Abstract:
A method of sacrificial layer etching of micromechanical surface structures, in which a sacrificial layer is deposited on a heatable silicon substrate and is structured. A temperature difference between the substrate and the vapor phase of an etching medium is established in such a way that exposed metal contacts made of aluminum alloys are not attacked at the same time and are not subsequently exposed to any risk of corrosion.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing micromechanical surface structures by means of a vapor-phase etching medium, avoiding aluminum contact corrosion. 
     BACKGROUND INFORMATION 
     In silicon surface micromechanics, a layer structure is used, consisting of a sacrificial layer, usually SiO 2 , on a substrate surface, usually a silicon substrate, with a layer of active silicon, usually polysilicon or single-crystal silicon (SOI), on top, in which structures, to be exposed later, are produced. Additional layers, e.g., buried polysilicon printed conductors, may also be present, but they do not play any role in the purely mechanical function of the components. In general, metal contact surfaces are provided on the active silicon layer for electric contacting of the components. Various aluminum alloys (AlSi, AlSiCu, etc.) containing aluminum as the predominant element have become established as contact materials in semiconductor technology, the repertoire of which has been used preferentially in surface micromechanics. 
     It is known that to produce unsupported micromechanical surface structures, they are first etched into the top layer of active silicon until the underlying sacrificial layer, usually SiO 2 , is reached (M. Bibel,  Physikalische Blätter [Physical Leters] , 1996, 52, pp. 1010-1012). To expose the structures created in this way, the sacrificial layer is dissolved, for example, by an isotropic wet etching method (PCT International Publication No. WO 92-03740), where gaseous hydrogen fluoride over an azeotropic hydrofluoric acid-water mixture can be used (M. Offenberg, B. Elsner and F. Lärmer,  Proc . 186 th    Electrochem. Soc. Meeting, Sensor General Session , Miami Beach, Fla., October 1994). 
     HF vapor is known to convert SiO 2  to volatile silicon fluorides, thereby dissolving it away under the structures: 
      SiO 2 +2H 2 O+4HF→SiF 4 +4H 2 O 
     The presence of water is necessary for this reaction to take place. It can be seen from this reaction equation that more water is formed than used. The important advantage of using gaseous hydrogen fluoride in comparison with aqueous hydrofluoric acid solutions is that with an optimum choice of measurement parameters there is no irreversible sticking of the exposed silicon structures to one another or to the substrate due to the surface tension of drying droplets of fluid in the subsequent drying of the substrate. 
     An important disadvantage of this vapor-phase etching process is that the gaseous hydrogen fluoride attacks not only SiO 2  but also the aluminum contacts that have been applied to the electronic components. In vapor-phase etching, the aluminum hydroxide fluorides produced cannot be removed but instead they remain as a rather thick insulating layer on the contact surface, which makes subsequent wire bonding of the aluminum contacts impossible. Washing off this layer would in turn result in sticking of the micromechanical surface structures already exposed and therefore is also impossible. Another problem with the resulting aluminum hydroxide fluoride layers is that they are hygroscopic, and water absorbed penetrates to the interlayer of metallic aluminum with the aluminum hydroxide fluoride layer, thus leading to progressive corrosion both during and after the etching of the sacrificial layer is completed. 
     The aluminum contacts may be protected during the etching of the sacrificial layer by means of additional layers such as lacquers that are impermeable to hydrofluoric acid, but this represents an additional step that is complicated and technically very difficult during the process of manufacturing the micromechanical surface structures because hydrofluoric acid diffuses very rapidly through protective polymer layers and thus can reach the metal surface. In addition, the contact protection would also have to be removed again after etching the sacrificial layer, i.e., if already exposed sensitive structures are present on the wafer surface, which leads to additional problems, in particular with regard to yield and reproducibility. 
     SUMMARY OF THE INVENTION 
     By establishing a temperature difference between the substrate and the vapor phase of the etching medium on the basis of the partial pressure composition of the vapor-phase etching medium, it is possible to control the chemical reactions during etching in an especially advantageous manner. A temperature difference between the substrate and the vapor phase permits selective etching of SiO 2  on the basis of the reaction below without attacking the exposed aluminum contacts on the substrate. The first reaction which can take place on the aluminum surface is then as follows: 
     
       
         Al 2 O 3 .3H 2 O+2HF→2Al(OH) 2 F+2H 2 O⇄2Al(OH) 3 +2HF 
       
     
     In the course of this reaction, the aluminum oxide hydrate breaks through at the surface. The hydroxide or hydroxide fluoride layer is hygroscopic. First, metallic aluminum is converted by the action of water under the influence of hydrofluoric acid to the oxide hydrate which can be fluorinated further by the following equation: 
     
       
         Al+3H 2 O+HF→Al(OH) 2 F+3/2H 2 +H 2 O⇄Al(OH) 3 +3/2H 2 +HF 
       
     
     Aluminum hydroxide and aluminum hydroxide fluoride are in a reversible chemical equilibrium. The conversion of aluminum to the corrosion product takes place essentially with the uptake of water, in contrast with SiO 2  etching, where water is formed. Both reactions, i.e., etching aluminum and SiO 2 , have in common the fact that they can take place only in the presence of water. Establishing a temperature difference between the substrate and the vapor phase of the etching medium permits, for example, rapid vaporization of the water formed on the substrate surface. Owing to the temperature difference, which is due to the partial pressure composition of the vapor-phase etching medium, water cannot condense on the substrate, and the parts of the surface which do not produce any water in the reaction with hydrofluoric acid, for example, remain dry and cannot be attacked. 
     In an advantageous embodiment of the method according to the present invention, the etching is performed at a temperature difference of 10-30 K, preferably 20 K, between the silicon substrate and the vapor-phase etching medium. The temperature of the vapor phase opposite the substrate is lower than the temperature of the substrate, so there is no condensation on the substrate surface. Consequently, the substrate surface is exposed to the vapor phase, but due to the higher substrate temperature, there cannot be any condensation on the substrate, and the parts of the substrate that cannot produce any water themselves in the reaction with hydrofluoric acid remain dry and cannot be attacked. This is true especially of the aluminum of the electric contacts which also does not release any water in the reaction with aqueous hydrofluoric acid, and therefore it does not react due to the absence of moisture. However, in the reaction with HF—H 2 O, the sacrificial SiO 2  layer reacts by forming water, some of which is bound as hydroxide in the form of Si(OH) 4 , as a precursor to additional reactions with HF to form volatile silicon tetrafluoride. The portion of the reaction water that is not bound as a hydroxide remains on the SiO 2  surface for a relatively short period of time and is rapidly evaporated because of the higher wafer temperature in comparison with the vapor phase. 
     In any case, even the transient presence of this reaction water accelerates the subsequent reaction of SiO 2  (or then Si(OH) 4 ) with HF, which supplies even more water for the reaction, until an equilibrium moisture content of the SiO 2  and Si(OH) 4  surfaces prevails. This acceleration of the SiO 2  etching process from initially very minor removal of material, which is initiated only by the water from the vapor phase, up to high etching rates results in achieving a high, quasi-steady-state silicon oxide etching rate after a start-up phase of approximately 5 to 7 minutes after the start of the process. 
     In another preferred embodiment, the temperature of the silicon substrate is at least 333 K, preferably 343-353 K, in particular 353 K ad at most 373 K. Consequently, there is no avalanche of dissolution reaction due to uptake of water from the gas phase on the dry aluminum surfaces that can form no reaction water and thus there is also no significant corrosion. At this temperature, additional protective mechanisms for the aluminum come advantageously into effect, resulting the fact that no fluorides remain on the surface. Thus, any delayed corrosion is effectively prevented even long after the actual etching of the sacrificial layer. The reliability of the components produced in this way is thus greatly improved. 
     Aluminum oxide hydrate, aluminum hydroxide or aluminum hydroxide fluoride present on the surface is dehydrated, i.e., water or the water of crystallization is removed in the form of hydroxides, or also HF; fluorine in the form of its fluorides is thus completely removed from the surface layer. 
     
       
         Al 2 O 3 .3H 2 O→Al 2 O 3 +3H 2 O 
       
     
     
       
         2Al(OH) 3 →Al 2 O 3 +3H 2 O 
       
     
     
       
         Al(OH) 2 F→AlO(OH)+HF 
       
     
     Structural compaction of the layer passivating the aluminum surface occurs in this dehydration, i.e., its pore density and permeability decrease, while its imperviousness and passivation with respect to water and HF increase. The chemical resistance of the layer passivating the aluminum surface, i.e., its protective effect, is thus increased in particular by the formation of compounds such as aluminates which are more chemically stable and inert. This effect is especially operative above a temperature range of 343-353 K. Up to a substrate temperature of 373 K, strong SiO 2  etching can still be performed, without any aluminum contact corrosion occurring. At temperatures in the preferred temperature range of 343-353 K, especially at 353 K, there is no longer any fluorination of the aluminum surface, i.e., the aluminum surfaces do not contain any fluorine atoms or ions after etching. 
     In another preferred embodiment, the vapor phase is adjusted so that another gas which is essentially chemically inert under the selected reaction conditions is used and is introduced into the etching apparatus. The establishment of a “quasi-azeotropic” mixture is thus regulated easily through the moisture in this gas. The chemically inert gas for dilution may also contain oxygen, so it is even possible to use air. 
     In another embodiment of the present invention, the partial pressure composition of the etching medium can be adjusted as a function of a temperature of the etching medium and/or as a function of a composition of components of the vapor phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an apparatus for carrying out the process according to the present invention. 
     FIG. 2 shows another embodiment of the cover of the apparatus in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a container  11  made of Teflon, for example, and containing an azeotropic mixture  23  of water and hydrofluoric acid in which there is arranged a silicon substrate  25 . It is completely surrounded by a heating jacket  15  through which water, held, for example, at the selected temperature by a thermostatic regulator (not shown), is circulated. This stabilizes the temperature of the Teflon container  11  and the azeotropic water-hydrofluoric acid mixture  23  at the desired level. Of course, any other type of heating, e.g., electric heating strips, hot air blowers, hot water jackets, etc. are also conceivable. It is important only that the walls of the Teflon container  11  and the water-hydrofluoric acid mixture  23  can be brought uniformly to the same temperature, so that condensation of moisture on the walls can be prevented and an especially homogeneous vapor phase can be produced. 
     Teflon container  11  has an inlet  12  and an outlet  13  through which water, for example, at a selectable temperature can be passed in the direction of arrow F. Teflon container  11  and heating jacket  15  are mounted in an insulating jacket  14  so that the temperature inside Teflon container  11  can be kept constant. Teflon container  11  is sealed by a cover  20  which has gaskets  22  made of Teflon, Viton or other gasket materials that are resistant to gaseous hydrogen fluoride. A heating device  21  is mounted in this cover  20  so that the silicon substrate  25  mounted on the inside of the cover  20  can also be heated. In this way, the silicon substrate  25  can be brought to the desired high temperature of 330-373 K, for example, preferably 353 K. Heating device  21  may again be hot water circulation, or electric heating films (not shown) may be used in combination with a thermocouple and an electronic temperature regulator. 
     The substrate temperature has a direct influence on the SiO 2  etching rate, so the most uniform possible temperature distribution over substrate surface  25  must be achieved. With the device according to the present invention, this is achieved with two heating films (not shown), an internal circular heating film and an external annular heating film that can be controlled electrically independently of one another. Through appropriate regulation of the heating power of the internal film element and the external film ring, a very uniform temperature distribution over substrate surface  25 , e.g., a 6″ silicon wafer, can be achieved with a uniformity of better than a 1 K temperature difference. The material of heating device  21  may be silicon, which has a good thermal conductivity and is stable with respect to gaseous hydrogen fluoride. 
     As an alternative, a Teflon plate may also be used, either carrying the heating elements or provided with a duct system through which hot water flows at a freely selectable temperature, regulated by a second thermostat. 
     FIG. 2 shows another possible embodiment of cover  20  of the device for carrying out the process according to the present invention. 
     Silicon substrate  25  is not clamped overhead against a top heating plate  20  but instead is placed on a heated bottom substrate plate  27 , which is connected to the top plate. Substrate plate  27  contains one of the heating devices  21  described previously. Cover  20  is heated by heating elements  21  to prevent condensation of HF gas. The temperature of heating plate  27  carrying silicon substrate  25  is monitored and has a homogeneous distribution over the substrate surface. Heating plate  27  and cover plate  20  are connected by at least two hollow Teflon webs  26 , for example. In the case of electric heating elements  21 , the electric current leads and the electric connection are carried to the outside in the form of wires (not shown) through the hollow webs, so that no hydrofluoric acid comes in contact with the wires. With hot water heating  21 , hot water flows through heating coil  21  from the cover plate and wafer heating plate through hollow webs  26 . 
     An azeotropic mixture is advantageously selected as the water-hydrofluoric acid mixture  23 , i.e., a mixture with an HF concentration of approximately 38%. With an azeotropic mixture, water and HF are vaporized in a constant ratio so that the concentration of the solution remains constant over a long period of time, i.e., the hydrofluoric acid concentration remains unchanged as the quantity of solution decreases. Thus, there are constant vapor etching conditions for many substrates to be treated over a long period of time, which leads to good reproducibility and very low maintenance. 
     Silicon substrate  25  to be treated is first inserted into cover  20  and is clamped by a Teflon holding ring (not shown). After a waiting time of 2 to 5 minutes, for example, to allow the wafer enough time to heat up to the preselected temperature of heating device  21 , the device is uncovered, and heatable cover  20  carrying the substrate is placed on it instead. With a start-up phase of approximately five minutes, the sacrificial SiO 2  layer is then removed under the structures that are to be exposed. After a process time of typically  20  minutes, an undercutting width of 5 μm has been reached and the process is concluded. Then heatable cover  20  carrying substrate  25  is removed from the etching apparatus and the latter is covered again. Silicon substrate  25  remains in heated cover  20  for a few minutes so that HF residues and any moisture present are removed completely. This prevents any subsequent corrosion. After this waiting time, silicon substrate  25  is removed from the apparatus and sent to the processes which follow the etching of the sacrificial layer. 
     With cover  20  which is shown in FIG. 2, silicon substrate  25  is simply placed on heating plate  27  without any further clamping, with gravity ensuring the contact. This also greatly simplifies the loading and unloading of the apparatus. 
     With another apparatus (not shown), the establishment of a defined vapor phase, i.e., a vapor phase with constant conditions, which is in equilibrium at a lower temperature than the temperature of the substrate, is achieved due to the fact that nitrogen or oxygen or air, for example, is humidified over a water-cooled bubbler and fed into etching apparatus  10 . Of course, any other gas such as argon, etc., that is essentially chemically inert under these conditions may also be used. In this case, the apparatus is not a closed system, but instead the gases flow through it continuously. The water supply in the nitrogen bubbler is heated to the temperature at which the vapor phase is to be in equilibrium. The nitrogen flow and the bubble size in the bubbler must be set so that an equilibrium can in fact develop in the gas phase. While the equilibrium temperature of the gas phase is set through the nitrogen flow and the bubbler temperature, the SiO 2  etching rate is controlled directly through the stream of dry hydrofluoric acid supplied independently. It is also possible to supply an additional stream of dry nitrogen through an additional apparatus (not shown) and to further reduce the moisture content of the gas phase. 
     In an additional apparatus (not shown), the mass transport to the substrate can be monitored and influenced. Under otherwise comparable boundary conditions, a higher flow of hydrofluoric acid leads to a higher SiO 2  etching rate, and a lower flow of hydrofluoric acid leads to a lower SiO 2  etching rate. It is important to ensure that the resulting reaction water can still be vaporized rapidly enough to prevent the formation of large droplets that could cause irreversible sticking of the resulting micromechanical surface structures to each other and to the substrate.