EP1181400B1 - Electrochemical etching installation and method for etching a body to be etched - Google Patents

Description

The invention relates to an electrochemical etching system, in particular a CMOS-compatible etching system for etching silicon wafers, and their use according to the preamble of the independent claims.

State of the art

An electrochemical etching system for etching an etching body is basically known to a person skilled in the art. Thus, US Pat. No. 4,220,508 describes an electrochemical etching cell for etching aluminum or aluminum alloys, wherein a plate made of this material is immersed in an electrobath. In this electrobath, a cathode and an anode are further provided, which consist of aluminum, stainless steel or another metal. However, an etching of an etching body, which consists at least superficially of silicon, is not discussed in this document.

On the other hand, WO 94/21845 presents an electrochemical etching cell for etching a silicon wafer. In this case, the anode and the cathode made of graphite plates. In Scripture the potential problem of a contamination introduced by the electrodes into the etched silicon wafer is neither mentioned nor discussed.

Electrochemical etching systems, for example for the production of porous silicon or for the surface porosification of silicon, usually consist of a 2-chamber system, between which a silicon wafer to be etched is clamped as a partition and wherein the two chambers are electrically coupled or connected only by the wafer , Furthermore, electrodes for supplying current are usually mounted in both chambers, which are usually made of platinum. Such an etching system is for example described in detail and in its essential details already by Fujiyama et al. in US 5,458,755.

In known etching systems, however, there is always the problem that at least the anodically connected electrode is at least slightly attacked and dissolved during operation, so that first the electrolyte and above the wafer to be etched is contaminated by the dissolved electrode material in the course of the etching process. However, such contamination, for example of platinum in a silicon production, is often unacceptable and significantly affects the etched wafer or etch body in its electrical or catalytic properties.

In particular, a silicon wafer on or in which an electrochemical etching process has produced a layer of porous silicon and which has been contaminated with platinum is unsuitable for use in CMOS fabrication (CMOS = complementary metal-oxide semiconductor) ,

Suggested solutions to this problem, which are based on a one-sided metallization of the wafer back and the use of a single-sided etching, wherein the back of the wafer to be etched forms the metal contact and only the front with the etching medium and the electrolyte and above with a platinum electrode in combination , are due to the thereby applied and required backside metallization and the necessary subsequent steps in the processing of the wafer (oxidation, layer depositions, etc.), which then this metallization in the way, unsuitable.

Also in JP-06275598 A, the problem of contamination of the electrolyte and thus of the wafer material to be etched is treated by the material of the dissolving electrodes. It is proposed to attach a bar of silicon wafer directly in front of the dissolving electrode. Since this barrier is conductive and contacts the electrode directly, it acts as an electrode itself. In this document, this measure is not proposed for both electrodes.

Advantages of the invention

The electrochemical etching system according to the invention for etching an etching body and its use with the characterizing features of the independent claims has the advantage over the prior art that it completely avoids contamination of the etching body, which at least superficially consists of silicon. This is especially true in the production of porous silicon from a silicon wafer. Thus, by this etching of the Etch body is not impaired in particular in its electrical or electronic or catalytic properties.

The advantage is achieved by having not only the surface of the positive metal electrode in contact with the electrolyte, but also the negative electrode in contact with the electrolyte made of a CMOS-compatible material, i. H. the first electrode material and the second electrode material of the first and second electrodes is a CMOS-compatible material and in particular no element selected from the group of platinum, gold, iridium, rhodium, palladium, silver or copper. Thus, the etching system according to the invention is particularly suitable for the production of porous silicon on a silicon wafer, wherein for example by the use of silicon electrodes contamination of the wafer with non-silicon substances such as platinum or palladium is prevented.

Under a CMOS-compatible material is understood in accordance with the general usage in semiconductor technology, a material that does not adversely affect the electrical properties of a circuit generated therewith.

Accordingly, a material which contaminates the etching body is understood to mean, in particular, a CMOS poison or a material which forms deep impurities in the etching body during its incorporation, ie impurities whose energy levels lie in the middle of the gap between conduction band and valence band of the material to be etched and thus providing a high transition matrix element for the recombination of Cause electrons and holes in the etch body ("recombination germ").

As electrode materials for the first or second electrode are compounds from the group of at least weakly conductive compounds of the elements silicon, carbon, nitrogen, oxygen, titanium, aluminum, boron, antimony, tungsten, cobalt, tellurium, germanium, molybdenum, gallium, arsenic and selenium, in particular SiC, SiN, TiN, TiC, MoSi ₂ or GaAs, in question, as well as pure electrode materials of the elements silicon, titanium, tungsten and molybdenum.

The concrete selection of the respective electrode material is advantageously carried out in each case taking into account the material of the etching body and the electrolyte used.

Further advantageous developments of the invention will become apparent from the measures mentioned in the dependent claims.

In addition, the first electrode and / or the second electrode and / or the etching body are advantageously flat, in particular in the form of wafers, wherein the electrodes for use as sacrificial electrodes are very advantageously also substantially thicker than the actual etching body, so that they are optionally treated , can be freed from contamination and reused. This advantageously achieves an extension of the replacement cycles of the electrodes.

The electrochemical etching cell is advantageously constructed such that a first chamber and a second chamber are provided, each of which at least partially filled with an electrolyte and which are spatially separated from one another via a separating device. It stands each of the both chambers, each having an electrode in electrical connection via an electrolyte, wherein the etching body is at least partially the separation device and very advantageous at the same time also the only, at least weakly conductive electrical connection between the two chambers and connected as the cathode or anode electrodes.

A further very advantageous embodiment of the invention provides that the electrochemical etching cell in addition to the already mentioned two chambers has a further third chamber or a further third chamber and a further fourth chamber, each filled at least partially with an electrolyte and each have a further Separating device of the first chamber and second chamber are spatially separated. In this case, the electrolyte in the third or fourth chamber is very advantageously in electrical connection only with the second or first electrode, which in turn at least partially serves as a separating device between the third and fourth chambers and respectively the first and second chambers.

In this context, it is particularly advantageous if the particular areal trained first and / or the second electrode in each case only with its surface facing the etching body with the contact with the etching body in contact electrolyte, so that a mixing of the electrolyte in the third or fourth chamber is avoided with the electrolyte in the first and second chamber. Thus, the electrolyte of the first or second chamber facing away from the first and / or second electrode for easier electrical contacting of the electrodes at least partially be superficially provided with a metallization or a doping or, for example, in the case of the construction of the electrode of several layers, made of a metal.

Furthermore, in each case an additional bath electrode immersed in an electrolyte located there, in particular a platinum or palladium electrode, can be provided in the third and fourth chambers for easy electrical contacting of the first and second electrodes via the respective electrolyte.

Moreover, the electrolytes in the individual chambers of the etching system according to the invention may advantageously also be different from each other, wherein the first and second chambers, in which the actual etching of the etchant takes place, advantageously with hydrofluoric acid or a mixture of hydrofluoric acid and ethanol, and the third and fourth Chamber are filled, for example, with dilute sulfuric acid as a contact electrolyte.

The individual chambers are also very advantageous separately filled with electrolyte and can be emptied separately, so that at any time a problem-free replacement of a contaminated electrolyte in each chamber is possible. In addition, a simple replacement of a consumed or contaminated first or second electrode serving as a sacrificial electrode is thereby made possible without any problems at any time.

Incidentally, the first and / or second electrode is advantageously contacted electrically via the electrolyte filled in the third or fourth chamber with a bath electrode located there, and thus with an external electrode Power supply connected, which impresses the etching system during operation, a current.

The easy interchangeability of the sacrificial electrodes i. The first and / or the second electrode also makes it possible, in a very advantageous manner, to examine in a simple manner the suitability of different electrode materials, such as graphite, for etching an etching body and to optimize the electrode materials for the respective material of the etching body.

In order to homogenize the etching of the etching body in the etching system according to the invention, a tunnel made of nonconductive material, in particular polypropylene, can furthermore advantageously be provided in a manner known per se.

drawings

The invention will be explained in more detail with reference to the drawings and the description below. FIG. 1 shows a first electrochemical etching system, FIG. 2 shows an alternative embodiment of the etching system, and FIG. 3 shows a third embodiment of the etching system.

embodiments

FIG. 1 shows as the first exemplary embodiment an electrochemical etching cell 1 according to the invention with four chambers, a first chamber 19, a second chamber 19 ', a third chamber 17 and a fourth chamber 18, which are each filled at least partially with an electrolyte. The first and second chambers 19, 19 'is for the actual etching of an etching body 15, for example, with a mixture of hydrofluoric acid and ethanol, while the third and fourth chambers 17, 18 are filled, for example, with dilute sulfuric acid as a contact electrolyte. The four chambers 17, 18, 19, 19 'thus define four electrolyte regions associated with the chambers 17, 18, 19, 19', a first electrolyte region 29, a second electro-lyte region 29 ', a third electrolyte region 27 and a fourth electrolyte region 28, one another spatially separated by separating devices, but at the same time allow an electrical connection of the chambers 17, 18, 19, 19 '.

More specifically, the first chamber 19 is spatially separated from the second chamber 19 'via a first separator 31, the first chamber 19 from the third chamber 17 via a second separator 32 and the second chamber 19' from the fourth chamber 18 via a third separator 33 spatially separated, so that no exchange of electrolyte between the chambers 17, 18, 19, 19 'occurs.

The first separator 31 is formed in a conventional manner by a Ätzkörperhalterung 11 made of Teflon or polypropylene, in the partially fitted or used the Ätzkörper 15, so that this superficially on the one hand with the electrolyte in the first chamber 19 and the other with the electrolyte in the second chamber 19 'is in contact. The etching body 15 is in the illustrated example a known per se, planar silicon wafer. The second separation device 32 and the third separation device 33 are each formed by an electrode holder 10 made of Teflon, in each of which a second electrode 13 'and a first electrode 13 is inserted, so that these surface at least partially on the one hand with the Electrolytes of the third and first chamber 17, 19 'and on the other hand with the electrolyte of the second and fourth chamber 19', 18 are in contact.

As a metallic contact electrode for contacting the first and second electrodes 13, 13 'is in the third and fourth chamber 17, 18 each have a platinum electrode or a palladium electrode as a bath electrode 34, 34' is provided, each immersed in the electrolytes located there. The bath electrodes 34, 34 'are further connected to a voltage source, not shown, which impresses an electric current in a manner known per se to the etching cell 1. In this case, in the illustrated example with respect to the etching body 15, the first electrode 13 or its side facing the etching body 15 is connected as an anode and the second electrode 13 'or its side facing the etching body 15 is connected as a cathode.

The first electrode 13 and the second electrode 13 'in the illustrated example consist of a planar silicon wafer or a silicon wafer, which is preferably substantially thicker than the silicon wafer used as the etching body 15. In general, the electrodes 13, 13 'with respect to the electrode material used in each case are preferably selected such that they consist of silicon at least superficially as the surfaces of the etching body 15. This ensures that the material of the first electrode 13 and the material of the second electrode 13 ', the etching body 15 is not contaminated during operation of the etching cell 1 and thus impaired after the etching in its electrical or catalytic properties.

During operation of the etching cell 1 now flows an external eingepägter current through the Badelektroden 34, 34 ', the Electrolytes, the first and second electrodes 13, 13 'and the etching body 15, wherein this is etched at least superficially in a Körperätzbereich 14'. At the same time, however, depending on the choice of the electrode material of the electrodes 13, 13 ', also the first and second electrodes 13, 13' at least superficially etched in an etching region 14, ie they serve as sacrificial electrodes in the etching process of the etching body 15. Due to their significantly greater thickness However, they are not etched through the etching body 15, but merely attacked on the surface, etched off or removed or, for example, porosified. In the case of wear, for example after the etching of a plurality of etching bodies 15, they can therefore be exchanged, reprocessed or, if necessary, regularly cleaned of adhering contaminations.

In detail, in the illustrated example, during the etching of a silicon wafer at its anodic side, ie in the case of the indicated polarity in the body etching region 14 ', porous silicon is formed, while at the same time on the anodic side of the first electrode 13 facing the etching body also an at least slight etching in a corresponding Elektrodenätzbereich 14 occurs, ie in the concrete example, a superficial formation of porous silicon. Incidentally, this also applies to the side of the second electrode 13 'facing away from the etching body 15, which assumes the role of the anode in the third chamber 17. At the same time, during the operation of the etching cell 1, the metallic bath electrode 34 anodically connected in the fourth chamber 18 also dissolves slightly, but only the side of the first electrode 13 facing away from the etching body 15 is contaminated with platinum, for example. Due to the spatial separation of the individual chambers 17, 18, 19, 19 'between which only an electrical connection via However, the electrodes 13, 13 'and the etching body 15 is between which but no electrolyte exchange is possible, however, this contamination remains away from the etching body 15. It can thus be removed in a preparation of the electrodes 13, 13 'from the corresponding side again.

Dissolution of silicon from one of the electrodes 13, 13 'in the electrolyte in the first or second chamber 19, 19' occurring during the etching, for example, is not critical for the etching body 15, since it consists of the same material and thus is not contaminated.

For easy replacement of the electrodes 13, 13 ', these are preferably connected via seals with the electrode holders 10 and screwed via closable window 16 in side walls of the etching system 1 with this. For a simple replacement of the etching body 15, a per se known quick release is provided.

For easy replacement of the electrolytes used and the electrodes 13, 13 'are also the chambers 17, 18, 19, 19' and the associated electrolyte regions 27, 28, 29, 29 'via respective devices known per se separately fillable and emptied ,

FIG. 2 illustrates a second exemplary embodiment of an etching cell according to the invention. This etching cell is essentially analogous to the etching cell 1 according to FIG. 1 in essential points, but has another embodiment of the contacting of the electrodes 13, 13 '. In this example, the third chamber 17 and the fourth chamber 18, the bath electrodes 34, 34 'and the electrolytes located in these chambers 17, 18 can be dispensed with. Instead For example, the first electrode 13 and the second electrode 13 'are each provided with a metallization 20 known per se on the side facing away from the etching body 15.

Alternatively, however, the electrodes 13, 13 'may also be provided on this side with a very high doping, so that a good electrical conductivity is ensured. Finally, the electrodes 13, 13 'can also consist of a layered body which has a metal layer on its side remote from the etching body 15 or consists of a metal. Further embodiments of the electrical contacting of the electrodes 13, 13 'provide that they are provided in a conventional manner on the side facing away from the etching body 15 with pin, network or surface contacts or, depending on the electrode material, particularly simple that the electrodes 13, 13 'in the first and second chambers 19, 19' partially immersed directly in the electrolyte and are electrically contacted directly at a non-immersed location. They thus serve as sacrificial electrodes instead of the known from the prior art platinum electrodes.

Finally, reference should be made to a third exemplary embodiment of the etching system according to the invention explained with the aid of FIG. 3, in which, in contrast to FIG. 1, additionally only a tunnel 30 of a non-conductive material, such as polypropylene, known per se is provided. This tunnel 30 is connected on both sides with the Ätzkörperhalterung 11 and surrounds an example circular wafer as the etching body 15 concentric. The tunnel 30 causes a homogenization of the flow lines in the etching system 1 and thus an excellent thickness homogeneity of the etching of Etching body 15, in particular in the etching of silicon to porous silicon.

Further, known in the art details of the preceding embodiments, which are described in detail, for example, already in US 5,458,755, is omitted.

LIST OF REFERENCE NUMBERS

1

etching cell

10

electrode holder

11

Ätzkörperhalterung

12

metal electrode

13

first electrode

13 '

second electrode

14

Elektrodenätzbereich

14 '

Körperätzbereich

15

etching body

16

window

17

third chamber

18

fourth chamber

19

first chamber

19 '

second chamber

20

metallization

27

third electrolyte area

28

fourth electrolyte area

29

first electrolyte area

29 '

second electrolyte area

30

tunnel

31

first separator

32

second separation device

33

third separator

34

bath electrode

34 '

bath electrode

Claims (18)

1. Electrochemical etching cell for etching an etching body (15) at least whose surface is composed of silicon, having at least one chamber which is at least partially filled with an electrolyte, and which is provided with a first electrode (13), at least whose surface has a first electrode material, and with a second electrode (13') at least whose surface has a second electrode material, with one of the electrodes (13, 13') being connected as a cathode and one of the electrodes (13, 13') being connected as an anode, and with the etching body (15) being in contact with the electrolyte, at least in places, characterized in that the first electrode material and the second electrode material is a material selected from the group of the at least slightly conductive compounds of the elements silicon, carbon, nitrogen, oxygen, titanium, aluminium, boron, antimony, tungsten, cobalt, tellurian, germanium, molybdenum, gallium, arsenic and selenium, in particular SiC, SiN, TiN, TiC, MoSi2 or GaAs or the elements silicon, titanium, tungsten or molybdenum.
2. Electrochemical etching cell according to Claim 1, characterized in that the etching body (15) is a silicon wafer.
3. Electrochemical etching cell according to Claim 1 or 2, characterized in that the first electrode (13) and/or the second electrode (13') and/or the etching body (15) are/is flat.
4. Electrochemical etching cell according to Claim 1 or 3, characterized in that the first electrode (13) and/or the second electrode (13') are/is in each case in contact, at least in places, with the electrolyte, which is in contact with the etching body, only by their or its surface facing the etching body (15).
5. Electrochemical etching cell according to at least one of the preceding claims, characterized in that a first chamber (19) and a second chamber (19') are provided, which are each at least partially filled with an electrolyte and are physically separated from one another by a first separating apparatus (31), with the first chamber (19) being electrically conductively connected to the first electrode (13') and with the second chamber (19') being electrically conductively connected to the first electrode (13), and with the etching body (15) forming the first separating apparatus (31), at least in places.
6. Electrochemical etching cell according to Claim 5, characterized in that a third chamber (17) is provided, is at least partially filled with an electrolyte and is physically separated from the first chamber (19) via a second separating apparatus (32), with the third chamber (17) being electrically conductively connected to the second electrode (13'), and with the second electrode (13') forming the second separating apparatus (32), at least in places.
7. Electrochemical etching cell according to Claim 5 or 6, characterized in that a fourth chamber (18) is provided, is at least partially filled with an electrolyte and is physically separated from the second chamber (19') via a third separating apparatus (33), with the fourth chamber (18) being electrically conductively connected to the first electrode (13), and with the first electrode (13) forming the third separating apparatus (33), at least in places.
8. Electrochemical etching cell according to Claim 5, characterized in that the first chamber (19) and the second chamber (19') are electrically conductively connected to one another only via the etching body (15).
9. Electrochemical etching cell according to Claim 6 or 7, characterized in that the first chamber (19) and the third chamber (17) are electrically conductively connected to one another only via the second electrode (13'), and/or the second chamber (19') and the fourth chamber (18) are electrically conductively connected to one another only via the first electrode (13).
10. Electrochemical etching cell according to Claim 5, characterized in that the first electrode (13) is flat and is electrically conductively connected on only one side to the electrolyte in the second chamber (19'), and/or in that the second electrode (13') is flat and is electrically conductively connected on only one side to the electrolyte in the first chamber (19).
11. Electrochemical etching cell according to Claim 5, characterized in that the first electrode (13) is flat and is electrically conductively connected on one side to the electrolyte in the second chamber (19') and on the other side to the electrolyte in the fourth chamber (18), and/or in that the second electrode (13') is flat and is electrically conductively connected on one side to the electrolyte in the first chamber (19), and on the other side to the electrolyte in the third chamber (17).
12. Electrochemical etching cell according to Claim 10, characterized in that that side of the first electrode (13) and/or of the second electrode (13') which faces away from the electrolyte in the first or second chamber (19, 19') is provided at least in places on the surface with a metallization (20) or heavy doping, or is composed of a metal.
13. Electrochemical etching cell according to at least one of the preceding claims, characterized in that the first electrode (13) is connected to a bath electrode (34), in particular a platinum electrode, via the electrolyte in the third chamber (17), and/or the second electrode (13') is connected to a bath electrode (34'), in particular a platinum electrode, via the electrolyte in the fourth chamber (18).
14. Electrochemical etching cell according to at least one of the preceding claims, characterized in that the electrodes (13, 13') are considerably thicker than the etching body (15).
15. Electrochemical etching cell according to at least one of the preceding claims, characterized in that the chambers (17, 18, 19, 19') are filled with different electrolytes, with the first and the second chamber (19, 19') in particular being filled with hydrofluoric acid and/or with a mixture of hydrofluoric acid and ethanol, and with the third and the fourth chamber (17, 18) being filled in particular with dilute sulphuric acid.
16. Electrochemical etching cell according to at least one of the preceding claims, characterized in that a tunnel (30) composed of non-conductive material, in particular polypropylene, is provided for homogenization of the etching of the etching body (15), with the tunnel being connected at both ends to an etching body holder (11), and concentrically surrounding the etching body (15).
17. Electrochemical etching cell according to at least one of the preceding claims, characterized in that the chambers (17, 18, 19, 19') can be filled and emptied separately.
18. Use of the etching cell according to at least one of the preceding claims for etching of silicon wafers in a CMOS-compatible production line.

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