INDIRECT LASER PATTERNING OF RESIST

Abstract: The present invention provides a method of forming a patterned resist mask over the surface of a layer to be selectively exposed. The method includes the steps of depositing a layer of resist over the surface, and removing the resist from selected areas of the surface. The resist is removed from the selected areas of the surface by heating an absorbing layer adjacent to the resist with a laser such that localised heating occurs in areas of the absorbing layer. The laser has an output at a wavelength that is absorbed by the absorbing layer, and the heated areas of the absorbing layer conduct heat to the layer of resist. The resist is formulated such that it is brittle in use and has a high thermal expansion coefficient, whereby rapid heating of a localised region of the resist causes shearing around the localised region.

Description: Indirect laser patterning of resist Technical Field The present invention relates generally to the field of semiconductor device fabrication and in particular, the invention provides an improved method of forming a resist mask over a surface. Background Art A variety of methods are known for etching features in the surface of semiconductors. Typically, a resist is placed over the surface to be etched, and then selected areas of the resist are removed in order to expose the surface in the desired pattern. A suitable etchant is then applied. The resist is substantially impervious to the etchant, and therefore only the exposed areas of the underlying layer are removed by the etchant. The remaining resist may then be washed away or dissolved with an appropriate solvent, leaving the underlying layer etched in the desired pattern. The selected areas of the resist may be removed by a number of methods. A common technique is photolithography, in which the resist placed over the surface to be etched is a photoresist. The photoresist is polymerised in selected areas by exposure to ultraviolet light in a pattern defined by a photographic mask. Those portions of the photoresist that have not been polymerised are dissolved and removed by the use of an appropriate chemical. This exposes the surface in the desired pattern, ready for etching. Photolithography requires specific equipment which may be expensive, and may occupy a large space in the manufacturing area. The need for an appropriate photoresist may also add significantly to costs. The additional step of manufacturing a photographic mask may also be expensive. Positioning the mask requires a time consuming and therefore expensive alignment step. Because photolithography is relatively slow due to the multiple processing steps required, the obtainable throughput of this process is low. Another technique used to selectively etch semiconductor devices is laser etching. In this technique a laser is used to directly etch a layer or to open a dielectric layer such as silicon oxide or silicon nitride which is then used as an etching mask. Laser etching can be less expensive than photolithography in the fabrication of semiconductor devices having relatively simple design, however, laser etching is a disruptive technique which is not suitable for some thin film applications, particularly where the top layers are required to be etched without damage to thin underlying layers. Laser etching can create significant amounts of heat both in the layer being removed and in the underlying layer(s), which increases the risk of damaging the thin film layers and degrading the performance of the semiconductor device. Disclosure of Invention The present invention provides a method of forming a patterned resist mask over the surface of a layer to be selectively exposed, the method including the steps of: a) depositing a layer of resist over the surface; and b) removing the resist from selected areas of the surface, by heating an absorbing layer adjacent to the resist with a laser such that localised heating occurs in areas of the absorbing layer, the laser having an output at a wavelength that is absorbed by the absorbing layer, the areas of the absorbing layer conducting heat to the layer of resist, and the resist being formulated such that it is brittle in use and has a high thermal expansion coefficient, whereby rapid heating of a localised region of the resist causes shearing around the localised region. By having a high thermal expansion coefficient, the resist expands sufficiently during heating to cause fracturing around the heated area, and the heated area of the resist shears away from the layer to be selectively exposed and out of the layer of resist, creating the required masking pattern in the resist. Ideally, this mechanism reduces the energy input that is required from the laser compared to conventional laser etching, as the selected area of resist does not have to be melted or ablated, merely sufficiently heated to cause expansion. Consequently, the amount of potentially damaging heat and energy that is generated and transmitted through the layer to be selectively exposed to underlying material is greatly reduced. In preferred embodiments, the semiconductor itself serves as the absorbing layer. In such embodiments, the resist is preferably substantially transparent to light at the frequency of the laser. Embodiments of the invention may have one or more layers between the absorbing layer and the resist. The one or more layers preferably conduct heat effectively from the absorbing layer to the resist. In embodiments where the laser passes through the one or more layers before being absorbed in the absorbing layer, the one or more layers are preferably substantially transparent to light at the frequency of the laser. The one or more layers may include silicon oxide or silicon nitride layers. Embodiments of the invention may incorporate the further step of urging the heated areas of resist away from the surface. The heated areas of resist may be urged away from the surface by applying a high velocity jet of liquid or gas to the heated areas of the resist. The heated area of the resist may be urged away from the surface by applying adhesive tape to the heated areas of the resist and then peeling the adhesive tape off the resist. Alternatively, the heated areas of the resist may be subjected to ultrasonic vibrations, for instance by immersion in an ultrasonic bath, in order to facilitate the removal of the resist from the selected areas. In cases where a laser is already used in a manufacturing process, embodiments of the present invention may use an absorbing layer chosen or modified such that it has a high light absorption at the operating wavelength of the laser. This removes the requirement for dedicated resist-patterning laser equipment. For example, a Nd:YAG laser operating at 1064 nm or frequency doubled to operate at 532 nm may be used in silicon solar cell production. The resist used in accordance with the invention is preferably based upon Novolac™ resin. The present invention may be applied in the manufacture of thin film silicon solar cells which consist of a glass substrate, n-type and p-type layers, and a transparent dielectric layer, the dielectric being the layer to be selectively exposed. For example, the method of the present invention may be used as part of a process to create metal contacts with the p-type layer of such a thin film silicon solar cell. In such embodiments, the method of the invention may further include one or more of the following steps, not necessarily in this order: baking the cell to soften the resist to smooth the edges around the exposed areas and to improve adhesion; etching the dielectric to expose areas of the p-type layer in the pattern defined by the resist mask; optionally washing or dissolving the remaining resist off the surface of the dielectric layer; and depositing metal over the surface of the cell, to form contacts with the p-type layer in the exposed areas. The etchant used in accordance with the preferred embodiment of the invention is dilute hydrofluoric acid, although any other suitable etchant to which the organic resist is substantially impervious may be used. Etching may be performed by reactive-ion plasma etching. Furthermore, the etchant preferably etches the dielectric layer much more rapidly than the p-type layer. This will minimise damage to the p-type layer when the etchant comes into contact with the p-type layer. Embodiments of the invention are particularly useful for patterned etching of dielectric layers located over thin semiconductor films such as when contacts are required to be made to the semiconductor through the dielectric layer. Brief Description of Drawings Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates a semiconductor structure upon which a layer of resist has been deposited; Figure 2 illustrates the structure of Figure 1 where the layer of resist has heated and fractured; Figure 3 illustrates the semiconductor structure of Figure 1 where a portion of the resist has been removed; Figure 4 illustrates the result of the etching of the dielectric layer of the structure of Figure 3; and Figure 5 shows the semiconductor structure of Figure 4 after the resist has been washed off and metal has been deposited over the dielectric and the selected areas of the p-type layer. Modes for Carrying Out the Invention Figures 1 to 5 portray an embodiment of the present invention in which a patterned resist mask is formed over the surface of a thin film solar cell as a step in the formation of a metal contact. Figure 1 illustrates the thin film solar cell 10 during manufacture. The cell 10 includes a glass substrate 11, a thin n-type layer 12, a thin p-type layer 13 and a dielectric layer 14. The thicknesses of the n-type layer 12 and the p-type layer 13 are preferably each between OJμm and 5μm, and the thickness of the dielectric layer 14 is preferably between 0.05μm and 2μm. In accordance with the present invention, a layer of resist 15, being brittle in use and having a high thermal expansion coefficient, has been deposited over the dielectric layer 14. Radiation 16 from a laser (not shown) is applied to a selected area 17 of the semiconductor layer 13, which has a high light absorption at the frequency of the laser. Localised heating occurs in the selected area 17 as a result of the incident radiation 16. The temperature rise in the surface of the semiconductor film is conducted through the dielectric layer 14 to the resist, causing localised expansion of the resist resulting in delamination, buckling and shearing of the resist. The total amount of energy required from the laser in order to remove the selected area of resist 17 is too small to damage the layers 13 and 12. The organic resist 15 used in this embodiment contains Novolac™ which is an organic resin. This resist has a high thermal expansion coefficient and is brittle in use. Figure 2 illustrates the device 10 at a later moment in time, when heat has been transmitted to resist layer 15. Due to the high thermal expansion coefficient of the resist 15, the resist has expanded to cause fracturing 20. Figure 3 illustrates the semiconductor device 10 at a later stage in the method, where the selected portion of resist has been removed. In accordance with the present invention, the heated area of resist does not heat sufficiently for it to melt or ablate. Because the Novolac™ resist 15 has a high thermal expansion coefficient, the heat generated by the laser is sufficient for the selected area of resist to expand sufficiently that it will shear upwards and out of the layer of resist 15. This process can be aided by applying high velocity liquid or gas jets or adhesive tape or ultrasonic vibration to the resist. This mechanism reduces the energy input that is required from the laser compared to conventional laser etching, as the resist does not have to be melted, merely sufficiently heated to cause expansion. In particular, the maximum temperature generated during this process is lower, limiting the possibility of damage to the other layers 12, 13 and 14. The next step in the formation of the metal contact is that of etching the dielectric layer 14 in the selected pattern defined by the resist mask 15 which has been formed in accordance with the present invention. Figure 3 shows the etchant 18 as it is first applied. Because the resist 15 is substantially impervious to the etchant 18, the etchant 18 only acts on the exposed surface of the dielectric 14. The etchant 18 used in accordance with such embodiments of the invention may be dilute hydrofluoric acid, or any other suitable etchant to which the organic resist 15 is substantially impervious. Furthermore, the etchant 18 preferably etches the dielectric layer 14 much more rapidly than the p-type layer 13. This will minimise damage to the p-type layer 13 when the etchant 18 comes into contact with it. Figure 4 illustrates the result of the etching of the dielectric layer 14. The resist 15 is substantially unchanged by the etchant, however the exposed area of the dielectric layer 14 has been completely etched away. The etching period is determined such that where the etchant has come into contact with the p-type layer 13, the etchant has had little effect on the p-type layer. Figure 5 shows the semiconductor 10 after the resist 15 has been removed by way of a suitable solvent. Alternatively, the resist may be left intact. Aluminium layer 19 has been deposited over the dielectric layer 14 and the selected areas of the p-type layer 13, hence forming contacts with the p-type layer 13 only in those selected areas where the dielectric layer 14 has been etched. In order to provide a good ohmic contact between aluminium layer 19 and p-type layer 13, the aluminium layer 19 may be heated. The effect of heating the aluminium layer 19 is to dissolve any thin interfacial oxide that may be present, and to alloy some aluminium into the p-type layer, forming a p region as part of each metal/semiconductor contact. When the semiconductor material is silicon, a small amount of silicon (perhaps 1-2%) is typically added to the aluminium metal to make the alloying process more reproducible. Although the invention has been described with reference to a particular application, it should be appreciated that it may have other applications. For instance, the present invention may be applied in any process where it is desired to form a resist mask over a layer to be selectively exposed. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims: CLAIMS: 1. A method of forming a patterned resist mask over the surface of a layer to be selectively exposed, the method including the steps of: a) depositing a layer of resist over the surface; and b) removing the resist from selected areas of the surface, by heating an absorbing layer adjacent to the resist with a laser such that localised heating occurs in areas of the absorbing layer, the laser having an output at a wavelength that is absorbed by the absorbing layer, the areas of the absorbing layer conducting heat to the layer of resist, and the resist being formulated such that it is brittle in use and has a high thermal expansion coefficient, whereby rapid heating of a localised region of the resist causes shearing around the localised region. 2. The method of Claim 1 wherein the layer to be selectively exposed serves as the absorbing layer. 3. The method of Claim 2 wherein the resist is substantially transparent to light at the frequency of the laser. 4. The method of Claim 1 wherein one or more layers are situated between the absorbing layer and the resist. 5. The method of Claim 4, wherein the one or more layers conduct heat effectively from the absorbing layer to the resist. 6. The method of Claim 4 or 5 wherein the laser passes through the one or more layers before being absorbed in the absorbing layer, and the one or more layers are substantially transparent to light at the freqviency of the laser. 7. The method of any one of Claims 4 to 6 wherein the one or more layers include a silicon oxide layer. 8. The method of any one of Claims 4 to 7 wherein the one or more layers include a silicon nitride layer. 9. The method of any preceding claim, further including the step of urging the heated areas of resist away from the surface. 10. The method of Claim 9 wherein the heated areas of resist are urged away from the surface by applying a high velocity jet of gas to the heated areas of the resist. 11. The method of Claim 9 wherein the heated areas of resist are urged away from the surface by applying a high velocity jet of liquid to the heated areas of the resist. 12. The method of Claim 9 wherein the heated areas of the resist are urged away from the surface by applying adhesive tape to the heated areas of the resist and then peeling the adhesive tape off the resist. 13. The method of Claim 9 wherein the heated areas of the resist are urged away from the surface by subjecting the resist layer to ultrasonic vibration. 14. The method of Claim 13 wherein the resist layer is immersed in an ultrasonic bath. 15. The method of any preceding Claim, wherein the absorbing layer is chosen or modified such that it has a high light absorption at the operating wavelength of a laser already in use in a manufacturing process. 16. The method of Claim 15 wherein the laser already in use is a Nd:YAG laser operating at 1064 nm. 17. The method of claim 15 wherein the laser already in use is a Nd:YAG laser frequency doubled to operate at 532 nm. 18. The method of any preceding claim wherein the resist is based upon Novolac™ resin. 19. The method of any preceding claim wherein the layer to be selectively exposed is a dielectric layer of a thin film silicon solar cell. 20. The method of any preceding claim applied in a process to create metal contacts with the p-type layer of a thin film silicon solar cell including a glass substrate, an n-type layer, a p-type layer, and a dielectric layer. 21. The method of Claim 20 further including the step of etching the dielectric to expose areas of the p-type layer in the pattern defined by the resist mask. 22. The method of Claim 21 further including the step of washing or dissolving the remaining resist off the surface of the dielectric layer. 23. The method of Claim 21 or 22, further including the step of depositing metal over the surface of the cell, to form contacts with the p-type layer in the exposed areas. 24. The method of Claim 21 or 22 or 23, further including the step of baking the cell prior to depositing metal to soften the resist to smooth the edges around the exposed areas and to improve adhesion. 25. The method of any preceding claim wherein etching is performed by dilute hydrofluoric acid. 26. The method of any one of claims 1 to 24 wherein etching is performed by reactive-ion plasma etching. 27. The method of any preceding claim wherein etching of the dielectric layer occurs much more rapidly than etching of the p-type layer. Substitute Sheet (Rule 26) RO/AU

Inventors: BARRETT NEIL, BASORE PAUL ALAN, YOUNG TREVOR LINDSAY, BARRETT, NEIL, BASORE, PAUL, ALAN, YOUNG, TREVOR, LINDSAY
Applicants: BARRETT NEIL, BASORE PAUL ALAN, PACIFIC SOLAR PTY LTD, YOUNG TREVOR LINDSAY, BARRETT, NEIL, BASORE, PAUL, ALAN, PACIFIC SOLAR PTY. LIMITED, YOUNG, TREVOR, LINDSAY
Application year: 1999
Main classification: H01L31
IPC codes: B23K26, G03F7, H01L21, H01L31