GB2359416A

GB2359416A - Etching unit for HF vapour etching of substrates

Info

Publication number: GB2359416A
Application number: GB0021935A
Authority: GB
Inventors: Franz Laermer; Andrea Schilp
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1999-09-07
Filing date: 2000-09-06
Publication date: 2001-08-22
Anticipated expiration: 2020-09-06
Also published as: GB0021935D0; GB2359416B; DE29915696U1

Abstract

An etching unit for substrates has a storage device (1), preferably a vacuum cartridge station which is capable of being evacuated, and a transportation device (9) for transferring substrates from the storage device (1) to etching devices (2,3,4,5). The etching units (2,3,4,5) may be HF vapour etching modules used to etch silicon wafers microstructured with a sacrificial layer of SiO<SB>2</SB>. There may be a cleaning station (6) connected to the transportation device (9) which may clean the substrate by oxygen plasma stripping or by applying ozone-containing gas. The transportation device may be a gripper station, and have heated parking positions (11) for temporary deposition of a substrate, and the unit may have an exit cartridge station (7). Wafers can therefore be transported in the unit whilst under vacuum, avoiding contamination.

Description

2359416 - 1 Etching unit for HF vapour etching The invention relates to an

etching unit for HF vapour etching of substrates while simultaneously avoldinca contamination, in accordance with the precharacterising part of the main claim.

Prior art

For selective removal of certain regions of an S102 sacrificial layer in a structured silicon wafer, for the production of surface micromechanical structures, much use is made in sensor manufacture of an FE vapour etching process known from DE 197 04 c 454 AI. Durine, this, the wafer to be processed is individually subjected to an c azeotropic aqueous hydrofluoric acid/water vapour mixture at a defined temperature.

However, this process is known to react in a critical manner to impurities in the wafer surface to be etched, and in particular in the oxide surfaces to be etched. Thus, even organic material coatinas or contamination of diverse composition, as results from a C1 relatively long contact between the wafers and the air in the clean room, can influence the etchine, rates of this HF vapour etching process.

In order to eliminate such impurities present on the surface of the wafers to be etched, they are conventionally first cleaned, before the actual HF gas phase etching, in an c W oxygen plasma stripper In a manner known per se. DE 199 24 058 furthermore already proposes carrying out an in-situ cleaning of the wafers to be processed, immediately before the etching, after or as an alternative to the oxygen plasma stripping, by an ozone treatment at elevated temperature.

Since it is very complex and costly to clean each wafer individually in an oxygen plasma stripper and immediately thereafter to supply it to the HF gas phase etching, C) 2 - it is conventional to clean on each pass a batch of wafers of for example 25 wafers, located in an entry cartridge, tocyether by oxygen plasma stripping, and thereafter to n introduce this batch of wafers Into a cartridge station of the HF vapour etching unit and to store it there.

Processing all the pre-cleaned wafers of a batch of wafers of this type typically takes 10 to 15 hours in known HF vapour etching units. Thus, the wafers which have not yet been processed and are in the cartridere station awaiting etching are increasingly contaminated again over time by their residence in a waiting position of the HF vapour etching unit, for example by organic impurities, and this can later result in derable dev'at'ons 'n the etching rate during etching. Thus, undesirable consi 1 1 1 1 c c discrepancies in the properties of the etched wafers can occur over one batch of wafers, and the cleaning of the wafers performed in the oxygen plasma stripper is to a certain extent a pointless exercise, in particular for the last wafers etched.

Advantages of the invention The etching unit according to the invention, havln--a,, the characterisint(y:l features of the main claim, has the advantage over the prior art that subsequent contamination for example of wafers which have already been cleaned in an oxygen plasma stripper and/or by ozone treatment before the actual HF vapour etching is avoided. Thus, each wafer or batch of wafers only has to be pre-cleaned once, and a consistent etching result can be expected over a batch of wafers comprisina a plurality of individual wafers. This means, overall, that there is a si-nificant improvement in quality and a uniformity of quality in the micromechanical structures produced in the wafers by the etching, Advantageous further developments of the invention emerge from the measures stated in the subclaims.

3 - Thus, it is particularly advantageous if the individual locks of the etching unit, the control of the aeration and evacuation procedures and the transfer of the substrates etched or to be etched between the individual stations of the etchinc, unit operate fully automatically and under computer control by way of a corresponding machine control. This leads to a considerable cost savin. and an increased throughput through the C-5 etchine, unit of substrates to be etched.

0 Furthermore, it is very advantageous if the gripper station actiner as a transportation c C) Z) device and the vacuum cartridge station acting as a storage device can be evacuated to a high vacuum. In this case, even after relatively long storage of a number of hours, there is no renewed contamination of already cleaned substrates.

To charge the gripper station, it is furthermore advantageous to use highpurity nitrogen from which, where appropriate, organic impurities have further been removed by filters. This prevents contamination of the gripper station during the aeration, and the time required to evacuate to a high vacuum is reduced. At the same time, in this case gas is not given off from the gripper station to a significant extent after the evacuation, that is to say any release of hydrocarbon compounds absorbed on the inside of the aripper station durina aeration is minimised and reduced to a tolerable level.

Drawing The invention will be explained in more detail with reference to the drawing and in C the description which follows. The figure shows a sketch of the principle of an HF vapour etching unit having a cleaning device, a transportation device, a plurality of c t> etching devices and a stora,e device.

0 Example embodiments - 4 The ffilure shows an etchinG unit 10 for etchincy structured silicon wafers which are c 11-P 111) pided in certain regions with a sacrificial layerOfSIO2 1 rov to be etched, in a preferably azeotropic, vapour hydrofluoric acid/water mixture at a defined temperature. Further details of this etching process, which is known per se to those skilled in the art, are described in DE 198 50 078.5.

The etching unit 10 has for this purpose first of all a vacuum cartridge station 1, acting as a storage device, and a gripper station 9 connected thereto by way of a storage lock 13 and having a gripper 8 acting as a transportation device. Furthermore, the etching W unit 10 has a plurality of HF vapour etching modules 2, 3, 4, 5 acting as etching W c devices, in which the actual I-IF vapour etching process, which is known per se, of the individual silicon wafers is carried out. These HF vapour etching modules 2, 3, 4, 5 are each connected to the gripper station 9 by way of etching locks 14, 15, 16, 17.

1;' Moreover, the gripper station 9 is optionally connected by way of a cleaning lock 18 to a cleaning station 6, which is known per se and serves as a cleaning device for the substrates to be etched and is for example constructed in the form of an oxygen plasma stripper and/or ozone cleaner. The supply and removal to the etching unit 10 of the wafers, or batches of wafers comprising a plurality of individual wafers in a wafer IP All) cartn -e, which are to be processed or have already finished being etched takes place 1 c by way of an exit cartridge station 7 which is connected by way of an entry lock 12 to the gripper station 9.

The cripper station 9 having the gripper 8 further has a plurality of parking positions t 1-7 In 11 for the temporary deposition of wafers, these parking positions preferably additionally each having a heating device for heating up the wafers parked there.

1 Additionally, the gripper station 9 can have besides these further heating devices in 0 order to heat up, for example, a wafer which is on the -ripper 8 and is in the course of being transferred.

When the etching unit 10 is operated, first of all for example a batch of wafers to be processed, comprising 25 silicon wafers which are located in a wafer cartridee and c which are each to be subjected, by an HF vapour etching in one of the vapour etching C) w modules 2, 3, 4, 5 according to the process known from DE 197 04 454 AI, to a s'fic'al c 1 11 acri 1 -layer etching ofS02 sacrificial layers to produce surface micromechanical structures in the silicon wafers, is first of all admitted to the gripper station 9 by way of the exit cartridge station 7 at outside atmospheric pressure with the gripper 8 by way of the entry lock 12, and is transferred from there by way of the cleanint, lock 18 into the cleaning station 6, which is also under atmospheric pressure.

Z There, the admitted wafer cartridge with the batch of wafers is first thoroughly cleaned c 0 in a manner known per se by oxygen plasma stripping and/or the application of an ozone-containing gas at elevated temperature in accordance with DE 199 24 058.2, and is thereby brought into a state suitable for the application of the HF vapour etching process in the HF vapour etching modules 2, 3, 4, 5.

To avoid subsequent contamination of the wafers of the wafer batch which have been prepared in this way before the etching in one of the HF vapour etching modules 2, c w 3, 4, 5, the cleaned wafers in the wafer cartridge are then first transferred by way of the gripper station 9 with the gripper 8 into the vacuum cartridge station 1, which is then evacuated by way of a turbomolecular pump connected to the vacuum cartridge station 1 to a hierh vacuum of preferably a pressure of less than 10-6 mbar.

0 The pre-cleaned wafers thus remain permanently under high vacuum for the entire Ilwaiting time" in the vacuum cartridge station 1 until the actual HF vapour etching in Z:1 --> the HF vapour etching modules 2, 3, 4, 5, so that contamination is prevented.

Furthermore, the vacuum cartridge station 1 also remains under this high vacuum C> 11.

during the entire time of processing the batch of wafers, that is to say until all the wafers of the respective batch of wafers have been etched.

6 - The actual HY vapour etching in the vapour etching modules 2, 3, 4, 5 takes place, as is conventional, under atmospheric pressure, that is to say the gripper 8 in the gripper station 9 operates constantly between high vacuum conditions in the vacuum cartridge station 1 and atmospheric pressure in the vapour etching modules 2, 3, 4, 5 in that the gripper station 9 is regularly aerated and re-evacuated during the wafer handling procedures and transfers of wafers.

Transportation of the individual wafers and the wafer cartridge is in particular always carried out such that first the locks respectively agected by the wafer transfer, between the gripper station 9 and the vapour etching modules 2, 3, 4, 5, the cleaning station 6 c l> and the exit cartridge station 7 are closed. For this purpose, the locks 12, 13, 14, 15, c 16, 17, 18 are for example constructed in the form of pneumatically actuable vacuum slide valves.

In detail, to transfer a wafer from the vacuum cartridge station 1, first the gripper station 9 is for example evacuated by way of a turbomolecular pump connected thereto by way of a suitable valve and is brought to a low pressure which is comparable with c the vacuum cartridge station 1. Then, the storage lock 13 is opened so that the gripper c 8 can be introduced into the vacuum cartridee station 1 in order to transfer one or more z:wafers from the vacuum cartridge station 1 to the gripper station 9.

After this transfer, the storage lock 13 is then closed again and the gripper station 8 is aerated for example by high-purity nitrogen which is largely free of organic impurities. For this purpose, preferably nitrogen which has been obtained by evaporation or "bubbling" from a liquid nitroeen reservoir is used. This nitrogen can if necessary be 1:1 Z-5 further freed of residual organic impurities by additional filters.

c) A preferred construction of the example embodiment described moreover provides that, during the time in which the gripper station 9 is undergoing the aeration c c - 7 procedure with nitrogen, in order to convey the wafer or wafers therein into a respective HF vapour etching module 2, 3, 4, 5, these wafers are already pre-heated to a temperature of preferably 70T to 200T. For this purpose, corresponding heating =1 devices known per se, in particular one or more radiation heaters or heating plates, are C) pded in the gripper station 9, and the wafers can be heated up in the erripper station rovi 1 1 1 9 on or with these heating, devices. This has the effect that the temperature of the wafers respectively introduced into the HF vapour etching, modules 2, 3, 4, 5 is not below 7TC. Such a temperature for the pre-heated wafers transferred to the HF vapour etching modules 2, 3, 4, 5 prevents local condensation of the vapour hydrofluoric acid/water mixture onto the wafer occurring during the actual HF vapour etching process, which leads to undesirable condensation droplets and local excess values for etchino, rates.

Furthermore, pre-heating of the wafers in the gri 9 has the effect that the W ipper station wafers are less susceptible to organic contamination, so that residual organic gases which may still have been introduced into the gripper station 9 with the nitrooen flushinc, eras cannot contaminate the wafer in the brief time available for the aeration c c procedure other than to a negligible extent.

In order to ensure the temperatures of 6CC to WC necessary for the HF vapour etching process in the HF vapour etching modules 2, 3, 4, 5 during etching of the wafers, these have heating devices which are incidentally also known per se, as described for example in DE 198 50 078.5.

Once atmospheric pressure has been reached in the gripper station 9 by aerating it with nitrogen, finally one of the etching locks 14, 15, 16, 17 for the respective HF vapour etching module 2, 3, 4, 5 provided for the etching of this wafer is opened, and the - 8 wafer to be processed is transferred to this HF vapour etching module 2, 3, 4, 5 by the gripper 8 and is deposited there.

0 Then, the lock of the AT vapour etching module concerned is closed once the gripper 0 8 has withdrawn into the empper station 9, and the wafer is subjected to an HF vapour phase etching in a manner known per se.

0 Once this etchine, is complete, the lock of the HF vapour etching module concerned CY towards the gripper station 9 is opened again, the gripper 8 takes the wafer from the vapour etching module and transfers it back to the gripper station 9. Then, the lock between the -ripper station 9 and the HF vapour etching module 2, 3, 4, 5 concerned is closed and the gripper station 9 is evacuated. When a pressure corresponding to the pressure of the vacuum cartridge station 1 has been reached, the storage lock 13 is, finally, opened and the wafer which has finished being etched is deposited by means of the gripper 8 in the wafer cartridge (entry cartridge) or alternatively in a second socalled exit cartridge located there.

Then, the gripper 8 takes a new wafer from the wafer cartridge (entry cartridge) and transports it, in a manner analogous to the statements made above, to the etching by way of the gripper station 9 and from there on into an HF vapour etching module 2, 3,4, 5.

The wafer cartridues used for the batch of wafers incidentally preferably comprise a material which can withstand a hieh vacuum, for example stainless steel or quartz ZD alass, which does not Csive off ias and thus is not itself a source of contamination.

The example embodiment described can furthermore be optimised as regards the sequence of handling procedures, which results in a minimisation of the time to be 9 - taken for each wafer processed. In particular, the necessary but time- consuming, procedures of pumping out and aeration are kept to a mini Z 1 1 imum.

Thus, in a preferred construction of the example embodiment described, a wafer which has finished being processed is not transported back into the vacuum cartridge station 1 from an HF vapour etching module 2, 3, 4, 5, but is transferred by the gripper 8 c c under atmospheric pressure into an atmospheric cartridge station, for example the exit c deye station 7, and is deposited there, for example in an exit cartridge. There the cartri. Z:1 wafers are in fact contaminated by organic impurities as time goes on, but this is no longer a source of problems.

C> In detail, in this case in order to unload a wafer from an HF vapour etching module 2, 3, 4, 5, first the lock concerned towards the gripper station 9, which is in the aerated state, is opened, the gripper 8 transfers the wafer into the gripper station 9, the lock towards the HF vapour etching module is closed and the entry lock 12 is opened. There, the gripper 8 then deposits the wafer which has finished being processed in an exit cartridge. After the entry lock 12 has closed, the gripper station 9 is evacuated again, in order subsequently to transport a new wafer from the wafer cartridge, which is in the vacuum cartridge station 1, into an HF vapour etching module 2, 3, 4, 5. This results in halving, the number of pumping-out procedures in the gripper station 9.

1 In this connection, the parkin., positions 11 which are provided in the gripper station c> 9 and on which a respective wafer can be deposited and where appropriate heated can also contribute to a further reduction in the number of pumping-out procedures required.

For this purpose, the gripper 8 for example, as described above, first transfers a wafer c from the wafer cartridge in the vacuum cartridge station 1 to a parking position 11 within the aripper station 9, where the wafer for example is already heated up by a C1 radiation heater or a contact heater (heating plate) while the gripper station 9 is still being aerated. After aeration of the gripper station 9 to atmospheric pressure, the gripper 8 then first transfers the preceding wafer which has finished being processed from an HF vapour etching module 2, 3, 4, 5 to a second parking position 11 within W _n the aripper station 9 or, alternatively, already to a cartridge station at atmospheric pressure, for example the exit cartridge station 7, before the new wafer waitina in its parking position 11 is then introduced into the HF vapour etching module.

c Only then must the gripper station 9 be evacuated again, in order either to transport the wafer which has already finished being processed, and which is waiting in its parking position 11, to the vacuum cartridge station 1 and either to deposit it in the wafer idge (entry cartridge) or in a second exit cartridge, and to take up a new wafer car-tri _. from the wafer cartridge (entry cartridge) and transport it to the gripper station 9. This C means that overall the number of pumping-out and aerating procedures per wafer operation is halved again.

The process described with parking positions 11 in the gripper station 9 is particularly c advantageous in particular when a plurality of connected HF vapour etching modules 2, 3, 4, 5 can be operated by way of a central gripper station 9. The "clusterinj of HF vapour etching modules 2, 3, 4, 5 on a gripper station 9 can thus considerably increase the throughput of wafers and improve the utilisation of central functions such as the vacuum cartridge station 1 or the gripper station 9. In this connection, it should further be noted that it is readily possible to control the transfer and pumping-out and aerating processes, and in particular to actuate the locks and valves to the pumps, fully automatically by way of a corresponding automated computer-controlled machine control.

If for example n HF vapour etching modules 2, 3, 4, 5 are arranged on a gripper C.) station 9 In a cluster, then for example n (n = 1 to 40) wafers will for example first be 11 broucyht successively by the gripper 8, with the gripper station 9 in the evacuated state, from the wafer cartridge (entry cartrl'dcce) located in the vacuum cartridge station 1 into n+l parking positions 11 provided therefor in the gripper station 9. These parking positions are advantageously combined with corresponding heating devices which perform pre-heating of the wafers to the introduction temperature necessary for the HF vapour etching. Then, the storage lock 13 will be closed and the gripper station 9 aerated. Once atmospheric pressure has been reached, then the n locks 14, 15, 16, 17 between the n vapour etching modules 2, 4, 5 and the gripper station 9 will first be opened and the n connected HF vapour etching modules 2, 3, 4, 5 discharged successively by the gripper 8. During this, the gripper 8 can unload each wafer which has already finished being processed and which is to be removed from the n vapour etching modules 2, 3, 4, 5 either into an additional exit cartridge at atmospheric pressure, for example the exit cartridge station 7, or alternatively transport each of the n unloaded wafers first into a parking position 11 within the gripper station 9.

If during this each time a wafer has been unloaded ftom an HF vapour etching module t 2- 2, 3, 4, 5 the latter is immediately reloaded with a new wafer from a parking position 11, then the number of parking, positions 11 necessary is reduced to n+l instead of 2n. At the same time, the time during which an HF vapour etching module 2, 3, 4, 5 has to be opened is minimised.

The clustering of a plurality of HF vapour etching modules 2, 3, 4, 5 is particularly C 0 expedient and advantageous if a high throughput of wafers is necessary. At the same time, it has the effect that the expensive central components of the etching unit 10, that is to say the vacuum cartridge station 1 and the gripper station 9, are utilised in as economic a manner as possible.

If the etchina unit 10 is provided with a cleaning station 6, it is furthermore expedient c c first not to carry out oxygen plasma stripping of the wafers to be processed before they - 1 1) are loaded into the vacuum cartridge station 1, and to admit the wafer cartridge to the gripper station 9 first by way of the exit cartridge station 7 as described. Thereafter, the gripper station 9 is then evacuated to high vacuum and the wafer cartridge, with the wafers to be processed, is transferred from the gripper station 9 by way of the cleanina lock 18 into the cleaning station 6, which is also under vacuum, and is stripped or cleaned there in an oxygen plasma in a manner known per se.

c Thereafter, the individual wafers are then transferred to the individual HF vapour etching modules 2, 3, 4, 5 for etching without again breaking the vacuum, as described above, by way of the gripper station 9. Since in this procedure the wafers do not leave Z_ the vacuum between the stripping or cleaning in the cleaning station 6 and the etching C W in an I-IF vapour etching module 2, 3, 4, 5, particularly contamination- free conditions are ensured.

List of reference numerals 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 Vacuum cartridge station First HF vapour etching module Second HF vapour etching modul Third HF vapour etching module Fourth HF vapour etching module Cleaning, station Exit cartridge station Gripper Gripper station Etching unit Parking position Entry lock Storage lock,g First etchin(y lock Second etchinc, lock Third etchine, lock Fourth etching lock Cleaning lock - 14

Claims

Claims

An etchine, unit havine, at least one etchin. dev'ce for etch ing substrates, in particular silicon wafers microstructured with a sacrificial layer of S102, in a vapour hydrofluoric acid/water mixture, at least one transportation device (9) for the substrates which is capable of bein:, evacuated and is connected to at least one of the etchinc, devices (2, 3, 4, 5) by way of at least one lock (14, 15, 16, 17) which is closable to be at least largely vacuum-tight, and having at least one storage device (1) which is capable of being evacuated and is connected to at least one lock (13) towards the transportation device (9), this lock (13) being closable to be at least largely vacuum-tight, a substrate in the storage device (1) being transferable by way of the transportation device (9) to one of the etching devices (2, 3, 4, 5).

2. An etching unit according to Claim 1, characterised in that the etching device is an HF vapour etching module (2, 3, 4, 5).

C> 3. An etching unit according to Claim 1, characterised in that the transportation device is a gripper station (9).

1 4. An etching unit according to Claim 3, characterised in that the z: A 1 1 1 gripper station (9) has at least one gripper (8).

c 1 5. An etching unit according to Claim 1, characterised in that the storage device is a vacuum cartridae station 6. An etching unit according to Claim 1, characterised in that the etching unit (10) has at least one loading device, in particular an exit cartridge station (7) which is connected to the transportation device (9) by way of at least one entry lock (12) which is closable to be at least largely vacuum-tl-ht.

- 7.

An etchin. unit accordina to Claim 1, characterised in that the locks IP => (12, 13, 14, 15, 16, 17, 18, 19) are vacuum slide valves which may be actuated pneumatically by way of a machine control.

8. An etching unit according to Claim 1, characterised in that a dev' 1 1 C) 1 1 1 ice for evacuating the transportation device (9) or the transportation device (9) and the storage device (1) is provided and has in particular at least one turbomolecular pump.

9. An etching unit accordinc, to Claim 1, characterised in that the C1 ty transportation device (9) or the transportation device (9) and the storage device (1) may be evacuated to a h] gh vacuum, in paiticular to a pressure less than C1 ^-6 rn bar.

10. An etching unit according to Claim 1, characterised in that an aerating device is provided for the transportation device (9) or the transportation device (9) and the storage device (1) and may be used to aerate the transportation device (9) and/or the storage device (1) in particular with high-purity or cleaned nitrogen.

An etching unit according to Claim 1, characterised in that at least 0 one cleanine, device, in particular a cleaning station (6), is provided, which is connected to the transportation device (9) by way of at least one cleaning lock (18) which is closable to be at least largely vacuum- tight.

c =5 12. An etchin(y unit according to Claim 11, characterised in that the C> cleaning station (6) has at least one device for cleaning a substrate by oxygen plasma stripping or for the application of an ozone-containing gas.

1. C 16 13. An etching unit according to Claim 1, characterised in that the transportation device (9) has at least one parking position (11) for temporary deposition of a substrate.

14. An etchina unit according to Claim 13, characterised in that the parking position (11) may be heated in partic ular by means of a radiation heater or a contact heater.

15. An etching unit according to Claim 11, characterised in that the cleaning station (6) has at least one device for heating a substrate and/or in that the etchincy devices have at least one heating device, in particular for heating a substrate C1 durinc, the etching.

16. An etching unit according to at least one of the preceding claims, characterised in that the loading device (7) may be charged with an entry cartridge having, a plurality of substrates.

17. An etchin. unit according to at least one of the preceding claims, characterised in that the cleaning station (6) may be loaded with the entry cartridge or with the entry cartridge and an exit cartridge, the entry cartridge and/or the exit cAC W artri ge being capable of being charged with a plurality of substrates.

18. An etching unit according to at least one of the preceding claims, characterised in that the etching unit (10) has at least one discharging device which is Z connected to the transportation device (9) by way of an exit lock which is closable to be at least largely vacuum-tight, and by way of which an etched substrate or an exit havin. etched substrates can be removed from the etching unit (10).

cartn,, 0 1 - 17 19. An etchina unit according to at least one of the preceding claims, characterised in that an at least largely automated, in particular computer-controlled machine control is provided for actuating the locks (12, 13, 14, 15, 16, 17, 18, 19) and the device for evacuating the transportation device (9) and for controlling the gripper (8) for the transfer of substrates between the storage device (1), transportation device (9), loading device (7), cleaning station (6) and discharging device.

20. An etchinc, unit substantially as hereinbefore described with reference Z to the accompanyinc, drawings.

C