Abstract:
Process for producing an SPM sensor having a holding element, a cantilever and a sensor tip which projects out of the surface of the cantilever and is delimited by three surfaces. According to the process, the starting material used is a (100)-silicon wafer. The main patterning process steps are carried out on the wafer back surface, so that an SPM sensor can be produced at low cost in a single batch run.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for producing an SPM sensor having a holding element, a cantilever and a sensor tip which projects out of the surface of the cantilever and is delimited by three surfaces at the free end of the cantilever. 
     Scanning probe microscopes (SPMs) are generally known and are used in practice to scan the surface of specimens using fine sensors of atomic resolution. SPMs include what are known as STMs (Scanning Tunneling Microscopes) and AFMs (Atomic Force Microscopes), which are likewise generally known. 
     All these known microscopes use sensors which comprise a micro-scale bending bar, referred to below as the cantilever, which at one end has a holding element and at the other end has a sensor tip, by means of which the specimen is scanned. This sensor tip, which is arranged at the free end of the cantilever, may be shaped in such a way that it does or does not project beyond the free end. The particular sensors used depends on installation in the corresponding microscope; there are situations in which the tip in the microscope is covered, so that alignment can only be carried out with difficulty. 
     SPM sensors of the type described above are known, for example, from U.S. Pat. No. 5,811,017. In this case, the starting material is a composite material comprising silicon on an insulator (Silicon on Insulator (SOI)), in which at least three lithography steps are required in order to fabricate an SPM sensor with holding element, rectangular cantilever and sensor tip made from silicon. The use of SOI materials as starting material is significantly more expensive than monocrystalline silicon. In this process, an expensive single-wafer dry-etching process is required for fabrication of two of the three surfaces which delimit the sensor tip. 
     U.S. Pat. No. 5,021,364 has likewise disclosed an SPM sensor, in which a silicon sensor tip is arranged, for example, on a nitride cantilever. The cantilever material is deposited and is therefore not a bulk material. In this case too, expensive single-wafer dry-etching processes are used to etch through the silicon membrane and to fabricate a cantilever or two of the three surfaces which delimit the sensor tip. 
     The present invention is based on the object of Proposing a further possible way of producing an SPM sensor, in which the latter can advantageously be produced in a single batch run. 
     SUMMARY OF THE INVENTION 
     The foregoing object is achieved by providing a process for producing an SPM sensor having a holding element, a cantilever and a sensor tip which projects out of the surface of the cantilever and is delimited by three surfaces. According to the process, the starting material used is a (100)-silicon wafer. The main patterning process steps are carried out on the wafer back surface, so that an SPM sensor can be produced at low cost in a single batch run. 
     A particular feature of the process consists in the fact that the production process is carried out starting from a monocrystalline (100)-silicon wafer, and therefore the entire production process can take place in a single batch run. 
     Furthermore, compared to the prior art only a few dry-etching processes and more inexpensive wet-chemical etching processes are carried out. Especially compared to the prior art, a rectangular cantilever is produced by a wet-chemical etching process and does not have to be patterned dry. The patterning, i.e. the application of the mask and the shaping (lithography steps), takes place exclusively from a single side of the wafer, namely the opposite side from the sensor tip, i.e. the wafer underside. The top side of the wafer is understood to mean the side on which the sensor tip is formed. Accordingly, the opposite side is the underside of the wafer. In the present context, thinning steps are not understood to mean shaping steps. The shaping of the wafer top side is restricted to a thinning step carried out over the entire surface, which is used to set the thickness of the cantilever and during which the sensor tip is formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below in conjunction with the accompanying drawings wherein  FIGS. 1  thru  10  show the individual steps of the process of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a (100)-silicon wafer  1  with a thickness of approx. 300 to 500 μm, the top side of which is covered with a silicon oxide layer  2  and the underside of which is covered with a silicon oxide layer  3 , for example by thermal oxidation, these layers having a thickness of a few 100 nm. 
     Next, a mask for production of the holding element  4  is fabricated by photolithography. For this purpose, a photoresist is applied to the wafer underside and patterned. Then, the silicon oxide layer  3  on the wafer underside is removed at the uncovered locations, so that only the region  5  remains in place on the wafer underside. The wafer top side had previously been covered by a protective resist. All the photoresists are removed again after the photomask has been transferred into the silicon oxide. These are generally standard, conventional processes. By way of example, in this case the photoresist is applied by the spinning process and is removed by means of standard solvents. The silicon oxide layer is removed by means of buffered hydrofluoric acid (HF). The result is illustrated in  FIG. 2 . 
     In the next step, which is shown in  FIG. 3 , a wet-chemical etching process, for example by means of aqueous potassium hydroxide (KOH) solution, is used to thin the substrate material from the wafer underside and thereby to produce the holding element  4 . The thickness of the silicon membrane  6  which is formed in the process is selected in such a way that it at least corresponds to the sum of the desired height of the sensor tip which is to be formed and of the desired final thickness of the SPM sensor cantilever which is to be formed. 
     Next, both sides of the wafer are oxidized, and a silicon nitride layer is applied to the wafer top side in a PECVD (Plasma Enhanced Chemical Vapor Deposition) process. In  FIG. 4 , the silicon oxide layer on the underside is denoted by reference numeral  7 , and the silicon nitride layer on the top side is denoted by reference numeral  8 . 
     Next, a mask is fabricated for the purpose of producing boundary surfaces of the free end of the cantilever which is to be formed, these surfaces simultaneously defining two faces of the sensor tip which is to be produced. For this purpose, a photoresist is applied to the pre-patterned wafer underside and is patterned. In the exemplary embodiment, the photoresist is applied in a spray-coating process and is patterned by means of projection lithography. In principle, however, other known measures are also possible. The photomask is transferred to the silicon oxide layer  7 , so that partial removal takes place in the region  9 . The silicon nitride layer  8  on the top side of the wafer is thinned slightly in the process. Then, the photoresist is removed. Next, the mask for production of the cantilever is fabricated as a result of a photoresist once again being applied to the pre-patterned wafer underside and being patterned in accordance with the desired shape. In this case too, the application is preferably carried out using the spray-coating process and the patterning is preferably carried out by means of projection lithography. Then, this photomask is likewise transferred to the silicon oxide layer  7 , so that the silicon oxide is partially thinned. Then, the photoresist is removed. As a result, a step is formed in the silicon oxide layer on the underside. In this context,  FIG. 5   a  shows a perspective view of the wafer underside with the silicon oxide layer  7 , which has various thickness regions, with the result that the step  10  in  FIG. 5  is formed. The regions  5  and  11  are thicker than the remaining region  12 . The region  9  is uncovered silicon. 
     The next process step is a deep silicon etch, preferably using the known ASE (Advanced Silicon Etching) process, producing vertical side walls on the back surface of the silicon wafer  11 , so that a recess  13  is formed in the silicon membrane  6  in the region  9  shown in  FIG. 5 . The etching mask used is the above-described silicon oxide mask for production of the boundary surfaces  14  and  15  ( FIG. 5   a ) of the free end of the cantilever which is to be formed. The depth of the recess  13  at least corresponds to the sum of the desired height of the sensor tip to be formed and the thickness of the SPM sensor cantilever to be formed. The thickness of the silicon membrane  6  is selected in such a way that the silicon membrane is not completely etched through in this step, as shown in  FIG. 6 . 
     Then, the silicon wafer is oxidized on the wafer underside, during which step the side walls  16  and the base surface  17  of the recess  13  are covered by silicon oxide  20  ( FIG. 7 ). 
     Next, the wafer underside is subjected to a targeted dry-etching process, which operates selectively with respect to silicon, and the silicon oxide layer on the base surface  17  of the recess  13  and also in the region  12  ( FIG. 5   a ) is removed. In the process, the cantilever mask is transferred to the underside of the wafer. This is followed by a wet-chemical etching step, for example carried out by means of KOH, in which a cantilever  18  is pre-patterned on the basis of the mask which has just been transferred. The etching depth and therefore the thickness of the pre-patterned cantilever  18  is selected to be greater (for example by 20%) than the final cantilever thickness of the SPM sensor to be produced. The result of this operation is illustrated in  FIGS. 7   a  and  7   b ,  FIG. 7   a  showing the underside of the silicon substrate with a cantilever  18  which rises up from the surface  19  of the substrate material and has the perpendicular surfaces  14  and  15 .  FIG. 7   b  shows a sectional illustration. The surfaces  14  and  15  are covered with a silicon oxide layer. The silicon oxide layer  11  is also present on the cantilever  18 . 
     Then, all the oxide and nitride layers  2 ,  20  and  8  are removed in hydrofluoric acid. Then, a further oxidation step is carried out, in order to completely cover the underside of the wafer with a silicon oxide layer  20   a . Then, the silicon oxide layer  2  on the top side of the wafer is removed by means of a dry-etching step which is chosen to be selective with respect to silicon. The result of this operation can be seen from  FIG. 8 . 
     Finally, the wafer  1  is subjected to a wet-chemical etching step. In the process, the silicon membrane  6  is thinned from the top side of the wafer. This process is stopped when the pre-patterned cantilever  18  has been thinned to the desired thickness. A tip  21  has formed on the open side, which is not covered by silicon oxide, of the cantilever  18 , this tip being defined by the surfaces  14  and  15  and a (111)-crystal plane  23 . The height of the tip  21  corresponds to the recess  13  ( FIG. 9 ) and is typically 5–25 μm. The thickness of the cantilever  18  is typically 0.5–10 μm. 
     Finally, the remaining silicon oxide layers are removed by wet-chemical means, so that an SPM sensor  22 , as illustrated in  FIG. 10 , having a holding element  4 , a cantilever  18  and a sensor tip  21  is formed. The tip which has been formed can be sharpened by means of a further low-temperature oxidation, for example carried out at below 1000° C., and final removal of the oxide layer which has grown on. 
     The materials which are preferred in connection with the process described above can also be replaced by materials which have a corresponding action and with which the person skilled in the art is familiar.