Stress tuned blazed grating light valve

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.

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.

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.

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.

What is needed is a blazed diffractive light modulator which provides higher contrast.

What is needed is a blazed diffractive light modulator which is easier to fabricate.

What is needed is a blazed diffractive light modulator which is more economical to fabricate.

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.

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.

FIG. 1 illustrates an isometric view of the preferred blazed grating light valve of the present invention.

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.

FIG. 2B further illustrates the single elongated element and the underlying substrate of the preferred blazed grating light valve of the present invention.

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.

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.

FIGS. 5A, 5 B, 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.

FIGS. 6A, 6 B, 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.

FIGS. 7A, 7 B, 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.

FIGS. 8A, 8 B, 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.

FIGS. 9A, 9 B, and 9 C illustrate a plan view and two orthogonal views, respectively, of a fabricated blazed grating light valve of the present invention.

FIG. 9D illustrates a cross-section of one of the fabricated elongated elements through the fabricated central blaze portion of FIGS. 9A, 9 B, and 9 C.

FIG. 10 illustrates an alternative elongated element and the underlying substrate of the present invention.

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.

One of the elongated elements 24 and a portion of the substrate 22 are further illustrated in FIG. 2 A. 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 .

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 .

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).

The elongated element 24 and the substrate 22 are further illustrated in FIG. 2 B.

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.)

A first cross-sectional view of the preferred blazed grating light valve 20 of the present invention is illustrated in FIG. 3 A. 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)).

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.

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.

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 .

A second cross-sectional view of the preferred blazed grating light valve 20 of the present invention is illustrated in FIG. 3 B. 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.

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.

A third cross-sectional view of the preferred blazed grating light valve 20 of the present invention is illustrated in FIG. 4 A. 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 .

A fourth cross-sectional view of the preferred blazed grating light valve 20 of the present invention is illustrated in FIG. 4 B. 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 .

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.

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.

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.

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.

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.

A first partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 5A, 5 B, 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.

A second partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 6A, 6 B, 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, 5 B, 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. 2 B). 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. 2 B).

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 .

A third partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 7A, 7 B, 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, 6 B, 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.

A fourth partially fabricated blazed grating light valve of the present invention is illustrated in FIGS. 8A, 8 B, 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, 7 B, 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 .

A fabricated blazed grating light valve of the present invention is illustrated in FIGS. 9A, 9 B, and 9 C. Fabrication of the fabricated blazed grating light valve 110 begins with fourth partially fabricated blazed grating light valve 106 (FIGS. 8A, 8 B, 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.

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. 9 B), 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.

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.

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.

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.