Abstract:
Multiple diffractive devices containing an environmentally delicate material such as photoresist are fabricated on a single substrate that is later cut to separate devices. One process shapes the material into multiple contoured regions on a single substrate. Each contoured region has the topography needed for a diffractive device and is separated from other contoured regions by saw streets. A seal layer, which may include an adhesion layer and a reflective layer is deposited over the contoured regions and extends into the saw streets. Cutting the substrate separates the diffractive devices with each diffractive device hermetically sealed between the seal layer and the substrate.

Description:
BACKGROUND  
         [0001]    Reflective diffractive elements generally have surfaces that are shaped so that reflected light forms a diffraction pattern that achieves a desired optical function. A reflection grating, for example, has lines spaced to create diffraction patterns from the incident light. A diffractive lens has a surface shaped to create a diffraction pattern that focuses a desired fraction of a particular frequency of light to a focal point. A general DOE (diffractive optical element) can be designed to create any optical pattern, including shapes such as rectangles or circles of even intensity, line profiles, spot arrays, illuminated images such as characters or numerals, and complex intensity profiles. The topography of the surface of the reflective diffractive element will generally depend on the optical function that the diffractive element performs and the frequency or frequencies of the light diffracted.  
           [0002]    Grayscale photolithography is one method for creating the desired topography for the surface of a diffractive element. With grayscale photolithography, a layer of a photosensitive material such as photoresist is exposed to a light pattern in which the intensity of the light at each position on the photoresist layer determines an exposure depth in the photoresist at that position. A development process can then remove the exposed photoresist to leave the photoresist layer with a topography corresponding to the grayscale light pattern. The photoresist can then be covered with a reflective material to form the surfaces of a reflective diffractive element.  
           [0003]    [0003]FIG. 1 shows a diffractive element  100  including a contoured photoresist layer  120  on a substrate  110 . A reflective layer  130 , typically made of a metal such as gold (Au), is deposited on photoresist layer  120  to provide the desired reflective surface.  
           [0004]    Another function of reflective layer  130  is to protect photoresist layer  120  from the environment. Meeting the application environment and reliability standards for diffractive element  100  generally requires a hermetic seal or encapsulation to environmentally protect that photoresist layer  120 . For example, to provide a hermetic seal, reflective layer  130  extends down the sides of photoresist layer  120  onto the sides of environmentally stable substrate  110 . Accordingly, substrate  110  has the size required for diffractive element  100 .  
           [0005]    When manufacturing gratings, simultaneous fabrication of multiple gratings is desirable. However, handling and fixturing of a large number of small substrates for individual coatings can be slow and expensive. If multiple gratings were formed on one substrate, the gratings could be separated at the end of the manufacturing process. However, simply cutting substrate  110  of FIG. 1 in half would expose edges of photoresist layer  120 , and the separate gratings would be subject to failure during environmental testing or during subsequent manufacturing assembly steps.  
           [0006]    A further problem with cutting grating  100  is that the adhesion strength of thin film layers  120  and  130  may not be sufficient to withstand the separation process. Delamination may occur. Further, the adhesion properties of layers  120  and  130  are convolved together with their performance, and sufficient improvement in the adhesion strength may be difficult to achieve without degrading the product performance.  
           [0007]    An alternative manufacturing process removes photoresist layer  120 , for example, in an etching process that removes the photoresist layer and etches into underlying substrate  110  to transfer the surface topography to substrate  110 . With no photoresist to protect, the requirement for a hermetic seal may change. However, such manufacturing processes are subject to variations and defects resulting from the need for very precise control of the etching process that removes the photoresist and shapes the substrate. Manufacturing processes that remove the photoresist thus may be unable to create diffractive elements having the same performance as diffractive elements such as diffractive element  100  that include contoured photoresist layers.  
           [0008]    Hermetic seals and efficient manufacturing methods are thus needed for the manufacture of precision diffractive elements.  
         SUMMARY  
         [0009]    In accordance with an aspect of the invention, a method for manufacturing diffractive elements containing polymer material such as photoresist or another environmentally delicate material produces multiple diffractive elements on the same substrate and then separates the diffractive elements. The substrate containing the multiple diffractive elements has saw streets that allow separation of the diffractive elements without damaging the delicate material. To provide hermetic seals that remain intact, a seal layer for the delicate material extends into the saw streets and remains intact on a top surface of the substrate after a separation process such as sawing or scribing. The saw streets are wide enough that in each separated diffractive element the sealing material extends beyond the edge of the photoresist on the top surface of the underlying substrate and thereby hermetically seals the photoresist.  
           [0010]    One specific embodiment of the invention is a process for fabricating diffractive devices. The process includes forming first and second contoured regions from a material such as a polymer on a surface of a substrate. On the substrate, a saw street that is free of the material separates the first contoured region from the second contoured region. The process deposits a seal layer that extends into the saw street on the substrate. The seal layer, which can include an adhesion layer of material such as chromium, titanium, tantalum, nickel, or nickel-chrome and a reflective layer of a metal such as gold, silver, aluminum, or dielectric reflectors generally covers the first and second contoured regions and serves to reflect light as required for the diffractive device. After forming the seal layer, cutting the substrate along the saw street separates individual diffractive devices. Each diffractive device thus includes a contoured region that is hermetically sealed between the seal layer and the substrate. The hermetic seal includes a portion of the seal layer that is on the top surface of the substrate.  
           [0011]    Another specific embodiment of the invention is a device that includes a substrate, a contoured region, and a seal layer. The contoured region can be made of an environmentally delicate material such as photoresist and has the topography required of a diffractive optical element. Generally, the contoured region is hermetically sealed between the substrate and the seal layer. The seal layer overlies the contoured region and adheres to the top surface of the substrate where the substrate extends beyond the contoured region. In some configurations, the contoured region has a second edge that is aligned with an edge of the substrate, and the seal layer extends down the second edge of the contoured region and onto the edge of the substrate. The seal layer can be reflective and such that reflections from the seal layer implement the function of the diffractive optical element. In one configuration, the seal layer includes an adhesion layer of a material such as chromium, titanium, tantalum, nickel, or nickel-chrome that adheres to the substrate and a reflective layer of a reflective material such as gold, silver, aluminum, or a dielectric stack.  
           [0012]    The device can further include a second contoured region having the topography required for another diffractive element. The first and second contoured regions with an intervening saw street are on the surface of the same substrate. hi this configuration, cutting the substrate along the saw street can create separate diffractive devices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a cross-sectional view of a conventional diffractive element containing a contoured photoresist layer.  
         [0014]    [0014]FIG. 2 is a flow diagram for a fabrication process in accordance with an embodiment of the invention.  
         [0015]    [0015]FIGS. 3, 4, and  5  are cross-sectional views of structures formed during a manufacturing process in accordance with an embodiment of the invention that simultaneously forms multiple diffractive elements.  
         [0016]    [0016]FIGS. 6 and 7 are cross-sectional views of diffractive elements after separation from the structure of FIG. 2. 
     
    
       [0017]    Use of the same reference symbols in different figures indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0018]    A manufacturing process for a diffractive element forms a combined structure containing multiple diffractive elements with saw streets separating the diffractive elements. A sealing layer that covers an environmentally delicate material in the diffractive elements extends into the saw streets of the combined structure. A separation process such as sawing or scribing then separates the diffractive elements for use, but leaves the sealing layer attached to remaining portions of the saw streets to maintain hermetic seals that protect the environmentally delicate material.  
         [0019]    [0019]FIG. 2 is a flow diagram of a fabrication process  200  for diffractive elements in accordance with an embodiment of the invention. Process  200  includes fabrication steps  210 ,  220 ,  230 ,  240 , and  250  that form structures such as illustrated in FIGS. 3, 4,  5 ,  6 , and  7 . Process steps  210 ,  220 ,  230 ,  240 , and  250  of FIG. 2 are thus described with reference to exemplary structures illustrated in FIG. 3, 4,  5 ,  6 , and  7 .  
         [0020]    Step  210  deposits a layer  320  of a material on a substrate  310  as illustrated in FIG. 3. Layer  320  can be of any material that is suitable for patterning that forms the topography of a diffractive optical element. In an exemplary embodiment of the invention, the material of layer  320  is a photosensitive material that can be pattern using known photolithographic techniques. Other examples of suitable materials for layer  320  are electron beam resists and materials that can be embossed, molded, or otherwise mechanically shaped using techniques such as ruling or ion milling.  
         [0021]    Substrate  310  is generally made of a material having a low thermal coefficient of expansion to reduce the effects of temperature changes on the dimensions of the diffractive elements being manufactured. Such dimensional changes would change the diffraction pattern and the optical performance of the diffractive elements. A material such as an ultralow expansion (ULE) glass is well suited for substrate  310 , but materials such as silicon, sapphire, or any other material having suitable structural, environmental, and thermal properties could also be used.  
         [0022]    In the exemplary embodiment, layer  320  can be made of any commercially available photosensitive material, and typically is deposited to a thickness between about 1 micron to about 100 microns depending on the type of diffractive elements being made. Methods for depositing and using such photoresist layers are well known in integrated circuit manufacturing processes.  
         [0023]    Step  220  of FIG. 2 exposes layer  320  as required to change the properties of a portion of layer  320  that will be removed. A variety of methods for such exposure are possible and the particular method employed is not critical to the present invention. U.S. Pat. No. 6,410,213 describes one known method for exposing a photosensitive material to form a desired surface profile.  
         [0024]    One suitable exposure process when layer  320  is a layer of photoresist involves a photolithographic process that projects onto the surface of photosensitive layer  320  light of the proper wavelength to activate photosensitive material. Layer  320  can be exposed with an electron beam when an electron beam resist is used. Exposure parameters, material parameters, and development parameters all can be varied to achieve the desired topography. Alternatively, multiple exposures with different radiation patterns can expose layer  320  to the proper depth at each point. In particular, a separate process can expose regions of layer  320  that correspond to the saw streets described further below.  
         [0025]    Step  230  is a development process that removes the irradiated portions of photosensitive layer  320  to leave regions  322  of unexposed material as shown in FIG. 4. As a result of the controlled exposure, each region  322  has a top surface contoured as required for a diffractive element. Optionally, the material in contoured regions  322  can then be cured or otherwise hardened to improve durability.  
         [0026]    Mechanical processes can be used instead of the exposure and development (steps  220  and  230 ) to form contoured regions  322 . One mechanical process, for example, stamps layer  320  to mechanically create the topology or pattern of regions  322 . Alternatively, a mechanical process can cover substrate  310  with a mold and then inject the material of layer  320  into contoured cavities of the mold to form regions  322 . In yet another alternative process, traditional grating ruling (scratching), ion milling, or other removal techniques can remove unwanted material from layer  320  to form regions  322 .  
         [0027]    Saw streets  324 , which separate contoured regions  322 , are areas of substrate  310  lacking the material of layer  320 . Many techniques are known for creating structures such as saw streets  324 . In the exemplary embodiment, the development process of step  230  removes portions of layer  320  between regions  322  to expose portions of substrate  310  and leave saw streets  324  between regions  322 . Alternatively, a process separate from exposure and development steps  220  and  230  can remove portions of layer  320  to open saw streets  324 . Laser ablation or a mechanical removal process, for example, could remove portions of layer  320  that are in saw streets  324 . In yet another alternative process, the initial deposition of layer  320  may be controlled to avoid saw streets  324 .  
         [0028]    Saw streets  324  have a width that is selected according to the needs of the design, the separation method to be used to cut substrate  310 , and the required size of a hermetic sealing area described below. In the exemplary embodiment of the invention, saw streets  324  are about 50 microns to 1000 microns wide to accommodate a 20-micron to 900-micron wide saw process positioned to a typical accuracy of about +/−25 microns.  
         [0029]    Step  240  deposits a seal layer or layers  530  on regions  322  and in saw streets  324  as illustrated in FIG. 5. In the embodiment of FIG. 5, seal layer  530  extends across saw streets  324  and is cut when a separation process cuts substrate  310  into separate diffractive elements. Sawing and other separation processes are generally aggressive mechanical processes that can delaminate, tear, or peel back layers at their interfaces. However, since saw streets  324  are free of the material required for forming the topography of the diffractive element, seal layer  530  can be chosen to increase the adhesion to substrate  310 .  
         [0030]    Layer  530  will generally include a stack of layers. Layers of metals such as chromium (Cr), titanium (Ti), tantalum (Ta), nickel (Ni), nickel-chrome (NiCr) and others are known to enhance the adhesion of subsequent layers and can be deposited on substrate  310  particularly in saw streets  324 . A top layer of a highly reflective and environmentally inert metal such as gold (Au) can complete layer  530 . The top layer could alternatively include another metal or a dielectric reflector. In an exemplary embodiment of the invention, layer  530  includes 1500-Å layer of gold on a 500-Å layer of chromium. The combination of layers has suitable adhesion strength to substrate  310  and still provides the desired conformal, high reflectivity layer  530  needed for product performance.  
         [0031]    In an alternative to structure  500  of FIG. 5, layer  530  can be patterned to expose portions of substrate  310  in the centers of saw streets  324 . In particular, layer  530  can extend far enough into saw streets  324  to form hermetic seals for contoured regions  322  but still provide gaps in saw streets  324  that are wide enough for a cutting process that does not cut or damage layer  530 .  
         [0032]    After deposition of layer  530 , step  250  separates individual diffractive elements. Many separation techniques such as sawing or scribing and breaking are known in the arena of integrated circuit manufacture and can be used to cut substrate  310  into individual grating pieces.  
         [0033]    In an exemplary embodiment of FIG. 5, a sawing process removes the material of substrate  310  and layer  530  between parallel surfaces  540  and  542  and between parallel surfaces  544  and  546 . Additional sawing along saw streets (not shown) that cross saw streets  324  may be necessary to separate individual diffractive elements. Sawing techniques and equipment that are well known in integrated circuit manufacture can cut substrate  310 .  
         [0034]    [0034]FIGS. 6 and 7 show cross-sectional views of diffractive elements  600  and  700  that result from the sawing of structure  500  of FIG. 5. Diffractive element  600  includes a substrate  610  and a reflective layer  630  that are portions cut respectively from edge portions of substrate  310  and layer  530  of FIG. 5. Between substrate  610  and reflective layer  630  is one of the regions  322  of environmentally delicate material. Diffractive element  600 , which comes from an edge of substrate  310 , inherits at least one edge  514  from substrate  310  and has at least one cut edge  540 . Accordingly, diffractive element  600  has a hermetic seal  534  where layer  630  extends past the edge of region  322  onto the edge  514  of substrate  610 . Near cut edge  540 , a portion  632  of layer  630  attaches to the top surface  612  of substrate  610  to provide a hermetic seal for contoured region  322  at the cut edge  540 . The size of the extension of substrate  610  beyond region  322  and the resulting hermetic seal at edge  540  depends on the width of saw streets  324  of FIG. 5, the width of the saw blade, and the accuracy of the cutting process.  
         [0035]    Diffractive element  700  of FIG. 7 includes a substrate  710  and a reflective layer  730  respectively cut from the center of substrate  310  and layer  530 . FIG. 7 shows two cut edges  542  and  544 , and diffractive element  700  may only have cut edges. At each cut edge  542  and  544 , a portion  732  of reflective layer  730  adheres to a top surface  712  of substrate  710  and forms a hermetic seal, where the top surface  712  extends beyond region  322 .  
         [0036]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.