GB2166254A - Method for the production of a metal patterning mask - Google Patents

Abstract

A negative photoresist is developed by a multiple puddle method, the concentration of the aqueous alkali developer being of a lower concentration than the minimum required to reduce the adhesion of the resist to the underlying substrate. The steps of: forming a puddle of developer on the resist, allowing a period for development to take place and washing the developer away, are repeated until the upper part at the photoresist overhangs the lower part at the edges exposed to the developer. The resist/substrate is then coated with metal (e.g. Al) and the unwanted metal overlaying the remaining resist material may be lifted off by immersion in a fuming nitric acid solvent.

Description

SPECIFICATION Metal patterning method This invention relates to a method of metal patterning.

An alternative to conventional etching to form metal patterns for microelectronic applications is known as the "lift-off" method. In such a method a substrate is coated with a masking layer in those areas where metal is not required, and a metal layer is deposited over the masking layer and exposed areas of substrate. The unwanted metal is then lifted-off by removal of the masking layer. An essential property of a lift off masking layer is that it be resistant to metal step coverage. If this is not so any holes in the masking layer, defining metal patterns to be retained after lift off, will be covered with a continuous layer of metal which will inhibit subsequent lift off of the surrounding masking layer. To prevent such bridging it is helpful to control amongst other parameters the cross sectional profile of the openings in the masking layer.

The cross sectional profile of the masking layer openings can be controlled by using a two layer resist the lower layer of which is etched so as to undercut the upper layer and provide an overhang at the mask edge. Alternatively a single resist layer and an etching process giving a tapered profile to the opening are used which also provides an overhang at the mask edge. One known method of achieving an overhang with a single layer resist involves chemically treating the surface of a positive photoresist with a chemical which alters the solubility of the upper portion of the resist layer. After irradiation, and development lower portions of the resist layer are removed more easily than the upper treated surface which forms an overhanging lip at the mask edges.Certain photoresists can achieve this by treatment with chlorobenzene which removes some of the low molecular weight components of the resist rendering the soaked portion of the resist less soluble in the developer solution.

Such procedures for producing lift-off masking resists having overhanging lips have hitherto only been possible when using positive resists where those portions of the resist irradiated are washed out on development. The use of negative resists as masking layers, where those portions of the resist irradiated are retained, has hitherto been impracticable. This has been due to the swelling of the photoresist which occurs on development of negative photoresists using conventional organic developer solutions. Such swelling reduces the accuracy with which holes in the mask defining the metal pattern to be retained can be formed.

It is now possible however to obtain negative photoresists which use a non-organic aqueous alkaline developer thus eliminating the image swelling during development which occurs with conventional organic solvent developed negative resists.

With a negative resist the portions of the resist exposed to UV light through a photomask become practically insoluble in the developer. Since the surface portions of the resist are irradiated to a greater extent than the underlying portions there will be a difference in solubility between them which on slight overdevelopment will result in the less soluble upper portion being undercut at the mask edge without the need for chemical treatment of the uper portions of the mask. Hitherto however negative resists have not been used for highly accurate lift off masking layers.

For a process to be commercially viable it is preferable that the quantities of materials, and in particular developer, used are as small as possible.

To avoid the need for immersion of semiconductor wafers in baths of developer for development it is preferable to use a puddle development process. In this method the developer is treated so as to give it a surface tension which enables a small quantity of it to form a film on a wafer. Once development has occurred the remaining solution is spun from the wafer. This considerably reduces the quantity of developer solution required to develop photoresist on wafers.

We have found however that whilst overdevelopment of a negative resist using standard developer solutions can give an overhang to the result edge such overhangs are insufficient for practical use with a lift off process. This problem can be overcome in theory by increasing the developer concentration. In particular we have found that beyond a certain limit too high a developer concentration will result in the masking resist peeling off the substrate. This problem cannot however be overcome by increasing the quantity of developer used whilst keeping the concentration below the limit at which the adhesion of the masking resist layer is affected since the quantity of developer is limited by the size and depth of puddle which can be retained on the wafer being processed.

According to the present invention there is provided a method of developing an irradiated negative resist using an aqueous developer solution whose concentration is less than the minimum concentration which reduces the adhesion of said resist to an underlying substrate wherein a film of said developer solution is deposited on said resist to form a puddle, development of said resist is allowed to occur and said puddle of developer solution is then removed from said resist and said deposition of developer solution, development and removal of developer is repeated.

By using a method according to the present invention it is possible to combine the economic advantages of puddle development with those of the use of a aqueous alkalic solution developed negative resist to give a resist mask edge having an overhang suitable for use with a lift off metal patterning process. Without the multiple puddle approach adopted by the present invention the use of negative resists for lift off processes would still be impracticable despite the development of higher definition negative resists using aqueous alkalic developer solutions. The success of the method is surprising in view of the know disadvantages which prevent the use of multiple develop ment cycles with positive resists.

Embodiments of the present invention will now be described with reference to the accompanying illustrations in which: Figure 1 shows a resist masking profile obtained using the method according to the present invention.

Figure 2 shows metal patterns obtained after using a lift off process according to the present invention.

To illustrate the method according to the present invention a 3 inch diameter silicon wafer was obtained and coated with a 1 .5#m layer of WX-303 resist (manufactured by Phillip A Hunt Chemical Corporation). The wafer and resist were then soft baked at 950C for 20 minutes. The resist was then exposed for 5 seconds to deep UV light of approximately 220 to 320 nm wavelength using a lamp power of 10mW/cm2. A quartz high resolution mask plate was used. The resist was then developed in an HPR402 puddle developer solution (manufactured by Phillip A Hunt Chemical Corporation) which is an alkali developer. The developer was diluted with deionised water to a strength of 2 parts developer to 1 part water.It was found that any increase in the developer concentration above this level resulted in significant loss of adhesion of the resist to the wafer. The developer solution was deposited on the wafer using a conventional automatic dispensing and coating system in this case an "Omnichuck" system (manufactured by Machine Technology Limited). Following development for 25 seconds the developer solution was spun from the wafer and the process repeated using a fresh puddle of developer for a further 25 seconds so that the activity of two puddles of developer was used to develop the resist fully without exposing the resist to concentrations of developer which would affect its adhesion to the underlying wafer.

Depending on the resist and developer used the number of puddles of developer may be increased to three or more. Similarly the concentration of developer used may be less than 2 parts developer to 1 part water if the number of puddles used is increased proportionately. Preferably though as few puddles as possible are used to achieve the desired degree of development.

Following full development of the resist, profiles such as those shown in Figure 1 were obtained.

The fully developed resists were then hard baked for 20 minutes at 1400C and coated with 0.5#m of evaporated Aluminium. The unwanted aluminium overlying the remaining resist was then lifted off by immersing the wafer in a fuming nitric acid solvent. Following lift off of the unwanted metal, metal patterns such as that shown in Figure 2 could be obtained. The resolution obtainable meant that patterns having adjacent feature gaps down to 1#m in size could be clearly defined.

Claims (6)

1. A method of developing an irradiated negative photoresist using an aqueous developer solution whose concentration is less than the minimum concentration which reduces the adhesion of said resist to an underlying substrate, wherein a film of said developer solution is deposited on said resist to form a puddle, development of said resist is allowed to occur, said puddle of developer solution is then removed from said resist and said deposition of developer solution, development and removal of developer solution is repeated.

2. A method according to Claim 1 wherein said developer solution comprises 2 parts developer to 1 or more parts water.

3. A method according to Claim 1 or Claim 2 wherein said deposition of developer, development and removal of developer solution is performed twice.

4. A method according to any one of the preceding claims wherein said deposition of developer solution, development and removal of developer solution is repeated a sufficient number of times for an upper part of said photoresist to overhang a lower part of said photoresist layer at edges of said layer exposed to said developer.

5. A method according to any one of the preceding claims wherein said developer solution is an aqueous alkali developer solution.

6. A method of developing an irradiated negative photoresist substantially as herein described and with reference to Figure 1.