Patent Publication Number: US-2007122077-A1

Title: Low drift planar waveguide grating sensor and method for manufacturing same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a planar waveguide grating (PWG) sensor which exhibits a low signal drift and an enhanced sensitivity due to the use of a fully dense silicon rich silicon nitride surface layer. In addition, the present invention relates to a method for manufacturing the PWG sensor with acceptable costs and high yields by utilizing well known semiconductor processes and tools.  
      2. Description of Related Art  
      The following abbreviations are herewith defined, at least some of which are referred to in the ensuing description of the prior art and the preferred embodiments of the present invention. 
          APCVD Atmospheric Pressure Chemical Vapor Deposition     CVD Chemical Vapor Deposition     FIB Focused Ion Beam     FSG Fluorine Doped Silica Glass     LPCVD Low Pressure Chemical Vapor Deposition     PECVD Plasma Enhanced Chemical Vapor Deposition     PWG Planar Waveguide Grating     PVD Physical Vapor Deposition     RIE Reactive Ion Etching     SACVD Sub-Atmospheric Chemical Vapor Deposition     SEM Scanning Electron Microscopy     SPR Surface Plasmon Resonance     UV Ultraviolet        

      Evanescent field-based sensors are fast becoming a technology of choice for accurate label-free detection of a biological, biochemical, or chemical substance (e.g., cells, spores, biological or drug molecules, or chemical compounds). The label-free detection technology typically involves using a PWG sensor or a SPR sensor to detect a change in the refractive index of liquid or gas immediately above the sensor. For example, the change in refractive index can arise from a concentration change, surface adsorption, reaction, or the mere presence of a biological or chemical substance at the sensor&#39;s surface. Several types of known PWG sensors are described next each of which have different structures and materials of construction. And, then a description is provided about what causes a problematical signal drift which adversely affects the sensitivity of those PWG sensors. The cause of this problematical signal drift is addressed by the present invention.  
      In general, the PWG sensor is made from a substrate, a monomode waveguide and a sub-wavelength period diffraction grating that is formed into either the substrate or the waveguide.  FIG. 1  (PRIOR ART) is a diagram which is used to help describe the basic elements and the basic functionality of one type of PWG sensor  100 . As shown, the PWG sensor  100  has the following elements: 
          Substrate  102  (patterned with a sub-wavelength period diffraction grating  104 ).     Waveguide  106 .     Chemically responsive surface chemistry layer  108 .     Chemically bound molecules  110  (targets  110 ) of interest.     Solution  112  containing the substance  114  (analyte  114 ) to be detected.        

      Typically, the thickness and refractive index of the waveguide  106  along with the characteristics (pitch, depth, and duty cycle) of the diffraction grating  104  are chosen to yield the highest possible sensitivity to a refractive index change which is caused by the interaction of the analyte  114  and target  110 . This sensitivity is defined as the shift in the reflected light  116  relative to the refractive index change (nm/refractive index unit). Because, the sensing principle involves the interaction of an evanescent wave emerging from the waveguide  106 , the sensed volume is typically limited to the first 150-200 nm above the surface of the waveguide  106 . A more detailed discussion about the structure and the functionality of this PWG sensor  100  can be found in U.S. Pat. No. 4,815,843. The contents of this patent are incorporated by reference herein.  
      In the past, a lot of work has been done to enhance the performance of the PWG sensor  100 . For example, it has been shown that the performance of the PWG sensor  100  can be enhanced by: (1) raising the index contrast between the substrate  102  and the waveguide  106  (see U.S. Patent Application 2005/0025421 and PCT Patent Application WO0235214); (2) producing a highly uniform diffraction grating  104  (see U.S. Pat. No. 6,873,764 B2); (3) lowering unfavorable interactions between the solution  112  containing the analyte  114  (molecules  114 ) of interest and the waveguide  106  which can result in resonant drift and other detrimental effects (see U.S. Pat. No. 6,332,363); and (4) increasing the amount of analyte  114  binding to the target  110  (receptor  110 ) by improving the surface chemistry layer  108  or its application (see U.S. Patent Application No. 2004/0043508A1).  
      In addition, a lot of work has been done in the past to make a disposable PWG sensor  100  so that one does not have to re-use the sensor  100  after performing an assay. This is desirable because if one re-uses the PWG sensor  100  then there is a possibility of cross-contamination. However, there are problems associated with manufacturing disposable PWG sensors  100  at a high yield and a low cost. And, there are problems associated with the performance of these disposable PWG sensors  100 . These problems are described next.  
      The prior art demonstrates that there has been a struggle to balance the cost and performance in making a disposable PWG sensor  100 . For instance, the PWG sensor  100  can have a low cost polymeric substrate  102  within which the sub-wavelength gratings  104  can be easily embossed or molded. However, the PWG sensor  100  which has a polymeric substrate  102  can suffer from a problematical optical signal loss that is due to absorption in the polymeric substrate  102 . To address this absorption problem, the PWG sensor  100  can be enhanced by depositing a thick oxide or organic modified oxide layer between the polymeric substrate  102  and the waveguide  106 . This type of PWG sensor  200  is illustrated in  FIG. 2  (PRIOR ART). As shown, the PWG sensor  200  has the following elements: 
          Polymeric substrate  202  (patterned with a sub-wavelength period diffraction grating  204 ).     Inorganic cladding layer  205 .     Waveguide  206 .     Chemically responsive surface chemistry layer  208 .     Chemically bound molecules  210  (targets  210 ) of interest.     Solution  212  containing the substance  214  (analyte  214 ) to be detected.        

      For a more detailed discussion about the structure and the functionality of PWG sensor  200 , reference is made to the following documents: 
          U.S. Pat. No. 5,369,722.     U.S. Pat. No. 6,804,445 B2.        

      The contents of these documents are incorporated by reference herein.  
      Referring back to  FIG. 1 , it can be difficult to manufacture the PWG sensor  100  which has a polymeric substrate  102 . In particular, there is a significant manufacturing challenge in keeping the polymeric substrate  102  flat to ensure a uniform coupling angle for the light  116  that is directed into and reflected out-off the PWG sensor  100 . And, there is a significant manufacturing challenge in depositing the waveguide  106  onto the polymeric substrate  102  without damaging the polymeric substrate  102 . To address these problems, the PWG sensor  100  can be enhanced by using a flat glass substrate  102  (which is more costly than a polymeric substrate  102 ) and then forming a polymeric layer onto the glass substrate  102  so the sub-wavelength diffraction grating  104  can be formed into the polymeric layer by embossing, UV curing, or molding. This type of PWG sensor  300  is illustrated in  FIG. 3  (PRIOR ART). As shown, the PWG sensor  300  has the following elements: 
          Flat glass substrate  302 .     Polymeric cladding layer  305  (patterned with a sub-wavelength period diffraction grating  304 ).     Waveguide  306 .     Chemically responsive surface chemistry layer  308 .     Chemically bound molecules  310  (targets  310 ) of interest.     Solution  312  containing the substance  314  (analyte  314 ) to be detected.        

      The sensitivity of the PWG sensor  300  can be further enhanced if low index polymer layers (not shown) are used instead of the polymeric cladding layer  305 . For a more detailed discussion about the structure and the functionality of these types of PWG sensors  300 , reference is made to the following documents: 
          U.S. Patent Application No. 2003/0017580.     PCT Patent Application WO 0235214.     U.S. Patent Application No. 2005/0025421.        

      The contents of these documents are incorporated by reference herein.  
      Referring again to  FIG. 1 , the PWG sensor  100  can be made where the diffraction grating  104  is directly formed into a flat glass substrate  102  by a photolithographic patterning and etching process. This type of PWG sensor  100  is costly but it is desirable because it uses a glass substrate  102  which does not have the flatness problem that is associated with a polymeric substrate  102 . In addition, this type of PWG sensor  100  is more durable than the PWG sensor  100  which has the polymeric substrate  102 . Because, the waveguide  106  can be deposited onto the glass substrate  102  without the strict restrictions on temperature and ion bombardment that are needed when the waveguide  106  is deposited onto a polymeric substrate  102 .  
      However, the use of a photolithography patterning and etching process to form a smooth and accurately reproduced diffraction grating  104  on the top surface of the glass substrate  102  can be challenging. For example, a PWG sensor  100  has been made where ˜0.25 to 0.5 micron linewidths which are required for a subwavelength diffraction grating  104  have been formed in a glass substrate  102  by using a holographic photolithography process (see PCT Patent Application Nos. WO9809156 and WO02082130 and U.S. Pat. No. 6,873,764 B2). Unfortunately, this type of photolithography patterning process can be costly and as such it is not be suitable to manufacture disposable PWG sensors.  
      Moreover, the etching process which is used to form the diffraction grating  104  within the glass substrate  102  can be a significant challenge itself. For example, silicate glass substrates  102  can be etched by wet etching in a solution containing hydrofluoric acid, or by dry etching in a fluorine containing plasma. However, only simple glass substrates  102  like fused silica may be cleanly etched in this manner. In contrast, most commercial glass substrates  102  have compositions which are complex and contain alkali metals, alkaline earths, aluminum oxide, or transition metal oxides that do not etch well. In particular, the etching of these complex glasses by either a hydrofluoric acid containing solution or a fluorine containing plasma typically produces a rough etch surface, because the fluoride salts of the alkali metals, alkaline earths, aluminum and transition metals are not removed. However, clean features (such as diffraction gratings  104 ) can be etched for some compositions of glasses using mixed halide gases such as CCl 2 F 2  when all of the etch products are volatile (see U.S. Pat. No. 6,873,764 B2). And, clean features (such as diffraction gratings  104 ) may also be plasma etched in some glasses under conditions where sputter etching is utilized (see J. Liu, N. I. Nemchuk, D. G. Ast, and J. G. Couillard, J. Non-Crystalline Solids 342 110 (2004)). Unfortunately, these plasma etching method are not practical for manufacturing large numbers of glass PWG sensors  100 .  
      The aforementioned problems associated with etching clean diffraction gratings  104  in glass substrates  102  can be overcome by depositing a silica layer (which can be easily etched) or a polymer layer onto the glass substrate  102  and then patterning that layer by wet or dry etching. This type of PWG sensor  400  is illustrated in  FIG. 4  (PRIOR ART). As shown, the PWG sensor  400  has the following elements: 
          Flat glass substrate  402 .     Discontinuous oxide or polymer layer  405  (which also forms a sub-wavelength period diffraction grating  405 ).     Waveguide  406 .     Chemically responsive surface chemistry layer  408 .     Chemically bound molecules  410  (targets  410 ) of interest.     Solution  412  containing the substance  414  (analyte  414 ) to be detected.        

      For a more detailed discussion about the structure and the functionality of PWG sensor  400 , reference is made to the following document: 
          PCT Patent Application WO02082130.        

      The contents of this document are incorporated by reference herein.  
      In many of the above PWG sensors, a surface layer (e.g., SiO 2 ) may be deposited on top of the waveguide (e.g., PVD deposited Nb 2 O 5 ) to facilitate the formation of a chemoresponsive layer. An exemplary PWG sensor  500  which has this surface layer is illustrated in  FIG. 5  (PRIOR ART). As shown, the PWG sensor  500  has the following elements: 
          Flat glass substrate  502 .     Polymer layer  505  (patterned with a sub-wavelength period diffraction grating  504 ).     Waveguide  506 .     Surface layer  507 .     Chemically responsive surface chemistry layer  508 .     Chemically bound molecules  510  (targets  510 ) of interest.     Solution  512  containing the substance  514  (analyte  514 ) to be detected.        

      For a more detailed discussion about the structure and the functionality of PWG sensor  500 , reference is made to the following document: 
          U.S. Patent Application No. 2004/0043508A1.        

      The contents of this document are incorporated by reference herein.  
      In all of the above PWG sensors, a key performance attribute is signal drift. Signal drift occurs when the high index waveguide (and if present the SiO 2  surface layer) is porous or cracked which allows the interaction of the solution and the waveguide material. These pores and root cracks are always present in PVD deposited metal oxide waveguides which are deposited over gratings in polymer substrates (see  FIG. 1 ), or deposited over polymer gratings which are on top of glass substrates (see  FIG. 3 ). The pores result from the deposition of the waveguide at a low temperature and under a low ion flux. Under such conditions, adatoms from the vapor arrive at the growth surface and stay where they fall. This produces a columnar grain growth where the packing of adatoms is not optimal, resulting in porosity (see, D. L. Smith, Thin Film Deposition: Principles and Practice, McGraw-Hill (1995) 159-161).  
      A coating scientist can produce a fully dense oxide coating/waveguide by PVD if they can heat the substrate (typically 250° C. or above) to provide energy for surface diffusion of the adatoms to sites of higher binding energy, and/or if they can use ion bombardment to transfer momentum and pack the adatoms more densely (see D. L. Smith, Thin Film Deposition: Principles and Practice, McGraw-Hill, New York (1995) pp 119-180). However, these methods cannot be used in applications where polymer substrates and/or polymer gratings degrade at temperatures below 200° C. and under ion bombardment. This porosity problem is solved by the present invention.  
      Moreover, since this PVD coating process is a line of sight process, root cracks are often caused by inadequate step coverage over the grating features. These root cracks may cause signal drift due to infiltration of water during assays. A FIB image which is shown in  FIG. 6  (PRIOR ART) illustrates two root cracks  602  that are in the waveguide layer  506  of the prior art PWG sensor  500 . As can be seen, this PWG sensor  500  has a UV curable polymer grating  504  formed in a UV curable polymer  505  which is located on a glass substrate  502 .  FIG. 7  (PRIOR ART) has several plots which illustrate the signal drift (grating resonance vs. time) for 96 PWG sensors  500  (shown in  FIGS. 5-6 ) that are located in a 96 well microplate which contains an aqueous solution. This root problem is solved by the present invention.  
      An engineering solution to cancel out the effects of signal drift caused by porosity and root cracks in PWG sensors  500  (for example) would be to reference the signal from within one half of each PWG sensor  500  to the other half, or from one PWG sensor  500  to other PWG sensors  500  that are incorporated in the wells of a microplate. For example, in the former solution part of each PWG sensor  500  may be covered or its surface chemistry altered to prevent binding of the target molecules. Then, these PWG sensors  500  would be interrogated. Alternatively, in the later solution a buffer solution can be added to all wells of a microplate, and the biological molecules only to some of the wells in the microplate. Then, the PWG sensors  500  in these wells would be interrogated. However, measurements of differential signal drift may only be used if each PWG sensor  500  in the microplate exhibits a similar signal drift when exposed to the same solutions. Unfortunately, as the plots in  FIG. 8  (PRIOR ART) indicate, this is not the case for PWG sensors  500 . These plots illustrate the intra-well referenced signal drift (grating resonance vs. time) for 96 PWG sensors  500  located in a 96 well microplate. Accordingly, there is a need to overcome the problematical signal drift that is associated with PWG sensors. This need and other needs are addressed by the present invention.  
     BRIEF DESCRIPTION OF THE INVENTION  
      The present invention relates to a PWG sensor which exhibits a lower signal drift and an enhanced sensitivity due to the use of a fully dense silicon rich silicon nitride surface layer. In the preferred embodiment, the silicon rich silicon nitride surface layer has a composition which includes Si and N, and optionally H, Ge and/or O, where a Si/N atomic ratio is greater than 0.75. In addition, the silicon rich silicon nitride surface layer has a refractive index that is greater than 2.45 and less than 3.2 at a wavelength of operation. The present invention also includes a method for manufacturing the PWG sensor with acceptable costs and high yields by utilizing well known semiconductor processes and tools. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete understanding of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:  
       FIG. 1  (PRIOR ART) is a cross-sectional diagram that shows the basic elements of a PWG sensor which is disclosed in U.S. Pat. No. 4,815,843;  
       FIG. 2  (PRIOR ART) is a cross-sectional diagram that shows the basic elements of a PWG sensor which is disclosed in U.S. Pat. No. 5,369,722;  
       FIG. 3  (PRIOR ART) is a cross-sectional diagram that shows the basic elements of a PWG sensor which is disclosed in PCT Patent Application WO 0235214;  
       FIG. 4  (PRIOR ART) is a cross-sectional diagram that shows the basic elements of a PWG sensor which is disclosed in PCT Patent Application WO02082130;  
       FIGS. 5-8  (PRIOR ART) are diagrams used to help describe the structure and functionality of a PWG sensor which is disclosed in U.S. Patent Application No. 2004/0043508A1;  
       FIGS. 9A and 9B  are diagrams of two different embodiments of a PWG sensor in accordance with the present invention;  
       FIG. 10  is a SEM cross-sectional image of a PWG sensor that is configured like the PWG sensor shown in  FIG. 9B ;  
       FIG. 11  is a plot illustrating the refractive index at 830 nm vs. cutoff wavelength for silicon rich silicon nitride which is used in the PWG sensors shown in  FIGS. 9A and 9B ;  
       FIG. 12  is a plot illustrating the refractive index at 830 nm vs. CF 4  flow for PECVD deposited FSG cladding which can be used in the PWG sensors shown in  FIGS. 9A and 9B ;  
       FIGS. 13A and 13B  are two diagrams of an exemplary  96  well microplate which can incorporate 96 PWG sensors like the PWG sensors shown in  FIGS. 9A and 9B ;  
       FIG. 14  is a series of plots that illustrate the signal drift (grating resonance vs. time) for 96 PWG sensors (see  FIG. 9B ) which are exposed to an aqueous solution in a 96 well microplate;  
       FIG. 15  is a series of plots that illustrate the intra-well referenced signal drift (grating resonance vs. time) for 96 PWG sensors (see  FIG. 9B ) which are exposed to an aqueous solution in a 96 well microplate;  
       FIG. 16  (PRIOR ART) illustrates two graphs which indicate the results of an experiment that was conducted to test known PWG sensors that are similar to the one shown in  FIG. 6 ; and  
       FIG. 17  illustrates two graphs which indicates the results of an experiment that was conducted to test new PWG sensors that are similar to the one shown in  FIG. 9B .  
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Referring to  FIGS. 9A and 9B , there are two block diagrams which illustrate two PWG sensors  900   a  and  900   b  configured in accordance with the present invention. As shown in  FIG. 9A , the first embodiment of the PWG sensor  900   a  has the following elements: 
          Substrate  902   a  (e.g., glass substrate  902   a , polymer substrate  902   a ).     Low index cladding  904   a * (e.g., polymer cladding  904   a ) that is patterned with a sub-wavelength period diffraction grating  906   a ).     Monomode waveguide  908   a ** (e.g., Nb 2 O 5  waveguide  908   a ).     Silicon rich silicon nitride surface layer  910   a.       Chemically responsive surface chemistry layer  912   a.       Chemically bound molecules  914   a  (targets  914   a ) of interest.     Solution  916   a  containing the substance  918   a  (analyte  918   a ) to be detected.        

      As shown in  FIG. 9B , the second embodiment of the PWG sensor  900   b  has the following elements: 
          Substrate  902   b  (e.g., glass substrate  902   b , polymer substrate  902   b ).     Low index cladding  904   b * (e.g., polymer cladding  904   b ) that is patterned with a sub-wavelength period diffraction grating  906   b ).     Silicon rich silicon nitride monomode waveguide  908   b.       Chemically responsive surface chemistry layer  912   b.       Chemically bound molecules  914   b  (targets  914   b ) of interest.     Solution  916   b  containing the substance  918   b  (analyte  918   b ) to be detected.     * It should be noted that the cladding layer  904   a  and  904   b  is optional. If it is not used, then the sub-wavelength period diffraction grating  906   a  and  906   b  would be formed in the substrate  902   a  and  902   b  (or the waveguide  908   a  and  908   b ).     ** It should be further noted that the waveguide  908   a  can be deposited by using one of the following: a sol-gel technique; a spin on glass technique; a CVD technique (including a PECVD technique, a LPCVD technique, a SACVD technique, or an APCVD technique); or a PVD technique (including a sputtering technique, an e-beam evaporation technique, and an evaporation technique).        

      In the first embodiment, the PWG sensor  900   a  has a monomode waveguide  908   a  that is covered by the silicon rich silicon nitride surface layer  910   a  (which is preferably 2-10 nm thick). And, in the second embodiment, the PWG sensor  900   b  has a monomode waveguide  908   b  that is made from silicon rich silicon nitride (which is preferably greater than 70 nm). In each embodiment, the preferred silicon rich silicon nitride (SiN x ) has a composition which includes Si and N, and optionally H, Ge and/or O, where a Si/N atomic ratio is greater than 0.75. In addition, the silicon rich silicon nitride has a refractive index that is greater than 2.45 and less than 3.2 at a wavelength of operation. Although the second embodiment of the PWG sensor  900   b  is discussed in more detail below, it should be appreciated that the same advantages associated with using the silicon rich silicon nitride material in PWG sensor  900   b  also applies to the first embodiment of the PWG sensor  900   a.    
      In the present invention, the SiN x  waveguide  908   b  (or the silicon rich silicon nitride surface layer  910   a ) is deposited by a step coverage process sufficient to prevent (or at least substantially prevent) problematical root cracks and/or pores from being formed in the grating structure  920   b  (the area associated with the cladding  904   b  (if any), the diffraction grating  906   b  and the waveguide  908   b ). For a vapor deposited SiN x  coating, this requires deposition at an elevated temperature and/or with ion bombardment. And, this type of deposition process requires a thermally robust substrate  902   b  and grating structure  920   b  which are not easily damaged by ion bombardment. Silicate glass is an economical substrate  902   b  that meets these requirements. For example, precision flat glass such as Corning code 7059, 1737 or 2000 could be used as the substrate  902   b.    
      A preferred grating structure  920   b  which can meet these requirements is described next. A preferred grating structure  920   b  is one that has a smooth and accurately reproduced diffraction grating  906   b  with a sub-micron pitch which is formed into the top surface of the substrate  902   b  (this is not the grating structure  920   b  shown in  FIG. 9B ). However, since it is difficult to cleanly wet etch or dry etch in fluorine plasma most commercial precision flat glass, a thin cladding layer  904   b  of a more easily etched material (within which the diffraction grating  906   b  can be etched) such as a doped or undoped silicon oxide (e.g., silica) can be deposited over the glass substrate  902   b  (this is the grating structure  920   b  shown in  FIG. 9B ). For instance, the silica cladding layer  904   b  can be deposited by using a PECVD, a SACVD or a LPCVD technique any of which are well known in the semiconductor industry. If the silica cladding  904   b  is doped with fluorine, germanium, boron, phosphorous and/or nitrogen, then it is possible to adjust the refractive index, thermal stress, and etch rate of the silica cladding  904   b . In addition, the silica cladding  904   b  if desired may be annealed to help stabilize the refractive index and hydrolytic stability.  
      After depositing the cladding  904   b , the diffraction grating  906   b  is etched therein. In the preferred embodiment, the diffraction grating  906   b  can be etched within the cladding  904   b  by using a photolithography process during which a resist is deposited over the cladding  904   b  and then the resist is patterned such that the cladding  904   b  can be etched through exposed areas in the resist to form the diffraction grating  906   b . For instance, a diffraction grating  906   b  with quarter micron linewidths could be economically formed within the cladding  904   b  by using: (1) a 248 nm projection lithiography (stepper) and reactive ion etch (or wet etch); or (2) a 4× or higher magnification i-line stepper and reactive ion etch (or wet etch).  
      At this point, a fully dense silicon rich silicon nitride waveguide  908   b  has been deposited over the diffraction grating  906   b  by a process which minimizes the formation of root cracks and/or voids. The preferred deposition process is CVD because it produces films with conformal coverage. As can be seen in  FIG. 10 , this conformal coverage leads to a fully dense waveguide  908   b  which has no cracks or voids (compare to  FIG. 6 ).  FIG. 10  illustrates a SEM cross-sectional image of a 140 nm thick silicon rich silicon nitride waveguide  908   b  which is deposited over a 55 nm deep diffraction grating  906   b  which is etched in a 200 nm thick FSG cladding layer  904   b  that is deposited on Corning code 1737 glass substrate  902   b.    
      The refractive index of the silicon rich silicon nitride can be adjusted from 1.9 to 3.2 at 830 nm by changing the concentration of silicon, nitrogen, and hydrogen (e.g., see W. R. Knolle, Thin Solid Films 168 (1989) 122). The raising of the refractive index causes the adsorption edge to move from near 300 nm to over 800 nm as the index rises from 1.9 to 3.2. Of particular interest for higher sensitivity PWG sensors  900   a  and  900   b  is the use of silicon rich silicon nitride compositions where the Si/N ratio exceeds the stoichiometric ratio of 3/4 (and preferably &gt;2.0). These silicon rich silicon nitrides when compared to previously used silicon nitrides have a higher refractive index, a lower stress, an absorption edge in the visible range, and more hydrogen bound to Si. And, the silicon rich silicon nitride composition can be set to maximize refractive index and minimize absorption in the waveguide  908   b  at the wavelength of interest.  FIG. 11  is a graph that shows the refractive index of silicon rich silicon nitride vs the cutoff wavelength where k&gt;1E-4 for films deposited from silane and ammonia in a typical parallel plate PECVD system over a range of reactant flows, reactor pressure, RF power, and electrode configurations. From this graph it can be seen that sufficiently low optical absorption can be achieved with a silicon rich silicon nitride that has a refractive index of over 2.75 at 830 nm. This feature permits the use of silicon rich silicon nitride as a high index waveguide  908   b  (or surface layer  910   a ).  
      To further increase the index contrast between the waveguide  908   b  and the cladding layer  904   b /substrate  902   b , one can lower the refractive index of the cladding layer  904   b . Fluorine is the most effective dopant for lowering the refractive index of a silica cladding layer  904   b . However, the moisture sensitivity of the silica cladding layer  904   b  increases with the fluorine content which puts a limit on the fluorine content. Ab initio configuration interaction calculations have predicted the lower limit of FSG stability against moisture at 10-12 at % F (see H. Yang and G. Lucosky, J. Vac. Sci. Tech. A16(3) 1525). A practical limit of ˜9 at % F, corresponding to a refractive index of 1.4150 at 830 nm was chosen for the PWG sensor  900   b  shown in  FIG. 10 . The FSG films  904   b  of this composition were deposited from controlled additions of a fluorine containing gas such as CF 4 , C 2 F 6 , or C 4 F 8  to a PECVD silicon dioxide process which also used silane and nitrous oxide.  FIG. 12  is a plot which shows the refractive index at 830 nm vs. CF 4  flow for PECVD deposition of an exemplary FSG film  904   b.    
      Example A: An experiment was conducted so the signal drift and theoretical sensitivity of the PWG sensor  900   b  (shown in  FIG. 9B ) could be compared to the prior art PWG sensor  500  (shown in  FIGS. 5-6 ). In order to conduct this experiment, a 96 well microplate  1300  similar to the one shown in  FIGS. 13A and 13B  was manufactured which had 96 PWG sensors  900   b  that where fabricated by PECVD depositing a 200 nm fluorinated silica glass (FSG) cladding layer  904   b  which had a refractive index of 1.41 at 830 nm onto the glass substrate  902  (Corning Code 1737 glass). A diffraction grating  906   b  pattern was then formed in the FSG cladding layer  904   b  by exposing a photoresist with a deep UV stepper, and a 65 nm deep diffraction grating  906   b  was etched into the FSG cladding layer  904   b  by using a RIE process and fluorocarbon gas CHF 3 . A ˜120 nm thick Si-rich silicon nitride monomode waveguide  908   b  with a refractive index of 2.75 at 830 nm was then PECVD deposited over the diffraction grating  906   b /FSG cladding layer  904   b , and an optional 5 nm thick silica surface layer was deposited over the waveguide  908   b  by PECVD (see  FIG. 9B ). The 96 well microplate  1300  was then formed by bonding a polymer holey well plate  1302  to the inorganic planar waveguide grating  1304  with a UV curable adhesive  1306 .  
      The signal drift of several PWG sensors  900   b  which were in contact with de-ionized water is shown in the plots illustrated in  FIGS. 14 and 15 .  FIG. 14  illustrates plots of signal drift (grating resonance vs. time) for 96 PWG sensors  900   b  which were exposed to an aqueous solution in microplate  1300 . As can be seen, these plots show that the inorganic PWG sensors  900   b  exhibit a drift of only 0.01 to 0.05 pm/min after 30 minutes of exposure to the aqueous solution. The higher initial drift is believed to happen while the PWG sensors  900   b  and aqueous solution attained thermal equilibrium. And,  FIG. 15  illustrates plots of intra-well referenced signal drift (grating resonance vs. time) for 96 PWG sensors  900   b  which where exposed to an aqueous solution in microplate  1300 . As can be seen, these plots show the intra-well referenced drift between the PWG sensors  900   b  as being only 0.005 to 0.015 pm/min after 30 minutes of exposure to the aqueous solution. TABLE 1 compares the drift rates of the PWG sensor  900   b  to the prior art PWG sensor  500  (see  FIGS. 5-8 ).  
                           TABLE 1                                   PWG sensor 900b       Observed numbers for       PWG sensor 500   (see  FIGS. 9B  and       most of the wells       (see FIGS. 5-8)   14-15)                                                    Total Absolute Drift   −60   pm to   10   to       (0 to 30 min)   −120   pm   15   pm       Absolute Drift Rate   0.150   pm/min to   0.01   pm/min to       (30 to 90 min)   0.25   pm/min   0.05   pm       Referenced Drift   0.01   pm/min to   0.005   pm/min to       Rate (30 to 90 min)   0.03   pm/min   0.015   pm/min                  
 
      As can be seen, the inorganic PWG sensor  900   b  had a lower and more uniform signal drift than prior art PWG sensor  500 . This is due to the conformal coverage of the waveguide  908   b  and the exceptional uniformity of the grating structure  920   b . And, since all the PWG sensors  900   b  in the wells drift in a similar manner, one can reference between the different PWG sensors  900   b  to cancel out the effect of signal drift. As can be seen, this greatly increased the sensitivity of the response of the PWG sensor  900 B.  
      In this example, the sensitivity of the inorganic PWG sensor  900   b  was determined by measuring the shift in the resonant wavelength as the index of refraction changed when liquid was added to the microplate  1300 . The refractive index of the solution was changed by adding glycerol to water. The resonant wavelength was determined for a series of water-glycerol mixtures containing 1-40 vol % glycerol. And, then the sensitivity of each PWG sensor  900   b  was obtained by performing a linear fit to the plot of the wavelength shifts vs. refractive index of the water glycerol solution. In this experiment, the average slope of the microfabricated PWG sensor  900   b  was 110 nm/RIU, vs. 80 nm/RIU for the prior art PWG sensor  500 . The PWG sensor  500  had a Nb 2 O 5  waveguide  506 , a 2 nm SiO 2  surface layer  507 , and a UV formed polymer grating  504 / 505  (see  FIGS. 5-6 ).  
      Example B: Another experiment was conducted so the assay performance of Fl-biotin interacting with immobilized Streptavidin could be compared by using 384 well microplates which had either inorganic PWG sensors  900   b  or prior art PWG sensors  500  incorporated therein.  
      The inorganic PWG sensor  900   b  array was fabricated by depositing onto a glass substrate  902   b  a 200 nm FSG cladding layer  904   b  which had a refractive index of 1.41 at 830 nm by PECVD. Then, grating patterns  906   b  were formed in the FSG cladding layer  904   b  by exposing a photoresist with a deep UV stepper, and 45 nm deep diffraction gratings  906   b  were etched into the FSG cladding layer  904   b  by using a RIE process and fluorocarbon gas CHF 3 . Then, a ˜148 nm thick Si-rich silicon nitride monomode waveguide  908   b  with a refractive index of 2.5 at 830 nm was deposited by PECVD over the diffraction gratings  904   b . The prior art PWG sensor  500  array was fabricated by forming diffraction gratings  504  in a UV cured polymer layer  505  located on a glass substrate  502 . Then, a Nb 2 O 5  waveguide  506  was PVD deposited over the UV cured polymer layer  505 . And, a 2 nm thick SiO 2  surface layer  507  was deposited over the Nb 2 O 5  waveguide  506 . Both types of PWG sensor arrays were then bonded to 384 holey well plates with a UV curable adhesive, and subsequently coated with aminoproyl silsesquioxane (APS) and poly(ethylene-alt-maleic anhydride) (EMA) to form the chemoresponsive layer  508 / 912   b  which was used to bind the Streptavidin  510 / 914   b.    
      TABLE 2 compares the Fl-biotin to Streptavidin assay performance between microfabricated 384 well microplates which contained the PWG sensors  900   b  and 384 well microplates which contained the prior art PWG sensors  500 .  
                                           TABLE 2                                               Approximate   Approximate                           Soak   Drift   Drift       Plate   Plate   AVE           Time   (pm/min)   (pm/min)       Number   Type   (pm)   STDEV   % CV   (ACE)   (rows ACE)   (rows BDF)                                                                    1   Prior Art   23.40   1.14   4.87   5   Hours   −0.60   −0.20       2   New   30.02   1.50   4.98   1   Hour   +0.05   +0.05       3   New   34.52   1.55   4.48   5.5   Hours   +0.15   +0.10       4   Prior Art   23.33   1.52   6.54   4.5   Hours   −0.35   −0.25           ( FIG. 16 )       5   New   29.09   1.61   5.52   50   Minutes   +0.15   0.00           ( FIG. 17 )       6   New   30.67   1.03   3.35   4.75   Hours   +0.075   +0.075                  
 
      In this experiment, three rows of wells were measured at once, with rows A, C, E measured first, and B, D, and F next. Upon addition of the Fl-biotin the grating resonance was observed to shift by an average of ˜30 pm for the inorganic 384 well microplates (which contained PWG sensors  900   b ) when compared to ˜23.5 pm for the 384 well microplates (which contained the prior art PWG sensors  500 ). The differences in the assay results between the inorganic 384 well microplates (which contained PWG sensors  900   b ) and the 384 well microplates (which contained the prior art PWG sensors  500 ) can be seen in the assay curves shown in  FIGS. 16 and 17 .  
       FIG. 16  (PRIOR ART) illustrates two graphs of a Fl-biotin on Streptavidin assay signal from column 12 rows ACE (top graph) and BDF (bottom graph) showing both the assay signal and the signal drift using the 384 well microplate which contained the prior art PWG sensors  500 . While,  FIG. 17  illustrates two graphs of a Fl-biotin on Streptavidin assay signal from column 12 rows ACE (top graph) and BDF (bottom graph) showing both the assay signal and the signal drift using the inorganic 384 well microplate which contained the new PWG sensors  900   b . In comparing the plots in  FIGS. 16 and 17 , a significant negative drift can seen with the prior art 384 well microplate prior to introducing the Fl-biotin, and the assay signal is seen to rise and roll off with time after the addition of the Fl-biotin. In contrast, the signal drift before the introduction of Fl-biotin is smaller, and the assay signal after the binding with Streptavidin is constant for the new inorganic 384 well microplate.  
      Referring back to TABLE 2, the average signal drift that is observed prior to addition of the Fl-biotin is shown in the two right columns. As can be seen, the signal drift is significantly lower for the new 384 well inorganic microplates than the prior art 384 well microplates. And, the difference in the signal drift in different rows is lower as seen by results for rows A, C, and E vs. B, D, and F. This difference is important to determine if referencing between wells (PWG sensors) could be utilized to reduce the signal drift. Also, it is important to note that the low drift rates where achieved after soaking the new inorganic microplates in a buffer for ˜1 hr vs. the 4.5 hrs plus required before the drift of the prior art microplates would stabilize.  
      Example C: This experiment was conducted to examine if the origin of the higher assay signal for the inorganic PWG sensors  900   a  and  900   b  was due to the higher index contrast of the waveguide structure, or to the surface interaction between the waveguide structure, the surface chemistry layer  912   a  and  912   b , and the solution containing the molecules  914   a  and  914   b  of interest. Eight microplates of either inorganic PWG sensors  900   a  or  900   b  as described above with respect to  FIGS. 9A and 9B  but with different waveguides and surface layers where tested and the Fl-biotin to Streptavidin assay results compared. The waveguide materials compared were the Nb 2 O 5  waveguides (PWG sensor  900   a ) as was typically used in prior art example PWG sensor  500 , and silicon rich silicon nitride (PWG sensor  900   b ). The surface layers compared were SiO 2  surface as was typically used in prior art example PWG sensor  500 , and silicon rich silicon nitride surface layers described in PWG sensors  900   a  and  900   b . Both silicon-rich silicon nitride and SiO 2  surface layers were deposited by both PECVD and PVD (sputtering). All eight microplates were coated with aminoproyl silsesquioxane (APS) and poly(ethylene-alt-maleic anhydride) (EMA). The results of this experiment are shown in TABLE 3.  
                                           TABLE 3                                   Waveguide       Cover   Assay                   Waveguide   Thickness       Thickness   Signal   CV       Sample   Waveguide   Index   (nm)   Cover   (nm)   (pm)   (%)                  1+   SiN x     2.75   121   SiO 2     2   22.4   7.1                       PECVD       2+   SiN x     2.75   121           31.3   5.2       3+   SiN x     2.50   148           30.5   7.8       4   Nb 2 O 5     2.30   168   SiO 2     7   23.5   8.3                       PVD       5+   SiN x     2.50   148   SiO 2     7   22.0   8.3                       PVD       6   Nb 2 O 5     2.30   168   SiO 2     7   23.5   9.6                       PECVD       7   Nb 2 O 5     2.30   165   SiN x     5   27.61   7.6                       PECVD       8   Nb 2 O 5     2.30   165   SiN x     9   30.61   8.2                       PVD                 +Samples 1, 2, 3 and 5 are similar to PWG sensor 900b except that they have a silica surface layer.             
 
      A clear distinction can be seen between the microplates with a SiO 2  surface layer and those with a silicon-rich nitride surface. In particular, the assay signal on microplates with SiO 2  surface layers averaged 22.9 pm, vs. 30.0 for the microplates with silicon-rich silicon nitride surfaces. No significant difference was observed between PVD and PECVD deposited SiO 2 , PVD and PECVD deposited silicon-rich silicon nitride, and between 2.75 index and 2.5 index silicon-rich silicon nitride. These results indicate that the sensitivity increase is due to the silicon-rich silicon nitride surface and this effect dominates the effects of the waveguide structure.  
      From the foregoing, it should be appreciated by those skilled in the art that the silicon rich silicon nitride surface layer or waveguide can also be used in any of the aforementioned prior art PWG sensors (see  FIGS. 1-5 ). As such, the present invention can be applied to PWG sensors that have a polymer substrate, a glass substrate with doped oxide cladding layer, or a glass substrate with a polymer grating layer. In addition, the present invention can be applied to PWG sensors that do not have the cladding layer.  
      Although several embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.