A METHOD FOR DEPOSITING LAYERS ON A SUBSTRATE

Abstract: A method is disclosed for depositing layers, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD), on a substrate, improves the stability of PECVD chambers when used for the fabrication of planar waveguide components. The method, which comprises that a substrate is placed on a carrier that is placed on the bottom electrode, improves the plasma cleaning process by substantially reducing build up of polymers during cleaning. Overall the method reduces chamber downtime and improves chamber parameter control and hence also improves control over deposited film parameters such as refractive index. In an embodiment of the invention the carrier is constituted of two parts, namely an outer carrier with a hole, and an inner carrier that fits in the hole.

Description: A method for depositing layers on a substrate. The present invention relates to a method for depositing layers, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD), on a substrate, said method comprising placing the substrate between a top electrode and a bottom electrode The technical field. When using a parallel plate PECVD deposition chamber a large part of the deposited material ends up on the chamber walls and the electrodes. This leads to a change in impedance of the system and therefore slightly changes in the properties of the deposited films. For many applications this is not a problem but for waveguide production, where precise control of refractive index is important this could lead to unreproducible results. It is therefore important to clean the chamber at regular intervals. This can be done either by a mechanical clean or an in situ chemical clean. The chemical clean is a special plasma process used to etch the deposited layer. An advantage of the chemical cleaning process is that it is performed without taking the system apart thus reducing down time of the chamber. However, the process suffers from a problem of different etch rates on different surfaces. Especially, the etch rate on the live electrode is approximately a factor of two higher than on the grounded electrode. Therefore the live electrode will be cleaned first. The aluminium walls of the chamber will be subjected to the plasma when all deposited glass has been removed. Continuing the cleaning process after this will result in a build-up of a polymer layer. As the cleaning process will not stop before the grounded electrode is cleaned, a fairly thick layer of polymers will be formed on the live electrode. A polymer layer will also be formed immediately on the grounded electrode in the place where the wafer has been lying since no glass is deposited here. After the cleaning process the chamber is therefore partly clean, partly covered by polymers. These polymers will affect the deposition process in a number of ways. Firstly, the deposited glass tends to peel off the polymer layers after a certain time giving rise to particle contamination in the chamber. This will affect the film quality. Secondly, the layer thickness of the polymers on the grounded electrode is not necessarily the same from time to time. Therefore the environment for the wafers will not be the same between different depositions and cleaning cycles. Using different substrate sizes will also not be possible, e.g. deposition on a 5" wafer after a 4" wafer will result in a different surrounding at the outer part of the 5" wafer as compared to the center 4" part. In conclusion two problems need to be solved: 1) The chamber conditions must be reproducible and stable during a deposition cycle, and 2) the cleaning process should generate as little polymers as possible. Finally, the cleaning process should be as short as possible to increase the system up time. Prior art disclosures. UK patent application GB 2312439 discloses a method for deposition of layers by PECVD for the fabrication of optical waveguides. It is disclosed that higher repeatability and control of the refractive index is obtained by placing the substrate on the live electrode and thus maintaining a negative bias on the substrate during deposition. Nothing is disclosed about how to prevent un-wanted build-up of layers of deposited material on the electrodes, cleaning procedures, how to prevent build up of polymers during cleaning or generally about how to increase chamber up-time and stable chamber conditions. Disclosure of the invention. It is an object of the present invention to provide a method for improving the stability and reproducibility of the chamber conditions in a PECVD reactor. It is a further object of the invention that the chemical plasma cleaning processes used to remove unwanted depositions of silica on the chamber walls should generate as little polymers as possible. Finally, it is an object of the present invention that the cleaning process should be as short as possible thereby helping to increase the system up time. Acceding to the invention there is provided a method that is defined in the preamble of claim 1. This method is characterised in that the substrate is placed on a carrier that is placed on the bottom electrode. In this way it is possible to reduce the down time of Plasma Enhanced vapour Deposition (PECVD) reactors. When using the method the PEVCD reactor remains in a steady and well-controlled state for longer periods of time resulting in higher yields for the deposition processes used. When as stated in claim 2, the carrier constitutes of 2 parts, namely an outer carrier with a hole, and an inner carrier so dimensioned, that it fits in the hole, it is possible to remove the substrate without removing the carrier, fx. by first lifting the inner carrier and then remove the substrate. It is also expedient, when as stated in claim 3, that the inner carrier has an area that is less than the area of the substrate. By this way it is easy to grip the substrate with an appropriate tool, and remove it from the carrier Further expedient embodiments for the invention are outlined in claims 4 - 10. The invention will now be described in greater detail with reference to the accompanying drawing, in which Fig. 1 illustrates a principle structure of the interior parts of a horizontal parallel plate PECVD chamber, and Fig. 2 illustrates an embodiment of the carrier. In the drawings the interior parts of a horizontal parallel plate PECVD chamber with bottom and top electrode are designated 1 ,2 respectively. As seen a substrate 4 is placed on a carrier plate 3, such as a large fused silica carrier 3. The diameter of the fused silica carrier 3 should match the diameter of the bottom electrode 1 to keep it free from unwanted deposited material. Stops 5 keep the substrate 4 in its designated position on the carrier plate 3 during loading and un-loading and during processing. The effect of the fused silica carrier 3 will now be explained: By placing the wafer 4 on a carrier 3 of a similar material as the layer deposited in the PECVD chamber, the impedance of the system will be less affected by the deposited material. Deposition on different substrate 4 sizes can be done either by using the same fused silica carrier 3 or another carrier 3. Also the cleaning process is carried out with a fused silica carrier 3. The cleaning process is then stopped as soon as the live electrode 2 has been cleaned. This way polymer formation is almost completely avoided. Fig. 2 shows a carrier that differs from the carrier in fig. 1 in that the carrier constitutes of 2 parts, namely an outer carrier 6 with a hole 7, and an inner carrier 8 dimensioned in such a way that it fits to the hole 7 of the outer carrier. It is noted that the inner carrier has a diameter or an area slightly less than the substrate processed. In this way it will be possible to remove the substrate from the carrier with an appropriate tool. Generally the most notable advantage of this method is the increased up time of the PECVD deposition system. Manufacturers of PECVD equipment recommend a plasma clean for every 5-6 μm of deposited material. This would render thick layer (4-15 μm) fabrication impossible. Further, the recommended cleaning time is similar to the deposition time. By using the method of the invention it is possible to deposit up to 200μm of silica glass before a plasma clean is necessary. A further advantage is improved reproducibility. The deposited material behaves much like the fused silica carrier 3 and therefore the plasma environment will not change during depositions. The polymer formation in the chamber is also dramatically reduced. This provides for considerably better glass quality than with conventional techniques. Many types of carriers can be used but it is generally preferred that both the dielectric and thermal properties of the carrier material should be similar to the dielectric and thermal properties of the material deposited in the PECVD chamber. Thus, if silica or doped silica is deposited, a fused silica carrier is preferred. It is preferred that the carrier is equipped with some means of holding the wafer in place when the wafer-on-carrier is loaded into or out of the PECVD chamber and during processing. In one embodiment the wafer positioning means consist of a recess in the carrier that fits the wafer. In another embodiment stops are placed on the carrier to hold the wafer in place.

Claims: CLAIMS 1. A method for depositing layers, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD), on a substrate, said method comprising placing the substrate between a top electrode and a bottom electrode c h a r a c t e r i s e d in that the substrate is placed on a carrier that is placed on the bottom electrode. 2. A method according to claim 1, c h a r a c t e r i s e d in that the carrier constitutes of 2 parts, namely an outer carrier with a hole, and an inner carrier so dimensioned, that it fits in the hole. 3. A method according to claim 2, c h a r a c t e r i s e d in that the inner carrier has an area that is less than the area of the substrate. 4. A method according to claim 1 -3, c h a r a c t e r i s e d in that the carrier substantially covers the area of the bottom electrode of the PECVD chamber. 5. A method according to claims 1-4, c h a r a c t e r i s e d in that the dielectric properties of the carrier are substantially similar to those of the layers deposited. 6. A method according to claims 1-5, c h a r a c t e r i s e d in that the thermal properties of the carrier are substantially similar to those of the layers deposited. 7. A method according to claims 1-6, c h a r a c t e r i s e d in that the carrier contains means for holding the substrate in place on the carrier. 8. A method according to claims 7, c h a r a c t e r i s e d in that the means for holding the substrate are provided by stops on the carrier. 9. A method according to claims 7, c h a r a c t e r i s e d in that the means for holding the substrate is provided by a recess in the carrier. 10. A method according to claims 1-9, c h a r a c t e r i s e d in that the carrier is made of fused silica.

Inventors: ANDERSEN LARS-ULRIK AAEN, EGGINTON PAUL NICHOLAS, ANDERSEN, LARS-ULRIK, AAEN, EGGINTON, PAUL, NICHOLAS
Applicants: ANDERSEN LARS ULRIK AAEN, EGGINTON PAUL NICHOLAS, IONAS AS, ANDERSEN, LARS-ULRIK, AAEN, EGGINTON, PAUL, NICHOLAS, IONAS A/S
Application year: 2000
Main classification: C23C16
IPC codes: C23C16