Abstract:
Method for etching a substrate wherein, after placing in an etching chamber, said substrate is treated with a mixture of HF and acetic acid. Acetic acid is introduced into the chamber first, followed by the hydrogen fluoride. Hydrogen fluoride is introduced via an intermediate stage during which the hydrogen fluoride is stored in an auxiliary chamber. By this means back-flow of a corrosive mixture consisting of hydrogen fluoride and acetic acid into the piping assembly for hydrogen fluoride is prevented and, thus, the life of the piping assembly concerned is appreciably prolonged and metal contamination on substrate treated later is prevented.

Description:
This application is a continuation-in-part of International Application No. PCT/NL99/00487, filed Jul. 29, 1999 and now abandoned, which claims priority to Dutch Application No. NL 1009767, filed on Jul. 29, 1998. 

   FIELD OF THE INVENTION 
   The present invention relates to a method and installation for etching a substrate. 
   BACKGROUND OF THE INVENTION 
   A method this type is known from EP 0.335.313-A-2. For etching silicon dioxide on the surface of semiconductor substrates, such a substrate is placed in a plastic-lined stainless steel etching chamber and then treated, if appropriate under reduced pressure, with a corrosive mixture of acetic acid, water or methanol (preferably 100%) and hydrogen fluoride. The chamber is first filled with the requisite amount of acetic acid, the requisite amount of hydrogen fluoride then being introduced. The various sources or feeds of hydrogen fluoride and acetic acid are each closable by means of valves. The piping system is made either of stainless steel or of plastic, such as perfluoroalkoxy (PFA). 
   Although a method of this type yields excellent etching results, there is the problem that, when the hydrogen fluoride shut-off valve is opened, on the one hand hydrogen fluoride flows into the etching chamber, but some acetic acid inevitably diffuses back via said shut-off valve into the piping system for HF. Such an amount is found to be very small in practice and not to have an adverse effect on the rate of etching, but it has been found that the upstream stainless steel piping system as a result is attacked as far as the BF source and that this can result in undesirable metal contaminants on the semiconductor substrate in subsequent etching operations. 
   This means that it is sometimes necessary to replace such a piping system and that in any event it is necessary to subject such a piping system to a regular check. 
   Both operations impede the process operation and are disadvantageous. 
   In EP 0.335.313-A-2 mentioned above, a method and device are described, wherein two etch gases from a ft and a second source are being supplied through a common conduit to an auxiliary chamber. In this auxiliary chamber a plasma is generated and this is transferred to the reactor. The etching gases used are CF 4  and O 2 . It is of importance that both gases are being supplied simultaneously and preferably continuously from the source through the auxiliary chamber to the process chamber. 
   Such a method does not give a solution for the problem mentioned above i.e. the aggressively of a mixture of HF and another etchant and/or catalyst such as acetic acid, formic acid or water. 
   SUMMARY OF THE INVENTION 
   The aim of the present invention is to circumvent this disadvantage and to provide a method which can be carried out continuously without ongoing checks and/or replacement of the piping being necessary. 
   One aspect of the present invention involves a method for etching a substrate. A substrate is placed in an etching chamber and at least one of a first etchant and a first etch catalyst originating from a first source is introduced into the etching chamber via an auxiliary chamber positioned within a first path. The introducing includes intermittently closing an inlet of the auxiliary chamber after introduction of the at least one of a first etchant and a first etch catalyst. The outlet of the auxiliary chamber is opened to discharge the at least one of a first etchant and a first etch catalyst into the etching chamber so that the inlet is closed when the outlet is opened. At least one of a second etchant and a second etch catalyst originating from a second source is introduced into the etching chamber via a second path. The substrate is etched and the etching chamber is flushed following the etching. The substrate is then removed from the etching chamber. 
   It is true that as a result of the use of an auxiliary chamber it is possible that when the outlet from the auxiliary chamber to the etching chamber is opened the other reactant, such as acetic acid, diffuses back into said auxiliary chamber, but this reactant is not able to diffuse back into the piping system for said one reactant, such as hydrogen fluoride. After all, the auxiliary chamber is closed off on that side. By removing both reactants from the auxiliary chamber (and the thing cab) before the next process, further back-diffusion in the direction of the HF feed can be precluded. It has been found that there is no attack on the upstream piping. 
   Consequently, it is possible to construct only the piping system from the auxiliary chamber to the etching chamber from relatively expensive plastic, but to construct the upstream part of the piping system with respect to the auxiliary chamber from inexpensive stainless steel material. This applies in particular if a ‘mass flow controller’ is used, which is not obtainable in plastic. 
   Back-diffusion can be completely precluded if, according to a preferred embodiment of the invention, following treatment in the etching chamber flushing is carried out via the auxiliary chamber. After closing the auxiliary chamber outlet valve, an elevated gas pressure can be applied in the auxiliary via the inlet valve using nitrogen or another inert gas, and the auxiliary chamber can be flushed with the aid of said nitrogen, back-diffusion being precluded. With this arrangement it is possible to use the auxiliary chamber as a lock, at least during the initial phase, when flushing with nitrogen as well, that is to say first to admit nitrogen into the chamber, to close the auxiliary chamber inlet and then to open the outlet to the etching chamber. 
   According to another possibility, a vacuum can be applied to both the auxiliary chamber and the etching chamber, followed by one or more flushing operations with an inert gas, such as nitrogen. It is possible to apply a vacuum to the auxiliary chamber and etching chamber, optionally followed by one or more flushing operations with nitrogen. 
   The invention is described above and below by way of example with reference to the etching of a semiconductor substrate with hydrogen fluoride and acetic acid, flushing being carried out with nitrogen. It will be understood that the present invention can be applied for etching or otherwise treating any other substrate with any other treatments where it is necessary that at least two reactants are present which are supplied from two different sources and can be mixed only at the time of the reaction. 
   According to a further advantageous embodiment of the invention, there is a bypass line around the auxiliary chamber. By this means a stable flow of hydrogen fluoride, on its own or mixed with nitrogen, is established, so that the amount of reactant which is briefly introduced into the auxiliary chamber can be metered using accurate time control. 
   According to an advantageous embodiment, it is possible to place an absorbent for the other reactant in the auxiliary chamber. If this other reactant is acetic acid, such an absorption volume could contain silicon. 
   Generally during an etch treatment wherein nitrogen fluoride is added as first etchant or reagens, first a second etchant and/or catalyst can be supplied to the etch chamber to pretreat the surface of the substrate. Only then hydrogen fluoride is added. 
   The invention also relates to an installation for carrying out the method described above. This installation comprises an installation for etching a substrate, comprising an etching chamber provided with an entry/exit opt for said substrate, an inlet/discharge opening for reactants connected to a piping system for the separate supply of at least two reactants, one feed comprising an auxiliary chamber provided with an inlet and outlet, each having a shutoff valve, the outlet being connected to the etching chamber and the inlet to the reactant feed. 
   According to an advantageous embodiment of the invention, the etching chamber is no longer constructed from a plastic-lined metal material, as in the prior art, but is made entirely of plastic. That is to say the plastic also takes the (reduced) pressures prevailing in the etching chamber. It has been found that there is still always a certain degree of porosity when steel is lined with plastic, whilst sealing the various parts constitutes a problem as a result of which attack takes place in the longer term. Moreover, the processing of such lined steel sheets is relatively expensive and it has been found that production from solid plastic components is less expensive and reduces such porosity, whilst no attack has been observed, so that regular checking of the various components is also not necessary. 
   Examples of plastics which may be mentioned are polyvinylidene fluoride, polypropylene, perfluoroalkoxy and polytetrafluoroethene. It must be understood that use of such a chamber made of plastic is not restricted to combination with the method described above, that is to say the invention also comprise use of plastic etching chambers in combination with the method and installation according to the prior art, that is to say not provided with the auxiliary chamber described above. It has been found that the plastic material has adequate strength even under reduced pressure and there is no risk of implosion. Moreover, it has been found that in contrast to prejudices existing in the prior art, gassing does not constitute a substantial problem, especially when reduced pressure is used. 
   It must be understood that the use, described above, of an etching chamber where the plastic is a structural component, that is to say is exposed to the pressure, can also be used in other systems without the use of the auxiliary chamber described above. Rights are explicitly claimed for such an application. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing: 
       FIG. 1  shows, diagrammatically, the piping system according to the invention with the etching chamber accommodated therein; and 
       FIG. 2  shows an example of an etching chamber which can be used with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1  an etching chamber into which the substrate to be treated is placed is shown diagrammatically by  1 . The etching chamber is provided with a connection  2  to various lines. One line is indicated by  3  and terminates at three valves  4 ,  7  and  10 . Valve  4  provides the connection to a pressure switch  5  and a pressure gauge  6 . Valve  7  is a safety valve, that is to say an overpressure valve which is connected to the discharge system of the installation. 
   A feed for 100% acetic acid is indicated by  9 . 
   Acetic acid is contained in a glass bottle  9  and is connected to shut-off valve  10  via a throttle opening  13 . 
   Etching chamber  1  is also connected to a valve  11 . The latter is, in turn, connected to a vacuum pump  12 . The latter is able, for example, to generate a vacuum of 100 mtorr. 
   Connection  2  is additionally connected to outlet valve  16  of an auxiliary chamber  17 . The inlet valve thereof is indicated by  15 . This auxiliary chamber is provided with a bypass line  18  in which a valve  19  is fitted and which opens into line  14  downstream of valve  11 . Bypass lie  18  is also provided with a branch in which a valve  20  and a flow controller  21  made of metallic material are fitted. This flow controller  21  is connected to a nitrogen source which is indicated diagrammatically by arrow  22 . 
   The inlet  15  of the auxiliary chamber  17  is connected to a feed for nitrogen and hydrogen fluoride. The nitrogen feed consists of a source  23  and the hydrogen fluoride feed of a source  24 . Valves  25 ,  27  are actuated downstream of said feeds, whilst said feeds can be connected to one another via a valve  26 . Flow controllers  28  and  29  made of stainless steel are fitted in the feed lines, as well as valves  35  and  36 . A pressure gauge  37  is also fitted. The valves mentioned in this description as well as other parts are controlled through a controller not shown. 
   In  FIG. 1  parts made of plastic material are indicated by ‘ = ’, whist parts made of stainless steel are shown by ‘///’. 
   The plastic material used can be, for example, PFA. 
     FIG. 2  shows an example of an etching chamber  1 . This etching chamber is provided with a entry plate  31  which can be moved up and down, in a manner which is not shown in more detail, in order to expose an opening  30  for the introduction of a substrate, such as a semiconductor wafer  32 , in the direction of arrow  33  and, in turn, for removal of said substrate from said chamber after treatment. A ring seal  38  is fitted.  39  indicates a flow distributor plate for distributing the gas flow from line  2  uniformly over the semiconductor substrate  32 . 
   In order to achieve that the etching chamber, made of plastic material can resist the outside pressure while processing at reduced pressure, the etching chamber comprises two main parts which are each made in its entirety out of a massive block of plastic material such that a main part does not comprise a weld. Further, care is taken that the wall thickness of the main parts is about 20 mm or more and more preferably about 40 mm or more. Finally, the main parts seal against each other employing sealing means such as an O-ring seal or a lip seal. The preferred number of main parts is two, but three or more constituent main parts are possible in alternative designs, as motivated by ease of manufacturing or other reasons. 
   The installation described above functions as follows: 
   After introducing a semiconductor substrate or wafer  32  into etching chamber  1  in accordance with arrow  33 , entry plate  31  is closed with the shut-off valves,  7 ,  10 ,  15 ,  19  and  20  closed, pump  12  is switched on with shut-off valves  11  and  16  open (for pumping out auxiliary chamber  17 ). By this means a vacuum is generated in etching chamber  1  and auxiliary chamber  17  and, with off valve  4  open, the reading can be read off on pressure gauge  5  and recorded electrically using pressure switch  6 . 
   If an adequate vacuum has been generated in etching chamber  1  and auxiliary chamber  17 , shut-off valves  11  and  16  are closed. Shut-off valve  10  is then opened and acetic acid flows, in an accurately time-controlled manner, via restriction  13  into etching chamber  1 . Providing said restriction enables control as a function of time, in order to achieve optimum metering. During or prior to the introduction of acetic add, auxiliary chamber  17  is filled with hydrogen fluoride originating from source  24 . For filing auxiliary chamber  17 , a stable hydrogen fluoride flow is first established. This is effected by allowing the hydrogen fluoride stream to flow via bypass line  18 , with shut-off valve  19  open, to pump  12 . As soon as a stable flow has been established, inlet  15  is opened (shut-off valve  19  closed) with outlet  16  closed and chamber  17  is filled, after which inlet  15  is closed. In the interim the etching chamber has been adequately filled with acetic acid and shut-off valve  10  is closed. Shut-off valve  4  is likewise closed. Shut-off valve  16  is then opened and, because the pressure in auxiliary chamber  17  is higher than the pressure in etching chamber  1 , hydrogen fluoride will pass into etching chamber  1 . It is possible that slight back-diffusion of acetic acid into auxiliary chamber  17  tales place, but this material is not able to diffuse further. Because the auxiliary chamber is constructed of resistant plastic material, no attack on the components is located therein can take place. 
   After etching is complete, shut-off valve  11  is opened, with outlet  16  open, and pump  12  is switched on and the mixture present in the etching chamber and auxiliary chamber is pumped out. Nitrogen originating from source  23  can optionally be admixed by opening shut-off valve  19 . As a result the concentration of aggressive medium becomes so low that attack on the pump and other components located downstream is not to be feared. With valve  11  closed, the auxiliary chamber  17  and etching chamber  1  are then filled with nitrogen by opening inlet  15  and outlet  16 ,  25 ,  28 ,  35 . By then closing inlet  15  and opening valve  11 , both chambers can be evacuated again. This flushing treatment can be repeated several times. 
   It is possible to allow pump  12  to run continuously. During etching there will then be no gas passing through said pump  12 , as a consequence of which the drawback of oil diffusing back could possibly rise. In order to prevent this, shut-off valve  20  is opened and a quantity of nitrogen originating from source  22 , which quantity is controlled via flow controller  21 , is discharged through pump  12 . 
   It will be understood that with the aid of the flow controllers  28  and  29  accurate metering of both nitrogen and hydrogen fluoride can take place. 
   A few values for the method described above may be mentioned by way of example. If the etching chamber has a volume of 1 liter, the auxiliary chamber can have a volume of approximately 30 cm 3 . After feeding acetic acid into the etching chamber  1 , the pressure is approximately 500 to 1000 Pa. Acetic acid can, for example, be supplied for 5 sec. in order to provide the correct dosage. 
   Although the invention has been described with reference to a preferred embodiment, numerous variants are possible. For instance, it is possible to install a further auxiliary chamber connected downstream of the first auxiliary member. It is also possible to introduce an absorbent volume into the auxiliary chamber  17 . This absorbent volume is capable of absorbing the small quantity of INIT which diffuses back via outlet  16  and thus to prevent further diffusion after closure of auxiliary chamber  17  by means of inlet  15 . An absorbent volume of this type can consist of a part silicon material. In this way, it is possible to carry out the process with an aqueous solution of HF. If, for example, an azeotropic mixture of 39% HF in water is used, the vapour has the same composition as the liquid and the composition of the liquid remains constant over time. Metering of this vapour is comparable to the metering of acetic acid and no longer has to take place via a flow controller, as described above, but can be realised via a restriction and/or can be controlled as a function of time and/or pressure. With an embodiment of this type the auxiliary chamber has to have a larger volume because the maximum vapour pressure is substantially lower than that of anhydrous HF. In such a case, all lines must be made of plastic because such an H 2 O/HF mixture is much more corrosive than HF without the presence of water. The auxiliary chamber is then important in order to prevent mixing of two reagents and to keep the composition constant. 
   These and further modifications are obvious to those skilled in the art after reading the above within the of the appended claims.