Focused ion beam for thin film resistor trim on aluminum nitride substrates

A method for trimming thin film resistors. A focused inert ion beam is employed to selectively remove portions of a resistive film deposited on a substrate.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates generally to thin film resistors on aluminum nitride 
substrates and, more particularly, to a method for trimming the thin film 
resistors. 
2. Discussion 
Thin film resistors are used extensively in integrated circuits having 
resistors which require very precise resistance values. By controlling the 
deposition of a thin film of resistive material on an insulating 
substrate, the resistivity of the film as well as the resulting resistance 
can be controlled to within five percent of a target resistance. For 
circuits requiring even more precise resistances, laser trimming is the 
established method for providing fine adjustments to the resistance of the 
thin film resistor. By using a laser beam to etch away a predetermined 
pattern, or "trim kerf", of the resistive material from the substrate, the 
available conductive path is reduced and the resistance is, therefore, 
increased. Active trimming processes in which actual circuit performance 
is monitored as the result of trim can be used to adjust resistance to 
within one tenth of one percent. Precision of the trim monitoring 
equipment normally becomes the limiting factor in resistance accuracy. 
When an aluminum nitride substrate is used, however, unpredictable results 
have been obtained from the laser trimming process. Decreases in 
resistance after laser trimming have been found to occur, possibly as a 
consequence of the transformation of the aluminum nitride into highly 
conductive aluminum metal. An improved method for providing fine 
adjustments to the thin film resistor formed on an aluminum nitride 
substrate is, therefore, required. 
SUMMARY OF INVENTION 
The present invention employs a focused inert ion beam to selectively 
remove portions of the resistive film from the substrate in forming a thin 
film resistor. The ion beam can be focused more precisely than a laser, 
thereby providing even more precision in resulting resistance values. 
Also, trimming thin film resistors formed on aluminum nitride substrates 
with a focused ion beam does not result in the formation of aluminum metal 
and provides predictable results.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, a thin film resistor made in accordance with 
the method of the present invention is shown generally at 10 and includes 
a thin film of resistive material 12 deposited on an insulating substrate 
material 14. The thin film 12, preferably tantalum nitride, nickel 
chromium (nichrome), cermet, or other such material, has a thickness of 
about 300 angstroms and is formed on substrate 14 by vacuum deposition, 
sputtering, or other such process as is known in the art. Substrate 14 may 
be aluminum nitride or any other similar insulating material, the method 
of the present invention being particularly suited for thin film resistors 
formed on aluminum nitride substrates. Conductor pads 16 and 17 are 
deposited on the thin film 12, spaced from one another and spaced from 
substrate 14 by the film 12. Conductor pads 16 and 17 enable electrical 
coupling of the resistor 10 to an integrated circuit. 
A focused ion beam is employed to selectively etch away or remove portions 
of the thin film 12 to form an appropriate trim kerf 18, thereby exposing 
a corresponding area 20 of the substrate 14. This results in reducing the 
available conductive path between conductor pads 16 and 17 and, therefore, 
in increasing the resistance of the resistor 10. The ion beam, which 
generates less heat than a laser beam, does not cause the aluminum nitride 
substrate to be transformed into aluminum metal. 
The ion beam is preferably Argon, having a diameter of 500 to 10,000 
angstroms. Utilizing a material having fairly low resistivity, such as 
tantalum nitride or nichrome, for the thin film requires greater resistor 
surface area but provides a more accurate trimming process. In resistors 
made in accordance with the teachings of this invention, resistances 
within a very small percent of a target resistance would be obtained. 
While the invention has been described in connection with the presently 
preferred embodiments, one skilled in the art will readily recognize that 
various modifications can be made thereto without departing from the 
spirit and scope of the invention as set forth in the appended claims.