6 31.4 Air- It is well known that in thin films, the resistance of the with the silicon body 11 which itself is a good thermal resistor changes with thickness but will not change with conductor. The body 11 is normally attached or brazed an increase in length as long as the thin film is kept to a metal header which also has power dissipating capasquare. The above characteristics are typical of films with bilities.

a thickness range of less than 100 angstroms to over Although the thin film resistors incorporating the pres- 10,000 angstroms. entinvention have been shown in the drawings as not Each of the factors set forth in the table above will being encapsulated, it should be appreciated that, if dealfect the TCR, that is, the temperature coefficient of resired, the thin film resistors can be encapsulated. This sistance of the resistor. The values set forth above show may be particularly desirable when cross-overs which that it is possible to obtain a TCR of zero ithi th pass over the resistors are desired. Encapsulation can be range of 60.1 to 68.6 percent silicon. By examining the accomplished by evaporating an oxide over the resistor, phase diagram for chromium-silicon, it is believed that a either silicon dioxide or some other suitable insulating suitable resistor can be formed having a substantially oxide.

zero TCR with the silicon content of the alloy ranging It is apparent from the foregoing that I have provided from 55 to 73 percent by weight with a minimum ohms a new and improved thin film resistor and material for per square readily obtainable from 1,000 to over 5,000 use in making such thin film resistors. As pointed out h per Squaw above, the thin film resistors have many excellent char- It will be noted in Examples 1 and 2 that the acteristics and are particularly adaptable for use in intestrate temperature is relatively high, that is, 600 C. f i g The 'f film resistors, can be readily during the depcsition of the thin film for the resistor utilized 1n der lices which must operate in a temperature which serves to stabilize the resistor as it is being deof to Wlthout "E dlfliculty and positecL In Example 3 the resistor is Stabilized by the vvlth substantially zero TOR. The chromium-silicon alloy heat treatment after the alloy has been deposited. It is Is palmculairly usefiil as thm film reslstor fh desirable to have the resistor stabilized at these high temrelatively. Inexpensive and h excellent charactenstlcs peratu'res because in conventional manufacture of intewhen for thm film reslstors' grated circuitry, the devices are heat treated to 550 to I claim: alloy the aluminum which is used for the leads to the In thm film reslstorg a body of lnsulatlng diodes 18, the transistor 19, and also to the resistors 26. tena1.liavmg.a Surface a l layer i alloy of chrome Utilizing this material, it has been found that it is and silicon. dlsposed P Said .Sllrface m a predeiermined possible to obtain a minimum sheet resistivity substantial- Pattern Sald alloy havmg a slhcon i rapgmg from ly above 1,000 ohms per square if desired. Thus, for eX- to 3 and lead means makmg elecmcal Contact ample, it has been possible to obtain a sheet resistivity wltzh the thlglayen of 5,000 ohms per square. It has been found that such a In a t film resistor, a body of msulatmg mater al resistor is very stable and has all the desirable characteri i a.surface a thm layer an alloy of i i and istics pointed out above smcon disposed on the surface in which the silicon con- Althongh the present invention has been described as tent ranges from 55 Percent m 73 percent the thin layer depositing the chromium-silicon alloy upon insulating 5 a thlikness randgullg from i 100 'ingstmms to material such as silicon dioxide or silicon, the material angs roms an eads makmg electncal Contact can also be readily deposited upon any other suitable inwith the h layersulating material such as glass or ceramic 3. A thin film resistor as in clalm 2 having a minimum By Way of example, in one integrated circuit of the of 1,000 ohms per square and having a substantial zero type shown in the drawing, the resistor 27 had a value of 7 200 ohms and the resistor 28 had a value of 500 ohms. A thin film resistor as in claim 2 having a Substan- The largest resistor 26 had a value of 15,000 ohms. tially Zero TCR ranging from to It has been found that resistors of the present type A thin film resistor as in claim 2 having an Ohms have three very desirable characteristics or advantages. per square ranging from 500 ohms per square to over The first is a high ohms per square value or, in other 5,000 ohms per square.

5 6 6. A resistor as in claim 2 which is stabilized to a tem OTHER REFERENCES paramre above 550 Hansen, M.: Constitution of Binary Alloys, McGraw- References Cited Hill, New York, 1958, pp. 560-562 relied on. UNITED STATES PATENTS 5 LARAMIE E. ASKIN, Primary Examiner. 3,310,711 3/1967 Hangstefer 317-101 E GOLDBERG A E FOREIGN PATENTS sszstant xammer.

Ad. 10,657 3/1913 Great Britain.

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