GB2200219A - Ultra-violet photo-ablative development of X-ray exposed photoresists - Google Patents

Abstract

Ultra-violet photoablation can be used as a non-chemical means of developing patterns that have been exposed onto photoresist by x-ray or electron beam lithography. The latent image in the resist or polymer is developed by ultra-violet photoablation to remove the exposed or unexposed regions. The lithography or pattern exposure can be accomplished by any damage inducing (i.e. chemical bond breaking) radiation or particles of sufficient energy and intensity. The photoablative development can be carried out by any electromagnetic radiation source of sufficiently high energy and intensity.

Description

ULTRA - VIOLET PHOTO - ABLATIVE DEVELOPMENT OF X~RAY PHOTORESISTS.

The present invention relates to a process for developing patterns in photoresists material.

Photoresist material is well known in the semiconductor industry and is to to protectively mask intricate patterns in integrated circuits during manufacture. Typically, a silicon substrate is coated with a thin layer of photoresist. Using microl ithography, a pattern is exposed in the photoresist, which, in the case of a so called postive resist, becomes more resistant to chemical solvents in the regions subjected to the light. On immersion in the appropricate solvent the unwanted photoresist is dissolved and hence removed from the substrate. This leaves a protected pattern on the resist, allowing the net stage of manufacture, typically doping of the silicon, to occur only in the selected areas.

In order to obtain finer line widths in the micro-circuits, the lithograpny is bein-, done with shorter and shorter wavelength light. Kuch attention is being given to using x-ray radiation of around 1 nm wavelength. Both positive and negative photoresists exist that function at this wavelength. These resists are developed chemically, which has a numoer of disadvantages, including undercutting of the pattern, swelling on immersion and shrinking on drying.

These cause problems with producing very fine lines ano in maintaining consistancy of line wi Ultra-violet photo-ablation is a well known phenomenon whereby certain organic polymers are etched away when eosed to very intense ultra-violet light. The process occurs without thermal phase changes occur In in the polmer left behind. The ultra violet light is usually provided by excimer lasers.Work on photo-abla tic in microcircuit fabrIcatIon has assumed that the pattern would be cirect;y etched into the photoresist, with the ultra-violet light being suitably modulated, or the photoresist masked so as to attack only the unwanted areas of photoresist. This has limitations on the linewidth possible, imposed by the wavelength of light used, which is from 157 nm to 308 nm though typically 193 nm radiation is used.

According to the present invention, the pattern e::osed in the photoresist material x-ray lithography is developed by means of direct photo-ablation using a powerfull ultra-violet source, typically a laser. The development relies on the fact that the change in the polymer caused by exposure to x-rays alters the rate at which photo-ablation occurs. Because there is a threshold of intensity 'below which photo-ablation does not occur for a particular polymer, it should be possible to choose an intensity of development which ablates the xray exposed regions, but not the unexposed (or vice versa).

The initial lithographic exposure may also be done at other wavelengths.

Any radiation that causes change or damage to the chemical structure, from ultra- violet to gamma ray radiation, could in principle be used for the initial iithography. Similarly damage by particals, such as electrons, neutrons and alpha particles could be employed for the lithographic step.

Photo ablation also occurs at x-ray wavelengths and, given the current rapid progress in research into intense x-ray sources such as soft x-ray lasers and undulators, it may in future become advantageous to use these shorter wavelengths for the ablation step.

A specific embodiment of the invention will now be described by way of example, with particular reference to the accompanying drawings in which: Figure i shows an elevational view of the development apparatus; Figure w shows a plan view of the apparatus for developing the resist; Figure 3 shows a cross section of the exposed resist before development; Figure 4 shows a cross section of the exposed resist after development by photo-ablation.

The silicon wafer 4, coated with a thin (typically O.5pm ) layer of an ray sensitive photoresist such as poly methyl methacralate 7 & S, has a pattern exposed on it by contact microlithography using x-rays, typically in the 1 nm region. The result of this is that the exposed regions of the photoresists 7 is substantially changed, # due it is thought to breaking of bonds In the polymer The development of this pattern into a relief profile is tnen cone usIng hi repltltion rate excimer laser 1. These produce light in the ultra-violet region between 157 and 308 nm but typically at 193nm (e.g. the Lamda Physik model 201 MSC).The result of the development is illustrated in figure 4, where the unexpose photoresist 8 remains or. the silicon substrate 4.

The exposed silicon wafer 4 is moved laterally on the stage 5, while the beam from the excimer laser 1 is scanned by a moving mirror 3. The scanning of the bean is perpendicular to the motion of the wafer 4 so that the entire area of the wafer can be covered. Mirrors 2 are used to direct the laser light from the laser I to the scanning mirror 3. The mirrors 2 and 3 and the moving stage 5 are all supported by the framework tj.

The wafer could be moving continuously in one direction, as on a conveyor belt and could be inside vacuum chamber. This would be of interest in that the wafer, having been exposed in a vacuum chamber, could then stay under vacuum for development and subsequent transfer to the doping station, where the process once again occurs under vacuum.

Claims (3)

GLA lY:

1. The use of ultra-violet photoablation to produce differential development between x-ray exposed and unexposed regions of photoresist or polymer material.

2. Differential development as in claim 1, where the exposure is to electron or other particle damage.

3. Differential development as in claims 1 and 2, but where the photoablation is done by x-ray or other intense, high energy electromagnetic radiation source.

6671 High Holborn, London WCIR 4TP. Further copies may be obtained from The Patent Office. Multiplay techniques ltd. St. Mary Cray, Kent Con. 1/87.