GB2121980A

GB2121980A - X-ray masks

Info

Publication number: GB2121980A
Application number: GB8216847A
Authority: GB
Inventors: John Christopher Greenwood; David William Satchell
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-06-10
Filing date: 1982-06-10
Publication date: 1984-01-04
Also published as: GB2121980B

Abstract

In semiconductor manufacture, where X-ray irradiation is used, a thin silicon membrane can be used as an X-ray mask. This membrane has areas on which are patterns to define the regions to be irradiated. These regions are of antireflection material 3. With the thin, in the order of 3 microns, membranes used, fragility is a problem. Hence a number of ribs 1 of silicon are formed integral with the membrane, and which are relatively thick, 5 to 10 microns. The ribs may be formed by localised deeper boron deposition followed by a selective etch. <IMAGE>

Description

SPECIFICATION X-ray masks This invention relates to X-ray masks in which the portion of the mask which passes the X-rays is a thin silicon membrane, and to the irradiation of semiconductor wafers using such masks.

In certain forms of integrated circuit manufacture it is necessary to irradiate selected areas of an integrated circuit wafer with X-rays, and for this purpose an X-ray mask is placed against the surface of the work to be irradiated.

The mask is of a material non-transparent to Xrays, but with regions which are transparent to Xrays. Silicon is a suitable material for this purpose as long as it has regions thin enough to pass Xrays, but such thin silicon is relatively fragile. Thus it is an object of the invention to provide an X-ray mask in which the X-ray transparent regions are of thin silicon and are adequately supported.

According to the invention there is provided an X-ray mask which includes a thin membrane of silicon having integral with it a number of ribs on one face of the membrane so that the latter is subdivided into a plurality of discrete areas, wherein the ribs are formed in the silicon by localised boron diffusion to a depth greater than that used for making the membrane areas, which diffusion is followed by selective etching to leave the membrane areas separated by the ribs.

According to the invention there is further provided a method of processing a semiconductor wafer employing X-ray irradiation, which includes placing a thin silicon X-ray mask over the wafer to be etched, wherein the wafer is a thin membrane of silicon whose thickness is of the order of 3 microns which has integral with it a number of ribs on one face of the membrane, wherein each of the ribs is thicker than the membrane, the thickness of the ribs being of the order of 5 to 10 microns, wherein the mask is so located on the wafer that the ribs overlay some at least of the sawing channels of the wafer and that each membrane region overlays but is slightly spaced from a region of the wafer to be irradiated, wherein the X-rayopaque antireflection pattern which defines the portion of the wafer to be irradiated is on the face of the mask which when in use faces away from the wafer, and wherein the X-ray irradiation passes through the X-ray opaque pattern at its non-opaque areas and therefrom via the membrane to the wafer to effect the irradiation.

An embodiment of the invention will now be described with reference to the accompanying drawing, in which Fig. 1 is a view from its underside of an X-ray mask embodying the invention, Fig. 2 is a sectionalised representation of a mask such as that of Fig. 1 in-operative position, and Fig. 3 illustrates the use of the mask with an X-ray source.

Thin silicon X-ray masks need to be aligned optically with the pattern on the semiconductor wafer being X-ray irradiated, but because of the high absorption of silicon in visible light, good anti-reflection coatings are needed. These are usually deposited layers of silicon monoxide or silicon nitride, the former being preferred due to its high refractive index. However, these deposited layers have intrinsic stresses which cause the thin silicon membrane to go slack. Note that this in the present context means a thickness of the order of 5 microns.

In the arrangement to be described the line-up area is separated from the rest of the membrane by ridges of thicker silicon made by local deeper boron diffusion, or by local holding up of the etching. In the first case the etching takes place preferentially in the regions which have not been subjected to the deeper diffusion. Thicker areas thus produced are also useful for reinforcing other areas of the membrane, i.e. at its edges.

Fig. 1 is a view from the underside of a mask made in the manner indicated above. Although not shown in Fig. 1, the edges of the mask are also thickened to facilitate handling the mask.

Fig. 2 shows a mask such as that of Fig. 1 in use. Here the mask 1 overlies a semiconductor wafer 2 to be irradiated by X-rays. The mask has a layer 3 on its upper surface of a suitable X-ray opaque material, hafnium. The pattern of this layer determines the pattern of irradiation of the area, indicated at 4, to be irradiated. In the area used for lining up the mask with a pattern already on the wafer an antireflection coating is on the upper surface to reduce difficulties due to visible light reflection.

The mask 1 is so located on the wafer to be irradiated that its ribs overlie the sawing channels of the wafer. These channels are the regions of the wafer at which it is cut into the separate chips for further treatment. These ribs are, due to their greater thickness, substantially impervious to Xrays.

Thus we provide a mask which is more robust than is usually the case with thin silicon masks.

When the mask is in use, the wafer to be treated is only touched by the mask at areas other than those on which the circuit is produced. This prevents damage either to the circuit or to the mask.

It is appreciated that silicon membranes or diaphragms with ribs are not novel per se, but the masks hitherto used had ribs which extended the full thickness of the wafer from which the mask is made, and a typical thickness in such a case is 400 microns. This should be contrasted with a mask as made in accordance with this invention where the ribs' thicknesses are in the range of 5~10 microns. This would be controlled accurately at the chosen thickness, so that image spread, IS in Fig. 3, is constant from mask to mask.

Claims

1. An X-ray mask which includes a thin membrane of silicon having integral with it a number of ribs on one face of the membrane so that the latter is subdivided into a plurality of discrete areas, wherein the ribs are formed in the silicon by localised boron diffusion to a depth greater than that used for making the membrane areas, which diffusion is followed by selective etching to leave the membrane areas separated by the ribs.

2. An X-ray mask which includes a thin membrane of silicon whose thickness is of the order of 3 microns, and a number of ribs of silicon integral with the membrane, wherein each of the ribs has a thickness which is of the order of 5 to 10 microns, so as to be thicker than the membrane, and wherein the ribs are formed in the silicon by localised boron diffusion.

3. An X-ray mask substantially as described herein with reference to the accompanying drawing.

4. A method of processing a semiconductor wafer employing X-ray irradiation, which includes placing a thin silicon X-ray mask over the wafer to be etched, wherein the wafer is a thin membrane of silicon whose thickness is of the order of 3 microns which has integral with it a number of ribs on one face of the membrane, wherein each of the ribs is thicker than the membrane, the thickness of the ribs being of the order of 5 to 10 microns, wherein the mask is so located on the wafer that the ribs overlay some at least of the sawing channels of the wafer and that each membrane region overlays but is slightly spaced from a region of the wafer to be irradiated, wherein the X-rayopaque antireflection pattern which defines the portion of the wafer to be irradiated is on the face of the mask which when in use faces away from the wafer, and wherein the X-ray irradiation passes through the X-ray opaque pattern at its non-opaque areas and therefrom via the membrane to the wafer to effect the irradiation.

New claims or amendments to claims filed on 23.9.82 Superseded claims None New or amended claims

5. An X-ray mask which includes a thin membrane of silicon having integral with it a number of ribs on one face of the membrane so that the latter is subdivided into a plurality of discrete areas, wherein the ribs are formed in the silicon by localised boron diffusion to a depth greater than that used for making the membrane areas, which diffusion is followed by selective etching to leave the membrane areas separated by the ribs, and wherein each of the ribs has a thickness which is of the order of 5 to 10 microns, so as to be thicker than the membrane.

6. An X-ray mask as claimed in claim 1, 2 or 5, and wherein the membrane areas each have a layer of hafnium deposited on the regions which are not intended to pass X-rays.

7. A method of processing a semiconductor wafer employing X-ray irradiation, which includes placing a thin silicon X-ray mask over the wafer to be etched, wherein the wafer is a thin membrane of silicon whose thickness is of the order of 3 microns which has integral with it a number of ribs on one face of the membrane, wherein each of the ribs is thicker than the membrane, the thickness of the ribs being of the order of 5 to 10 microns, wherein the ribs have been formed in the silicon by localized boron diffusion to a depth greater than that used for making the membrane areas, which diffusion was followed by selective etching to leave the membrane areas separated by the ribs, wherein the mask is so located on the wafer that the ribs overlay some at least of the sawing channels of the wafer and that each membrane region overlays but is slightly spaced from a region of the wafer to be irradiated, wherein the X-rayopaque antireflection pattern which defines the portion of the wafer to be irradiated is on the face of the mask which when in use faces away from the wafer, and wherein the X-ray irradiation passes through the X-ray opaque pattern at its non-opaque areas and therefrom via the membrane to the wafer to effect the irradiation.

8. A method as claimed in claim 7, wherein the membrane areas of the mask each have a layer of hafnium deposited on the regions which are not intended to pass X-rays.