GB2227701A

GB2227701A - Selective etching process

Info

Publication number: GB2227701A
Application number: GB8902727A
Authority: GB
Inventors: Stephen Rolt; Chester John Jones
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-02-07
Filing date: 1989-02-07
Publication date: 1990-08-08
Anticipated expiration: 2009-02-07
Also published as: GB2227701B; GB8902727D0

Abstract

A refractory surface film, e.g. of silicon nitride, on e.g. an integrated circuit is selectively removed by exposure to pulsed ultra-violet laser radiation at an energy density of at least 300 mJ cm<-2>. Each laser pulse removes a predetermined thickness of the surface film. Contact metallisation can be applied to the circuit via the windows so formed. The use of an argon fluoride excimer laser in a vacuum or a helium atmosphere is disclosed.

Description

SELECTIVE ETCHING PROCESS This invention relates to integrated circuits, and in particular to a method and apparatus for the selective removal of refractory surface films for integrated circuits.

Integrated circuits are conventionally provided with a passivating surface coating of a refractory material, typically silicon nitride. whilst this surface coating provides effective protection for the underlying circuit devices, particularly under high temperature conditions, the refractory nature of the coating renders subsequent inspection of the circuit, e.g. for failure analysis, extremely difficult. Attempts to remove refractory coatings using conventional techniques have not proved satisfactory and have generally resulted in damage to the underlying circuit.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a method of selectively removing a refractory surface film from an integrated circuit, the method including exposing selected regions of the surface film to pulsed ultra-violet laser radiation at an energy density of at -2 least 300 mJ cm whereby to evaporate and remove the surface film and expose the underlying circuit.

According to the invention there is further provided a method of contacting an integrated circuit, a method including providing a refractory insulating surface filter on said circuit, exposing selected regions of said film to pulsed ultra-violet radiation at -2 an energy density of at least 300 mJ cm whereby to evaporate the film from said regions and define windows exposing the circuit, and applying contact metallisation to the circuit via said windows.

We have found that, at ultra-violet wavelengths, refractory materials such as silicon nitride are strongly absorbing, whilst silicon is reflective. Thus, the refractory layer is ablated whilst the underlying circuit is substantially undamaged by the procedure.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic diagram of a selective etching apparatus; Referring to the drawing, the selective etching apparatus includes a pulse mode ultra-violet laser 11, e.g. an argon fluoride Eximer laser, the output of which is coupled via a mask 12 to a converging lens system 13. The output beam from the lens system 13 is directed via a beam steering mirror 14 on to a selected area of a workpiece 15. Typically the workpiece comprises an integrated circuit, or a silicon wafer on which a plurality of integrated circuits are disposed. The lens system concentrates the laser output so as to provide a focussed high intensity pattern on the workpiece 15.

The configuration of this pattern corresponds to the configuration of the mask 12; i.e. an image of the mask is projected on to the workpiece. Typically, the pattern covers an area of a few square millimctrcs and has features with a resolution of 0.5 microns or less.

The arrangement thus provides a high spatial resolution over a wide area. The beam steering mirror may be adjusted to align the pattern with particular features of the workpiece. In one embodiment of the invention, the pattern projected on to the workpiece may comprise an array of dots whereby etching of a silicon nitride film on an integrated circuit may be performed to provide an array of contact openings. Metallisation may subsequently be applied to the structure to contact the integrated circuit via the etched openings in the insulating film.

To effect removal of material from the workpiece surface, the laser is operated in a pulsed mode. The pulse repetition frequency may be between 1Hz and 500Hz. Conveniently we employ repetition rates of 20 to 100Hz. Alternatively, etching may be performed by a series of single shot steps.

The intense local heating provided by each laser pulse causes evaporation or ablation of a surface refractory film, typically of silicon nitride, from the workpiece surface. We have found for example that, for -2 an energy density of 400 mj cm , each laser pulse etches or ablates silicon nitride to a depth of 100A Thus, from a knowledge of the film thickness, the number of laser pulses required to etch through the film and expose the substrate can readily be calculated. The process is self-terminating as, at the wavelengths employed, the underlying silicon substrate is reflective and is thus substantially undamaged by the laser beam.

The etch depth (dt) to which material is ablated by said laser pulse is given, to a first approximation, by the relationship A t = to ln(E/Eo) where E is the energy density, Eo is the threshold energy density for which ablation occurs and t is a 0 constant. For silicon nitride we have found the -2 threshold energy density to be about 300 mJ cm Further, when etching silicon nitride, we prefer not to -2 employ energy densities above 450 mJ cm 2 as this can lead to damage of the substrate. Typically we employ an -2 energy density in the range 300 to 400 mJ cm We prefer to perform the etching or ablation process either under vacuum or in an atmosphere of an inert gas, and as helium which is transparent to the laser radiation. This permits dispersal of debris and prevents contamination of the newly etched surface.

The process may be used for the removal of refractory surface layers to facilitate inspection and examination of an integrated circuit. The process may also be employed for circuit modification wherein the remaining surface layer is employed ass a metallisation mask for the deposition of interconnections on the circuit.

Although the laser etching process has been described with particular reference to integrated circuits, it will be appreciated that it is not limited to the treatment of semiconductor devices.

Claims

CLAIMS.

1. A method of selectively removing a refractory surface film from an integrated circuit, the method including exposing selected regions of the surface film to pulsed ultra-violet laser radiation at an energy density of at least 300 mJ cm 2 whereby to evaporate and remove the surface film and expose the underlying circuit.

2. A method as claimed in claim 1, wherein the laser radiation is provided by an eximer laser.

3. A method as claimed in claim 2, wherein the laser is an argon fluoride laser.

4. A method as claimed in claim 1, 2 or 3, wherein the surface film comprises silicon nitride.

5. A method of contacting an integrated circuit, a method including providing a refractory insulating surface filter on said circuit, exposing selected regions of said film to pulsed ultra-violet radiation at -2 an energy density of at least 300 mJ cm 2 whereby to evaporate the film from said regions and define windows exposing the circuit, and applying contact metallisation to the circuit via said windows.

6. A method of selectively removing a refractory surface film substantially as described herein with reference to and as shown in the accompanying drawing.

7. An integrated circuit treated by a method as claimed in claim 5.