Abstract:
A light modulator includes elongated elements arranged parallel to each other and suspended above a substrate. The light modulator operates in a first diffraction mode and in a second diffraction mode. In the first diffraction mode, an incident light diffracts into at least two diffraction orders. In the second diffraction mode, the incident light diffracts into a single diffraction order. Each of the elongated elements comprises a central blazed portion, a first outer blaze transition, and a second outer blaze transition. The central blaze portion couples the first outer blaze transition to the second outer blaze transition. Each of the central blazed portions comprises a reflective surface. Selected ones of the central blazed portions comprise a first conductive element. The first outer blaze transition and the second outer blaze transition are coupled to the substrate. The substrate comprises a second conductive element. The elongated elements produce the first diffraction when a first electrical bias, preferably a zero electrical bias, is applied between the first conductive elements of the selected ones of the elongated elements and the second conductive element. A relative height of the blazed portions are adjusted to produce the second diffraction when a second electrical bias is applied between the first conductive elements of the selected ones of the elongated elements and the second conductive element.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of light modulators. More particularly, this invention relates to the field of light modulators where an incident light is modulated to produce a blazed diffraction.  
         BACKGROUND OF THE INVENTION  
         [0002]    Bloom et al. in U.S. Pat. No. 5,311,360, entitled “Method and apparatus for modulating a light beam,” teach a grating light valve which operates in a reflection mode and a diffraction mode. The grating light valve includes elongated elements suspended above a substrate. In the reflective mode, reflective surfaces of the grating light valve cause incident light to constructively combine to form reflected light. In the diffractive mode, the reflective surfaces of the grating light valve are separated by a quarter wavelength of the incident light to produce diffracted light. When the grating light valve is in the diffractive mode, the grating light valve predominantly diffracts light into a plus one diffraction order and a minus one diffraction order but also diffracts a small amount of light into higher diffraction orders.  
           [0003]    Bloom et al. further teach an alternative grating light valve which operates in the reflection mode and in a blazed diffraction mode. The alternative grating light valve includes the elongated elements suspended above the substrate. For the alternative grating light valve, the elongated elements include off-axis neck portions at ends of each of the elongated elements. In the reflection mode, the elongated elements are parallel causing incident light to reflect from the elongated elements and, thus, produce the reflected light. In the blazed diffraction mode, each of the elongated elements is rotated about an axis defined by the off-axis neck portions to produce a blazed diffraction.  
           [0004]    Because the light modulator is switched between the reflection mode and the blazed diffraction mode and because the reflection mode diffracts small quantities of light into the same angles as does the blazed diffraction mode, a contrast between the nonactivated state and the activated state is less than an optimum contrast. Further, the off-axis neck portions are critical to operation of the light modulator which necessitate tight tolerances for the off-axis neck portions making the light modulator relatively difficult to fabricate and also relatively expensive to fabricate.  
           [0005]    What is needed is a blazed diffractive light modulator which provides higher contrast.  
           [0006]    What is needed is a blazed diffractive light modulator which is easier to fabricate.  
           [0007]    What is needed is a blazed diffractive light modulator which is more economical to fabricate.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is a light modulator. The light modulator includes elongated elements arranged parallel to each other and suspended above a substrate. The light modulator operates in a first diffraction mode and in a second diffraction mode. In the first diffraction mode, an incident light diffracts into at least two diffraction orders. In the second diffraction mode, the incident light diffracts into a single diffraction order, which is at a diffraction angle different from diffraction angles for the at least two diffraction orders.  
           [0009]    Each of the elongated elements comprises a central blazed portion, a first outer blaze transition, and a second outer blaze transition. The central blaze portion couples the first outer blaze transition to the second outer blaze transition. Each of the central blazed portions comprises a reflective surface. Selected ones of the central blazed portions comprise a first conductive element. The first outer blaze transition and the second outer blaze transition are coupled to the substrate. The substrate comprises a second conductive element.  
           [0010]    The elongated elements produce the first diffraction when a first electrical bias, preferably a zero electrical bias, is applied between the first conductive elements of the selected ones of the elongated elements and the second conductive element. A relative height of the blazed portions are adjusted to produce the second diffraction when a second electrical bias is applied between the first conductive elements of the selected ones of the elongated elements and the second conductive element.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 illustrates an isometric view of the preferred blazed grating light valve of the present invention.  
         [0012]    [0012]FIG. 2A illustrates an isometric view of a single elongated element and an underlying substrate of the preferred blazed grating light valve of the present invention.  
         [0013]    [0013]FIG. 2B further illustrates the single elongated element and the underlying substrate of the preferred blazed grating light valve of the present invention.  
         [0014]    [0014]FIGS. 3A and 3B illustrate a cross-sectional view of the preferred blazed grating light valve of the present invention in a non-activated state and in a fully activated state, respectively, with incident light at normal incidence.  
         [0015]    [0015]FIGS. 4A and 4B illustrate a cross-sectional view of the preferred blazed grating light valve of the present invention in the non-activated state and in the fully activated state, respectively, with incident light at an oblique incidence.  
         [0016]    [0016]FIGS. 5A, 5B, and  5 C illustrate a plan view and two orthogonal views, respectively, of a first partially fabricated blazed grating light valve of the present invention.  
         [0017]    [0017]FIGS. 6A, 6B, and  6 C illustrate a plan view and two orthogonal views, respectively, of a second partially fabricated blazed grating light valve of the present invention.  
         [0018]    [0018]FIGS. 7A, 7B, and  7 C illustrate a plan view and two orthogonal views, respectively, of a third partially fabricated blazed grating light valve of the present invention.  
         [0019]    [0019]FIGS. 8A, 8B, and  8 C illustrate a plan view and two orthogonal views, respectively, of a fourth partially fabricated blazed grating light valve of the present invention.  
         [0020]    [0020]FIGS. 9A, 9B, and  9 C illustrate a plan view and two orthogonal views, respectively, of a fabricated blazed grating light valve of the present invention.  
         [0021]    [0021]FIG. 10 illustrates an alternative elongated element and the underlying substrate of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    The preferred blazed grating light valve is illustrated in FIG. 1. The preferred blazed grating light valve  20  includes a substrate  22 , elongated elements  24 , first posts  26  (one shown), and second posts  28  (one shown). The substrate  22  includes a first conductor  30 . The elongated elements  24  each include a central blazed portion  32 , a first outer blaze transition  34 , and a second outer blaze transition  36 . One of the first posts  26  and one of the second posts  28  couple each of the elongated elements  24  to the substrate  22 . Each of the elongated elements  24  are also preferably coupled at first and second ends (not shown) to the substrate  22 . Each of the central blazed portions  32  comprise a reflective surface  38 , which is preferably conductive.  
         [0023]    One of the elongated elements  24  and a portion of the substrate  22  are further illustrated in FIG. 2A. The elongated element  24  includes the first outer blaze transition  34 , the central blazed portion  32 , the second outer blaze transition  36 , and the reflective surface  38 . Preferably, the first outer blaze transition  34 , the central blazed portion  32 , and the second outer blaze transition  36  are each about a third of the length of the elongated element  24  between the first and second posts,  26  and  28 . The elongated element  24  is coupled to the substrate by the first and second posts,  26  and  28 .  
         [0024]    Preferably, the elongated elements  24 , and the first and second posts,  26  and  28 , comprise silicon nitride. Preferably, the reflective surface  38  comprises aluminum. Alternatively, the reflective surface  38  comprises a different metal. Further alternatively, the reflective surface  38  comprises a multi-layered dielectric mirror. The substrate  22  includes the first conductor  30 . Preferably, the substrate  22  comprises silicon. Alternatively, the substrate  22  comprises a different semiconductor material or an insulating material. Preferably, the first conductor  30  comprises doped poly-silicon. Alternatively, the first conductor  30  comprises a metal. For a visible spectrum application, the elongated element  24  preferably has a width of about 4.25 μm and a length of about 200 μm between the first and second posts,  26  and  28 .  
         [0025]    The first and second outer blaze transitions,  34  and  36 , cause the central blazed portion  32  to tilt so that a near side  33  of the central blazed portion  32  tilts downward. The tilting of the central blazed portion  32  occurs when the elongated elements  24  are released from an underlying layer during fabrication. The tilting is caused by an internal tensile stress within the elongated element  24  and by rigidities of the elongated element  24  at a first transition  37  between the first outer blaze transition  34  and the central blazed portion and at a second transition  39  between the central blazed portion  32  and the second outer blazed transition  36 . The internal tensile stress is counteracted by first and second anchors (not shown).  
         [0026]    The elongated element  24  and the substrate  22  are further illustrated in FIG. 2B.  
         [0027]    The elongated element  24  preferably comprises a central portion  40  and first and second outer portions,  42  and  44 . The first outer portion  44  is preferably coupled to the substrate  22  at the first end  46  and the first post  26 . The second outer portion is preferably coupled to the substrate  22  at the second end  48  and the second post  28 . Preferably, the first and second outer portions,  42  and  44 , are also coupled to the substrate  22  by the first and second anchors,  29  and  31 , located proximate to the first and second ends,  46  and  48 , respectively. Preferably, the first and second anchors,  29  and  31 , have an oval cross-section with a long axis of the oval cross-section oriented parallel to a length of the elongated elements  24 . By orienting the long axes of the first and second anchors parallel to the length of the elongated elements  24 , the first and second anchors,  29  and  31 , are relatively stiff in a tension direction defined by the internal tensile stress within the elongated elements  24 . Preferably, lengths of the first and second outer portion,  42  and  44 , are about as long as the central portion  40 . Alternatively, the lengths of the first and second outer portion,  42  and  44 , are longer or shorter than the central portion  40 . The first and second outer portions,  42  and  44 , assure uniform fabrication of the elongated elements  24  and the first and second posts,  26  and  28 . (Note that due to small feature sizes in FIG. 2B the tilt of the central blazed portion  32  is not shown.)  
         [0028]    A first cross-sectional view of the preferred blazed grating light valve  20  of the present invention is illustrated in FIG. 3A. The first cross-sectional view  50  illustrates the preferred grating light valve  20  in a non-activated state. The central blazed portions  32  of the elongated elements  24  preferably comprise a rectangular body  52  and a metal reflector  54 . The rectangular body  52  preferably comprises silicon nitride and the metal reflector  54  preferably comprises aluminum. Each of the central blazed portions  32  is preferably at a blaze angle γ with respect to a grating plane  56 . Each of the central blazed portions  32  preferably has a height difference of a quarter wavelength λ/4 of an incident light I between a high edge of the central blazed portion  32  and a low edge of the central blazed portion  32 . The central portions  32  are preferably on a grating pitch A. The blaze angle γ is given by the expression: γ=arctan (λ/(4A)).  
         [0029]    In the non-activated state, there is preferably a zero electrical bias between the elongated elements  24  and the first conductor  30 . The incident light I of the wavelength λ illuminates the preferred blazed grating light valve  20  normal to the grating plane  56 . For discussion purposes, the diffraction orders are based on a second grating pitch  2 A, which is twice the grating pitch A.  
         [0030]    In the non-activated state, the incident light of the wavelength λ is diffracted into a zeroth diffraction order D 0  and a second diffraction order D 2 . The zeroth diffraction order D 0  is normal to the grating plane  56 . The second order diffraction D 2  is at a second order diffraction angle θ 2  given by the expression: θ 2 =arcsin (λ/A). For the preferred blazed grating light valve  20 , the second order diffraction angle θ 2  is less than about 10°. Thus, for the preferred blazed grating light valve  20 , the second order diffraction angle θ 2  is approximately four times the blaze angle γ.  
         [0031]    Neglecting a first light loss due to absorption by the metal reflectors  54  and a second light loss by the incident light I passing through gaps between adjacent pairs of the elongated elements  24 , half of the incident light I is diffracted into the zeroth diffraction order D 0  and half of the incident light I is diffracted into the second diffraction order D 2 .  
         [0032]    A second cross-sectional view of the preferred blazed grating light valve  20  of the present invention is illustrated in FIG. 3B. The second cross-sectional view  60  illustrates the preferred grating light valve  20  in an activated state. Preferably, to produce the activated state, alternate ones of the elongated elements  24  are moved toward the substrate  22  by applying an electrical bias between the first conductor  30  and the metal reflectors  54  of the alternate ones of the elongated elements  24 . In a fully activated state, the electrical bias moves the alternate ones of the elongated elements  24  by the quarter wavelength λ/4 of the incident light I. This results in pairs of the central blazed portions  32  forming a fully activated height difference of a half wavelength λ/2 of the incident light I while maintaining the blaze angle γ.  
         [0033]    In the fully activated state, the incident light I of the wavelength λ is diffracted into a first order diffraction D 1  having a first order angle θ 1 . The first order angle θ 1  is given by the expression: θ 1 =arcsin (λ/2A). For the preferred grating light valve  20  as described here, the first order angle θ 1  is approximately twice the blaze angle γ.  
         [0034]    A third cross-sectional view of the preferred blazed grating light valve  20  of the present invention is illustrated in FIG. 4A. The third cross-sectional view  70  illustrates the preferred blazed grating light valve  20  in the non-activated state with the incident light I at an oblique angle θ i  to the grating plane  56 . In the non-activated state, the incident light I is diffracted into an oblique zeroth order diffraction D 0   ′  and an oblique second order diffraction D 2   ′ . The oblique zeroth order diffraction D 0   ′  is at an oblique zeroth order angle θ 0   ′  with respect to the normal to the grating plane  62 , which is equal to the oblique angle θ i . The oblique zeroth order angle θ 0   ′  and oblique angle θ i  are given by the expression: θ 0   ′ =θ 1 =arcsin (λ/2A). The oblique second order diffraction D 2   ′  is at the oblique angle θ i .  
         [0035]    A fourth cross-sectional view of the preferred blazed grating light valve  20  of the present invention is illustrated in FIG. 4B. The fourth cross-sectional view  72  illustrates the preferred blazed grating light valve  20  in the activated state with the incident light I at the oblique angle θ i  to the grating plane  56 . In the fully activated state, the incident light I is diffracted into an oblique first order diffraction D 1   ′ , which is normal to the grating plane  56 .  
         [0036]    A first advantage of the preferred blazed grating light valve  20  is that the preferred blazed grating light valve  20  provides a blazed diffraction in the activated state while quickly switching between the non-activated state and the activated state. This is because the elongated elements are translated rather than rotated.  
         [0037]    A second advantage of the preferred blazed grating light valve  20  is that in the non-activated state none of the incident light I is diffracted into the first diffraction order D 1  for the normal incidence and none of the incident light I is diffracted into the oblique first order diffraction D 1   ′  for the oblique incidence. In a display application where the preferred blazed grating light valve  20  produces an array of pixels and where a bright pixel corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this provides a dark pixel of an image. In a telecommunications application, where the preferred blazed grating light valve  20  operates as a switch and where an on-state of the switch corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this provides an off-state for the switch.  
         [0038]    A third advantage of the preferred blazed grating light valve  20  is that, in the activated state, the incident light I is diffracted into a single diffraction order which is either the first diffraction order D 1  for the normal incidence or the oblique first order diffraction D 1   ′  for the oblique incidence. In the display application where the preferred blazed grating light valve  20  produces the array of pixels and where the bright pixel corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this simplifies display optics since only the single diffraction order is collected to produce the bright pixel. In the telecommunications application, where the preferred blazed grating light valve  20  operates as the switch and where the on-state of the switch corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this provides efficient utilization of the incident light I since the incident light I is diffracted into the single diffraction order.  
         [0039]    A fourth advantage of the preferred blazed grating light valve is that because, in the non-activated state, none of the incident light I is diffracted into either the first diffraction order D 1  for the normal incidence or the oblique first order diffraction D 1   ′  for the oblique incidence and because, in the activated state, the incident light I is diffracted into the single diffraction order, the preferred blazed grating light valve  20  provides a high contrast ratio between the non-activated state and the activated state. Typically, this contrast ratio is on an order of a thousand to one. In the display application where the preferred blazed grating light valve  20  produces the array of pixels and where the bright pixel corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this produces a high contrast image. In the telecommunications application, where the preferred blazed grating light valve  20  operates as the switch and where the on-state of the switch corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this produces a high discrimination between the on-state and the off-state.  
         [0040]    A fifth advantage of the preferred blazed grating light valve  20  is that, because the activated state diffracts the incident light I into the single diffraction order, a depth of focus of either the first diffraction order D 1  for the normal incidence or the oblique first order diffraction D 1   ′  for the oblique incidence is relatively long compared to a diffractive light modulator which diffracts useful light into multiple diffraction orders. In the display application where the preferred blazed grating light valve  20  produces the array of pixels and where the bright pixel corresponds to either the first diffraction order D 1  or the oblique first order diffraction D 1   ′ , this allows for simpler optics. In a printing application, which is a type of display application where the bright pixel is typically used to illuminate a cylindrical drum, the longer depth of focus provides a sharper printed image.  
         [0041]    A first partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 5A, 5B, and  5 C. Fabrication of the first partially fabricated grating light valve  80  begins with a silicon substrate  82 . Next, a field oxide layer  84  is formed on the silicon substrate  82  by preferably heating the silicon substrate in an oxygen atmosphere. Preferably, the field oxide layer has a thickness of about 1.0 μm. Following this, a conducting layer  86  is deposited on the field oxide layer  84 . Preferably, the conducting layer  86  has a thickness of about 0.35 μm and comprises doped poly-silicon deposited using an LPCVD (low pressure chemical vapor deposition) process. Subsequently, an etch stop  88  is formed on the conducting layer  86 . Preferably, the etch stop  88  comprises a second field oxide layer formed by heating the poly-silicon in the oxygen environment. Alternatively, the etch stop  88  comprises a deposited oxide or a deposited nitride. Preferably, the etch stop  88  has a thickness of about 200 Å. Next, a sacrificial layer  90  is deposited on the etch stop  88 . Preferably, the sacrificial layer  90  comprises poly-silicon deposited using the LPCVD process. Preferably, the sacrificial layer  90  has a thickness about 1.0 μm. Alternatively, the sacrificial layer has a thickness greater than or about equal to a wavelength λ of the incident light I.  
         [0042]    A second partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 6A, 6B, and  6 C. Fabrication of the second partially fabricated grating light valve  92  begins with the first partially fabricated blazed grating light valve  80  (FIGS. 5A, 5B, and  5 C). Fabrication of the second partially fabricated grating light valve  92  comprises first and second etching steps using photolithography and a semiconductor etching technique, such as plasma etching. The first etching step etches shallow stress inducing features  96  into the sacrificial layer  90 . The second etching step etches post holes  94  into the sacrificial layer  90  and also etches anchor holes (not shown) into the sacrificial layer  90 . The anchor holes form the first and second anchors,  29  and  31  (FIG. 2B). The second etching step also etches sacrificial layer edges (not shown) where first and second ends,  38  and  40 , of each of the elongated elements  24  couple to the substrate  22  (FIG. 2B).  
         [0043]    It will be readily apparent to one skilled in the art that the semiconductor etching technique are likely to produce facets at exposed comers of the post holes  94  and at exposed comers of the stress inducing features  96 .  
         [0044]    A third partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 7A, 7B, and  7 C. Fabrication of the third partially fabricated blazed grating light valve  100  begins with the second partially fabricated blazed grating light valve  92  (FIGS. 6A, 6B, and  6 C). Fabrication of the third partially fabricated blazed grating light valve  100  comprises depositing a resilient material  102  on the second partially fabricated grating light valve  92  and then depositing a metal  104  on the resilient material  102 . Preferably, the resilient material  102  comprises silicon nitride. Preferably, the resilient material  102  coats surfaces of the post holes  94  and the anchor holes of the second partially fabricated grating light valve  92 . Alternatively, the resilient material  102  more substantially fills the post holes  94  and the anchor holes. Further alternatively, the resilient material fills the post holes  94  and the anchor holes. (Note that FIGS. 7A and 7B depict the resilient material  102  filling the post holes  94  as a simplification for more easily understood illustrations.) Preferably, the resilient material  102  has a thickness of about 920 Å and is deposited using an LPCVD process. Preferably, the resilient material has an internal tensile stress of about 1 GPa. Preferably, the metal  104  comprises aluminum. Preferably, the metal  104  has a thickness of 500 Å. Preferably, the metal  104  is deposited using a physical vapor deposition technique, such as sputtering or evaporation.  
         [0045]    A fourth partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 8A, 8B, and  8 C. Fabrication of the fourth partially fabricated blazed grating light valve  106  begins with the third partially fabricated blazed grating light valve  100  (FIGS. 7A, 7B, and  7 C) and comprises etching the metal layer  104  and the resilient material  102  to form fabricated elongated elements  24 A supported by the sacrificial layer  90 .  
         [0046]    A fabricated blazed grating light valve of the present invention is illustrated in FIGS. 9A, 9B, and  9 C. Fabrication of the fabricated blazed grating light valve  110  begins with fourth partially fabricated blazed grating light valve  106  (FIGS. 8A, 8B, and  8 C) and comprises etching the sacrificial layer  90  to completion using a xenon difluoride etch. This produces fabricated elongated elements  24 A comprising fabricated central blazed portions  32 A coupled to first and second fabricated posts,  26 A and  28 A, by first and second fabricated blaze transitions,  34 A and  36 A. The first and second fabricated posts,  26 A and  28 A, couple the fabricated elongated elements  24 A to a fabricated substrate  22 A.  
         [0047]    [0047]FIG. 9D illustrates a cross-section of one of the fabricated elongated elements  24 A through the fabricated central blaze portion  32 A. The first and second fabricated blaze transitions,  34 A and  36 A (FIG. 9B), cause the fabricated cental blaze portion  32 A to orient to a desired blaze angle γ′ upon release of the fabricated elongated elements  24 A by the xenon difluoride etch.  
         [0048]    It will be readily apparent to one skilled in the art that suitable electrical connections for the fabricated blazed grating light valve  110  comprise bond pads, which are well known both in structure and fabrication. Further, it will be readily apparent to one skilled in the art that the fabricated blazed grating light valve  110  is a particular embodiment of the present invention and that accordingly the preferred blazed grating light valve  20  more generally describes the present invention.  
         [0049]    A first alternative elongated element and the underlying substrate  22  of the present invention are illustrated in FIG. 10. The first alternative elongated element  24 B comprises an alternative central blazed portion  32 A coupling first and second alternative blaze transitions,  34 A and  36 A. The first and second alternative blaze transitions comprise a symmetrical step at ends of the alternative central blazed portion  32 A causing the near side  33  of the alternative central blazed portion  32 A to tilt downward while causing a far side  35  of the alternative central blazed portion  32 A to tilt upward.  
         [0050]    It will be readily apparent to one skilled in the art that other various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined by the appended claims.