Abstract:
The invention relates to a method of structuring of a substrate by providing a polymerization starter layer on the substrate, applying a radiation field on the polymerization starter layer for selectively reducing a density of polymerization starters of the polymerization starter layer, applying monomers and then polymerizing of the monomers, the polymerization being initiated by the starters of the polymerization starter layer, and structuring the substrate using the polymerized monomers as a mask.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of structuring of substrates, and more particularly without limitation, to semi-conductor and micromachined devices. 
   2. Background and Prior Art 
   Microchip manufacturing relies on photolithography, a process similar to photography, to define the shape and pattern of individual surface features. A film of photo-reactive polymer, known as a photoresist, is applied to the surface of a silicon wafer where a latent image is generated in the photoresist by exposure to light. 
   Photolithography can also be used for producing a three dimensional topography. For this purpose several photolithographic steps have to be performed in a sequence. The difficulty with two or more layers of topography is that the layer alignment is difficult to achieve. This difficulty limits the precision of the structuring of the substrate. As a consequence certain structures such as lenses having a defined focal point can to date not be produced by means of photolithographic multilayer processes. 
   Another field of application of photolithography is micromachining of miniature mechanical or other devices. It is a particular challenge for such applications to produce steps, slopes or other surface variations. The ability to coat the substrate with a uniform thickness of photo resist has been identified as one of the toughest challenges in the area of pattern definition for high topography structures (cf. SPIE Micromachining &amp; Microfabrication Symposium &#39;96, “Imaging and Resist Technologies for the Micromachining Industry”, David Craven). 
   There is therefore a need for providing of an improved method of structuring of a substrate, in particular for producing miniature steps, slopes or other surface variations of the substrate and for semiconductor manufacturing. 
   SUMMARY OF INVENTION 
   The present invention provides for a method of structuring of a substrate which enables to fabricate three dimensional topologies without a need for multiple layers of photolithography. 
   In accordance with a preferred embodiment of the invention this is accomplished by immobilizing a layer of polymerization starters on the substrate surface. The immobilization of the polymerization starters can be accomplished by providing a coating on the substrate surface with which the polymerization starters can form covalent bonds. For example if azomonochlorsilane (AMCS) is used as a polymerization starter an appropriate coating of the substrate surface is SiOx. 
   The density of the polymerization starters which are immobilized on the substrate surface is then modulated by selectively reducing the density of the polymerization starters. This can be done by applying a suitable radiation field, such as a UV radiation field which depending on its intensity cracks more or less of the polymerization starters. Due to the density distribution of the polymerization starters on the surface of the substrate a subsequent polymerization step results in a corresponding three dimensional structure. This structure can be used as a complex mask for a subsequent structuring step. 
   In accordance with a further preferred embodiment of the invention the polymerization starter layer is a mono-molecular layer of starter molecules. Such a mono-molecular layer is advantageous as it enables a very high spatial resolution of the three dimensional topography. 
   In accordance with a further preferred embodiment of the invention a radiation source emitting a substantially homogeneous radiation field is employed. The homogenous radiation field is modulated by means of a gray mask or a grating mask in order to provide an inhomogeneous radiation field for selectively reducing the density of the polymerization starters. 
   In accordance with a further preferred embodiment of the invention the homogeneous radiation field is modulated such that the resulting structure which is produced on the surface of the substrate has the form of a lens with a defined focal point. 
   In particular the invention enables to produce semiconductor and micromachined devices having a multiple layer topography but only a single crystal growth edge. This is in contrast to the prior art where the production of multiple layer three dimensional topographies requires several photolithography steps. The sequence of photolithography steps results in so called growth edges in the crystal growth. In other words each photolithography step results in an additional growth line or growth edge when a crystal is grown between the lithography steps. 
   The present invention enables to fabricate a semiconductor or micromachined device in a single crystal growth step as multiple step lithography is made superflous by the complex three dimensional mask in accordance with the present invention. 
   Examples of such devices include a slider ABS surface, padded slider, recessed P 3  structure on a magnetic recording head as well as micro/nano structures with multiple layer topography. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     In the following a preferred embodiment of the invention will be described in greater detail by making reference to the drawings in which: 
       FIG. 1  shows a substrate with a polymerization starter layer, 
       FIG. 2  is illustrative of the application of a radiation field for selectively reducing the density of the polymerization starters, 
       FIG. 3  shows the substrate after polymerization, 
       FIG. 4  is illustrative of a processing step using the structure provided by the polymerization as a mask, 
       FIG. 5  shows the substrate after the processing step of  FIG. 4 , 
       FIG. 6  shows the substrate after deposition of a layer of material having uniform thickness such as by crystal growth, 
       FIG. 7  shows an example of a gray mask, 
       FIG. 8  shows an example of a grating mask. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a substrate  100 . Substrate  100  is a silicon wafer or another kind of wafer material. On top of substrate  100  a mono-molecular layer of polymerization starter molecules  102  is applied. The polymerization starters  102  are immobilized on the surface of substrate  100 . Preferably the immobilization of the polymerization starters  102  is accomplished by covalent bonds which are formed between the polymerization starters  102  and the substrate  100 . 
   To facilitate the formation of covalent bonds between the polymerization starters  102  and the surface of substrate  100  there is a coating  104  on substrate  100  onto which the polymerization starters  102  are applied. The polymerization starters  102  form covalent bonds with the coating  104 . 
   Preferably the coating  104  consists of a layer of SiOx to secure optimal polymerization conditions. The polymerization starter  102  is selected depending on the surface conditions of substrate  100  and coating  104 . For example azomonochlorsilane (AMCS) is used as a polymerization starter which is applied onto coating  104  substrate  100  by dip or contact coating of the polymerization starter solution. The SiOx coating  104  ensures good starter bonding for immobilization of the polymerization starter molecules  102  on the substrate  100 . 
     FIG. 2  illustrates the application of a radiation field on the polymerization starters  102  in order to modulate the density of the active polymerization starters  102 . The radiation source  106  provides a homogeneous radiation field  108 . For example radiation source  106  is a UV light lamp. The UV light which is emitted by the radiation source  106  is transformed into homogeneous radiation field  108  by means of a gray mask or other appropriate optical means which are as such known from the prior art of photolithographic processes. 
   A gray mask  110  is used for modulation of the homogeneous radiation field  108 . Gray mask  110  has areas  112  which absorb the UV light which falls on these areas. This means that no UV light of the homogeneous radiation field  108  penetrates the areas  112  of the gray mask  110  such that the polymerization starters  102  which are located underneath the areas  112  are not subjected to UV radiation. As a consequence the density of polymerization starters  102  under these areas  112  remains unchanged. 
   Gray mask  110  has areas  114  which absorb some of the UV light of the homogeneous radiation field  108 . As a result the polymerization starters  102  underneath the areas  114  are subjected to UV light having a reduced intensity as compared to the original intensity of the homogeneous radiation field  108 . The intensity of the UV light impinging upon the surface of the substrate  100  is determined by the absorption index of the areas  114 . As a consequence the density of the polymerization starters underneath areas  114  is reduced correspondingly. Depending on the intensity of the UV light impinging upon the surface of the substrate  100  a greater or a lower number of polymerization starters  102  are cracked and thereby deactivated such that a reduced density of active polymerization starters  102  remains in areas  114 . 
   Further gray mask  110  has areas  116  which are transparent for radiation field  108  and which do not absorb the UV light of radiation field  108 . Depending on the intensity of the homogeneous radiation field  108  the polymerization starters  102  underneath areas  116  are cracked. If the intensity of the homogeneous radiation field  108  is sufficiently high all of the polymerization starters  102  are removed underneath areas  116 . 
   By means of gray mask  110  the homogeneous radiation field  108  is modulated such that the UV exposure of the polymerization starters  102  is also locally modulated. In areas where the local intensity of the modulated radiation field is zero the original density of the polymerization starters is unchanged; in other areas where the intensity of the modulated radiation field is greater than zero but below the intensity of the original homogeneous radiation field  108  the population of the polymerization starters  102  is reduced depending on the local radiation field strength. In areas where the intensity of the modulated radiation field is equal to the original intensity of the homogeneous radiation field  108  most or all of the polymerization starters are cracked and thus deactivated. 
   It is to be noted that the modulation of the homogeneous radiation field  108  is not limited to a step function but that any other kind of modulation function can be realized by using a corresponding gray mask. The gray mask can have any gray level image in order to implement a desired modulated radiation field. 
   After the UV exposure polymerization is started by applying monomers onto the surface of substrate  100 . For polymerization of the monomers an active polymerization starter  102  is required. As a consequence no polymerization occurs in areas where the population of the polymerization starters  102  has been reduced to zero by the UV exposure of the modulated radiation field. 
   In other areas where the polymerization starters  102  have been subjected to UV exposure with a reduced intensity due to the absorption of the gray mask  110  there is a correspondingly limited number of polymerizations; as a consequence the extension of the resulting structures  118  into the vertical direction is also reduced as compared to the vertical extend of structures  120  where the original polymerization starter population has survived the UV exposure step due to the shielding of the completely absorbing areas of the gray mask. 
   Preferably a wash step is performed after polymerization to wash off any free polymer chains from the substrate surface. 
   As apparent from  FIGS. 2 and 3  the modulation of the intensity of radiation field  108  by gray mask  110  results in a corresponding modulation of the vertical extension of the resulting structure formed by the polymerization step of  FIG. 3 . This way any required three dimensional topography formed by the locally varied polymerization can be formed on top of substrate  100 . 
   This complex three dimensional topography can be used as a mask for a subsequent processing step as illustrated in  FIG. 4 . In  FIG. 4  a process step such as ion mill, reactive ion etch (RIE) or wet etch is applied for removing material from the surface of substrate  100 . The surface of substrate  100  is locally protected by mask  122  formed by the structures  118  and  120  provided by the polymerization step of  FIG. 3 . As a consequence the three dimensional topography of mask  120  is translated into the resulting structure of the surface of substrate  100 , as shown in  FIG. 5 , because the degree of material removal from the surface of substrate  100  is modulated in accordance with the local thickness variations of mask  122  in the vertical direction. 
   Subsequently a material deposition step can be performed in order to manufacture a semi-conductor or micro machined device. In the example considered in  FIG. 6  a layer  124  of uniform thickness is deposited on the surface of substrate  100  such as by crystal growth. As compared to prior art photolithography techniques layer  124  has a complex three dimensional topography without any growth edges other then the surface of the substrate  100 . Such a three dimensional structure without any additional growth edges cannot be realized by prior art multi layer photolithography techniques. 
   As a further advantage the resulting three dimensional topography can be manufactured with a high degree of precision as the tolerances which are an unavoidable consequence of the multi layer photolithography steps of the prior art are eliminated. 
     FIG. 7  shows an example of a gray mask  126 . Black areas of gray mask  126  absorb all of the impinging radiation, white areas are completely transparent for the radiation and gray areas absorb some of the radiation such that a corresponding modulation of the radiation field is provided. 
   In order to realize gray mask  126  of  FIG. 7  a grating mask can be used. Such a grating mask  128  is shown by way of example in  FIG. 8 . Black areas are realized by full chromium (Cr) coverage while the areas with black bars symbolizing Cr bars only reduce the radiation intensity. The production of such grating masks is as such known from the prior art. 
   It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.