WO1989004052A1

WO1989004052A1 - Exposing substrates to ion beams

Info

Publication number: WO1989004052A1
Application number: PCT/GB1988/000916
Authority: WO
Inventors: William B. Thompson; Patricia G. Blauner; David Felsenthal; Robert S. Vincent; Henry C. Kaufman
Original assignee: Oxford Instruments Limited
Priority date: 1987-10-22
Filing date: 1988-10-21
Publication date: 1989-05-05

Abstract

Ion staining of a mask substrate (22) in a mask repair system by ion bombardment is controlled by introducing in the path of the ion beam, above the substrate, a material which interacts with the ion beam to avoid the staining inherent in ion irradiation of the mask substrate. In an ion beam containing, for example, gallium ions fur use in mask repair, and in particular for sputtered erosion of undesired opaque portions of a mask on a glass substrate, a protective layer or zone formed, for example, by water vapour, is introduced in the path of the ion beam. The introduced material reacts with the ions in the ion beam to avoid significant ion staining of the underlying glass substrate (22). The protective layer, in general, reduces the erosion rate and radiation damage to a substrate surface bombarded by energetic ion beams. As a result the protective layer is useful in minimising substrate damage in ion beam imaging, and in controlling ion implantation.

Description

OXFORD INSTRUMENTS LIMITED

EXPOSING SUBSTRATES TO ION BEAMS The present invention relates to methods and apparatus for exposing substrates to ion beams. Researchers in the last few years have made a considerable effort to refine the use of focussed ion beams in the repair of semiconductor masks. These masks are generally made of chrome or molybdenum suicide o patterns, approximately 1000A thick, on a glass substrate. During manufacturing opaque and clear defects unavoidably occur in the masks. Opaque defects typically consist of excess opaque material outside the desired pattern while clear defects consist of missing opaque material within the desired pattern. Until recently, repair of opaque defects was typically achieved by a laser system wherein the excess chromium was melted and then evaporated. However, as the dimensions of the features on the masks have shrunk to sub-micron levels, the laser technique has become inadequate. Presently, focussed ion beams offer an improved means of mask repair because they can achieve beam spot sizes of less than O.lμ and can be accurately positioned using electrostatic deflection. Focussed ion beams can repair masks either by eroding (sputtering) away excess material or, through interaction with an applied gas, by depositing opaque material.

The present invention finds particular application to opaque repairs. In opaque repairs, in addition to sputtering material from surfaces, the bombarding ions also create damage by implanting themselves into the glass mask substrate and by rearranging the surface structure of the substrate. Gallium ions are commonly used for ion beam mask repair and have been observed to damage the mask by leaving a greyish brown stain in the glass substrate which mask inspection machines may detect as a defect and which affects the quality of the images printed with the mask. The stain represents a region of the mask that is less transparent to light than the unbombarded glass itself. Staining thus leads to inspection difficulties and degrades the quality of the mask. Currently, the only means of removing such stains involve post-processing of the mask utilising an unwanted, extra step.

Ion beams can also be used, for example during mask repair, to image areas of a substrate by detecting secondary particles created by the impacting ions. During this imaging, undesirable substrate damage can occur from the incident ions.

In a further application, ion beams are used for implanting ions of a particular element into the substrate crystal structure, just below the s bstrata surface. For example, P or N type dopants may be added to a surface or subsurface layer of a semiconductor wafer by ion implantation. In most semiconductor applications the dopant concentration and its variation with depth are important parameters to be able to specify and control.

In accordance with one aspect of the present invention, a system for exposing a substrate to an ion beam comprises an ion beam source; ion beam control means for controlling the application of the ion beam from the source to the substrate; and supply means for supplying material to the path of the ion beam in the vicinity of a substrate to interact with the ion beam so as to reduce the affect of the ion beam on the substrate. in accordance with a second aspect of the present invention, a method for exposing a substrate to an ion beam comprises applying an ion beam to a substrate in response to a control; and introducing material in the path of the ion beam in the vicinity of the substrate to interact with the ion beam to reduce the affect of the ion beam on the substrate.

The present invention enables the radiation effect or damage to an ion bombarded substrate, typically a photomask to be minimised. For example, a system according to the invention can introduce a volatile or gaseous material into a vacuum chamber containing the substrate and apply the material directly above the substrate in the path of the incident ion beam. In the preferred example, a stainless steel tube is suspended over the mask surface and is fed by a manifold which regulates the flow of a volatile material, such as water, from a reservoir. Valves leading to the manifold are pneumatically operated by computer control to appropriately apply the volatile material.

In the case of focussed ion beam mask repair, the damaged area is viewed on a CRT by the detection of secondary electrons or ions created by the ion beam striking the mask surface. The operator outlines the area to be repaired on the CRT and the coordinates of that outline are recorded in^' computer memory. The computer then commands the appropriate valves to open, allowing the introduction of water vapour into the work chamber above the substrate. The ion beam under computer control is directed to trace out the area to be repaired causing sputter removal of unwanted opaque material. After pattern repair, the computer stops the water flow and a secondary electron or ion image of the repaired area is obtained to assure the operator of a successful repair.

It is believed that the incident ion beam's energy which can have values between 5 and 200 keV, is partially attenuated by adsorbed layers of the volatile material on the surface of the substrate. This attenuation of the ion beam energy has at least three effects. First, the reduction in energy of the ions which do enter the substrate shortens the implantation depth of the ions. Second, the radiation damage to the substrate is reduced. Third, the energy deposited in the layers of the adsorbed material can decompose the material on the substrate surface producing chemical interaction which reduce the staining of the mask.

In the preferred embodiment of the invention, the volatile material used is water. Water has the advantage of being easily handled while it does not contaminate the vacuum system, ion source, or photomasks. Water, in addition to providing a protective layer, decomposes into hydrogen, oxygen, and hydroxide ions. In the application to mask repair with gallium ions, it is believed that the introduction of water vapour res ts in the formation of gallium hydroxide, a liquid at Si?-?, which vaporises in the vacuum chamber, thus removing gallium from the substrate surface and reducing tHe degree of staining and other substrate damage. The vapour phase material may also be applied to the area of ion impact during imaging from detection of secondary particles. The application of this material reduces the substrate damage which results from the ion beam's interaction with the substrate during imaging. A vapour phase material may also be added in the ion beam path in ion implantation applications to provide a control over the implantation process.

An example of a system and method according to the present invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a diagram of the apparatus; and, Figure 2 is a flow chart representing the steps of mask repair practiced by the apparatus of Figure 1.

Figure 1 illustrates apparatus for ion beam imaging and photomask repair. The apparatus includes a focussed ion beam source 28 and associated optics placed above a vacuum, work chamber 20 for the application of an ion beam focussed to submicron dimensions at a substrate 22, such as a photomask, within the chamber 20. The optics of source 28 controls deflection of the ion beam within the chamber 20 to follow a prescribed course over the substrate 22 as is known in the art. The ion beam source 28 and vacuum chamber 20 are differentially pumped, to evacuate them of air, the ion beam passing between them through a small aperture.

A stainless steel tube 2 with 1mm inside diameter is suspended in a fixed position with its orifice approximately 0.03 inches (0.076cm) over the surface of the substrate 22. The substrate 22 is normally an opaquely patterned mask to be repaired by opaque material removal at a point of ion beam application. The mask is mounted on an X-Y positioning stage 24 and moves in response to computer activated control 23.

The tube 2 is fed from a manifold 4 which, with appropriate valves, controls the flow of a volatile material through tube 2 from a reservoir 10 during the mask repair process. The manifold 4 is used to control the flow rate of the volatile material held in a storage reservoir 10 via valves 16 and 18 under control of a computer 32. A vacuum pump 8 is connected to the manifold 4 by a computer controlled, pneumatically operated valve 12 and a variable leak valve 14. Valve

14 in conjunction with the pump 8 serves to control the pressure of the volatile material in the manifold 4. A low energy electron flood beam 26 may be applied to the substrate 22 during ion bombardment to achieve charge neutralisation of insulating substrate materials, as is known in the art.

Figure 2 illustrates the process of mask repair. in a step 40 the coordinates of the portion of the substrate or mask 22 to be repaired are recorded in a scan control memory of computer 32. In accomplishing this,, the ion beam is scanned over the damaged area of the mask generating secondary electron emission from the point of beam application. An electron detector 30 senses these electrons and controls the intensity of a spot on a CRT screen 31 at a position controlled by computer 32 to correspond to the position of the ion beam. In this manner, the CRT screen 31 is used to view the ion scanned area. An operator outlines the area of the mask 22 to be repaired by sputtering using, for example, a mouse control 25. This process records the X and Y coordinates of that area into the scan control memory of computer 32. The system then stands in a wait state 42 until given a command by an operator to proceed.

In step 44, in which an operator initiates further processing, valves 14, 16 and 18 are opened and valve 12 is closed by the computer 32. In this valve configuration, the material vapour is introduced into the vacuum chamber 20 through the tube 2. The gas flows from the manifold 4 into the work chamber 20 and over the substrate 22. Leak valve 14 limits the vapour pressure to avoid excess vapour pressure at the tube orifice. This causes the pressure in the work chamber 20 to rise. The pressure in the manifold at typical operating conditions is approximately 2 Torr for the exemplary tube to substrate distance and tube diameter given above.

When the pressure has reached a preset value, step 46 is entered. In step 46, the ion beam is deflected over and interacts with the material of the substrate 22 in the operator defined portion of the pattern to be repaired. The ion beam is applied for a preset computer controlled amount of time. The computer software controls the beam deflection, the step size between beam application points, and the beam's dwell time there. The pattern is repeated until each point has been exposed for a preset amount of time as is known in the art.

To complete the repair process, step 48 shuts down the system after computer controlled scan of the repair area by the ion beam is complete. Step 48 turns the ion beam from its beam source 28 off, closes valves 14, 16 and 18, and opens valve 12. After the pressure in the vacuum chamber 20 has dropped below a preset value, dependent upon the type of electron detector 30, an image of the repaired area is obtained in step 50 by secondary electron imaging to assure the operator of the completion of a successful repair.

In several experiments, the process of Figure 2 was practiced with water, both tap and pure water, in the reservoir 10. The repaired areas in the experiments, particularly with pure water as the applied material, showed a significant improvement in transmitting light at frequencies used for mask exposure compared to the case where mask repair was done without water. In imaging applications, vapour phase material is added to the vacuum chamber 20 to reduce the rate of substrate damage from the applied ion beam. In this application, the vapour is applied using the same process as utilised in ion beam repair except that pressures are adjusted to accommodate the requirements of an individual detector 30.

In a further application, a vapour phase material, such as water vapour, is applied during ion beam implantation to act as a control mechanism for the depth of ion penetration. In this application, the apparatus and process as described above may be used to apply a vapour to the region of ion beam application to control the ion beam penetration. In this case, the substrate 22 is a material such as a semiconductor wafer and the ion beam comprises ions to be implanted, as is known in the art.

Claims

CLAIMS 1. A system for exposing a substrate (22) to an ion beam, the system comprising an ion beam source (28) ; ion beam control means (32) for controlling the application of the ion beam from the source (28) to the substrate (22); and supply means (2,8-16) for supplying material to the path of the ion beam in the vicinity of a substrate (22) to interact with the ion beam so as to reduce the affect of the ion beam on the substrate (22) .

2. A system according to claim 1, wherein the material is adapted to reduce radiation damage to the substrate.

3. A system according to claim 1 or claim 2, wherein the supply means introduces a material having components selected from the group comprising hydrogen and oxygen.

4. A system according to claim 3, wherein the material comprises water.

5. A system according to any of the preceding claims, wherein the supply means introduces a vapour phase material.

6. A system according to claim 5, when dependent on claim 4, wherein the material is vapour phase water.

7. A system according to claim 5 or claim 6, further including means (14) for controlling the vapour pressure of the vapour phase material.

8. A system according to any of the preceding claims, •further including means for imaging the substrate in response to the ion beam.

9. A system according to any of the preceding claims, wherein the means for producing an ion beam includes means for scanning the ion beam across a predetermined area of the substrate (22) .

10. A method for exposing a .substrate (22) to an ion beam, the method comprising the steps of applying an ion beam to a substrate (22) in response to a control; and introducing material in the path of the ion beam in the vicinity of the substrate to interact with the ion beam to reduce the affect of the ion beam on the substrate (22) .

11. A method according to claim 10, wherein the material is adapted to reduce radiation damage to the substrate.

12. A method according to claim 10, wherein the material reduces staining of the substrate by the ion beam.

13. A method according to claim 10, for implanting ions into the substrate (22) , the introduction of material controlling the ion implantation.

14. A method according to claim 13, wherein the material introduction step controls the depth of implantation of ions into the substrate (22) .

15. A method according to any of claims 10 to 14, wherein the ion beam causes erosion of substrate material, the introduced material reducing the amount of erosion.

16. A method according to any of claims 10 to 15, wherein the introduced material includes components selected from the group comprising hydrogen and oxygen.

17. A method according to claim 16, wherein the material includes water.

18. A method according to any of claims 10 to 17, wherein the material is in the vapour phase.

19. A method according to any of claims 10 to 18, wherein the substrate (22) comprises a substantially transparent material.

20. A method according to claim 19, wherein the substrate (22) is glass.

21. A method according to claim 19 or claim 20, wherein a substantially opaque material is applied to the surface of the substrate facing the ion beam in a predetermined pattern.

22. A method according to claim 21, wherein the material applied to the substrate surface is selected from the group consisting of chromium and molybdenum suicide.

23. A method according to any of claims 10 to 22, wherein the substrate (22) is a photomask.

24. A method according to any of claims 10 to 23, wherein the ion beam includes gallium ions.

25. A method according to claim 24, when dependant on claim 12, wherein the introduced material reduces the generation of staining by gallium material on the substrate (22) .