Reflection type liquid crystal device, manufacturing method therefor, and projection display system

An active matrix reflective liquid crystal device includes an array substrate which includes switching elements corresponding to pixels and an array of pixel electrodes connected to the switching elements; and an opposing substrate which has a transparent electrode opposite the array of pixel electrodes with a liquid crystal layer inserted therebetween. Each of the pixel electrodes includes an array of electrode studs, (e.g., divided electrode elements). The regions between the electrodes are filled with an insulating material, and the surface of the stud array is planarized by chemical-mechanical polishing (CMP). A dielectric light reflective film is formed on the planarized surface of the stud array, and a liquid crystal molecule alignment film is deposited thereon. Thus, a reflection type liquid crystal device is provided having a planarized light reflective face for a reduced reflection loss, along with a method for manufacturing the same, and a projection display system using such a reflection type liquid crystal device.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a reflection type liquid crystal device
 suitable for use as a light valve or an optical modulation device for a
 projection display system and to a manufacturing method therefor, and to a
 projection display system employing such a liquid crystal device.
 2. Description of the Related Art
 Recently, a type of projection display that uses an active matrix type
 reflective liquid crystal device as a light valve or as an optical
 modulation device has been developed for use in a high image quality
 display. Such a projection display is disclosed, for example, in Japanese
 Unexamined Patent Publication (Patent Kokai) Nos. 08-248425, 07-209621 and
 08-328034.
 In a projection display using a reflective liquid crystal device, light
 emitted by a light source is separated into the three primary colors: red,
 blue and green. The light elements of the respective colors are
 transmitted to corresponding reflective liquid crystal devices where they
 are optically modulated. The colored light elements reflected by the
 liquid crystal devices are recombined and the resultant light is magnified
 by an optical lens system and is projected onto a screen to thereby
 display a color image.
 An active matrix type reflective liquid crystal device has an array
 substrate and an opposing substrate disposed opposite the array substrate
 at a predetermined space. The array substrate includes switching elements
 which are field-effect transistors (FETs), provided in a matrix
 correspondingly to pixels, pixel electrodes connected to the switching
 elements and arranged as a matrix, and storage capacitors connected to the
 pixel electrodes for holding the charge on the electrodes. A transparent
 electrode is provided on the opposing substrate. A liquid crystal layer is
 inserted into the space between the array substrate and the opposing
 substrate.
 Light is incident on the side of the opposing substrate, and the liquid
 crystal selectively changes the polarization state of the incident light
 in response to a voltage applied to the pixel electrodes. The incident
 light is reflected by the pixel electrodes that also serve as light
 reflectors, the reflected light passing through the transparent electrode
 and emerging from the liquid crystal device.
 FIGS. 1 and 2 show a conventional active matrix type reflective liquid
 crystal device that is similar to that shown in FIGS. 1 and 2 of the
 above-mentioned Japanese Unexamined Patent Publication (Patent Kokai) No.
 08-248425.
 The array substrate includes a silicon substrate 1, on which field-effect
 transistors (FETs), are provided in a plurality of regions defined by
 field oxide isolation regions 12. The FETs, which are provided
 correspondingly to pixels, each include a gate insulating film 2 made, for
 example, of silicon dioxide, a gate electrode 4 made, for example, of
 polysilicon, a drain region 6, a source region 8, and a channel region 10
 extending between the drain 6 and the source 8.
 A storage capacitor 16 for holding a charge is formed on silicon dioxide
 film 14. The storage capacitor 16 includes two polysilicon layers that
 serve as capacitor electrodes and a dielectric layer made of silicon
 dioxide film that is sandwiched between the polysilicon layers. A silicon
 dioxide film 18 is formed to cover the silicon dioxide film 14 and the
 capacitor 16. A drain electrode 20 and a source electrode 22 made, for
 example, of aluminum are formed in openings in the silicon dioxide films
 14 and 18. The drain electrode 20 is connected to a data line 21 that
 extends perpendicular to the drawing in FIG. 1, and the gate electrode 4
 is connected to a gate line 5 (see FIG. 2) that is orthogonal to the data
 line 21.
 The source electrode 22 has an extension 23 which extends on the silicon
 dioxide film 18 to overlap the capacitor 16. The upper capacitor electrode
 is connected to the source electrode extension 23 by a via 19, made, for
 example, of tungsten, that passes through the silicon dioxide film 18.
 A silicon dioxide film 24 is deposited on the drain electrode 20 and the
 source electrode 22, and a light absorption layer 26 is formed thereon.
 The light absorption layer 26 is a composite layer including, for example,
 a titanium underlayer, an intermediate aluminum layer and a titanium
 nitride upper layer, and prevents undesirable light reflection and light
 transmission to the FETs.
 A through hole is formed in the light absorption layer 26 to permit the
 passage of a via 30. A silicon nitride film 28 that is substantially about
 4000 .ANG. to about 5000 .ANG. thick is formed on the light absorption
 layer 26. Pixel electrodes 32 made of aluminum and having a thickness of
 1500 .ANG. are formed on the film 28 at a space or gap of approximately 1
 .mu.m. The pixel electrodes 32 are also used as light reflectors
 (reflection mirrors), and the array of the pixel electrodes 32 forms a
 light reflective plane.
 The via 30, made of tungsten, is formed so that it passes through the
 silicon dioxide film 24, the light absorption layer 26 and the silicon
 nitride film 28. The FET source electrode 22 and the pixel electrode 32
 are connected to each other by the via 30. A liquid crystal molecule
 alignment film 33 is formed on the array of the pixel electrodes 32.
 As shown in FIG. 2, the pixel electrodes 32 are arranged as a matrix to
 correspond to the individual pixels. Spacers 34, which are shaped like
 pillars about 2 to 3 .mu.m tall and are made, for example, of silicon
 dioxide, are provided at selected positions between the pixel electrodes
 32.
 Disposed on the pillar-shaped spacers 34 is an opposing substrate
 comprising a glass substrate 40 on which is located a transparent opposing
 electrode or common electrode 38 coated with a liquid crystal molecule
 alignment film 37. The transparent electrode is made, for example, of ITO
 (indium tin oxide). A liquid crystal layer 36 is inserted between the
 array substrate and the opposing substrate.
 The aluminum pixel electrodes 32 are formed by depositing aluminum on the
 entire surface of the silicon dioxide film 28, and by etching the aluminum
 layer using a photolithographic process. Then, a liquid crystal molecule
 alignment film, such as polyimide film, is formed to cover the array of
 the pixel electrodes 32, and rubbing (e.g., polishing) of the alignment
 film is performed.
 FIG. 3 is an enlarged cross-sectional view of portion A enclosed by the
 circle shown in FIG. 2. As shown in FIG. 3, a height difference (e.g., a
 groove) exists between the pixel electrodes 32. Since silicon nitride film
 is not fully resistant to RIE (Reactive Ion Etching), which is used for
 aluminum etching, the silicon nitride film is also more or less etched
 during the aluminum etching.
 As a result, a groove or a height difference equal to the sum of the
 thickness of the pixel electrode 32 and the depth of the cut in the etched
 silicon nitride film is formed in the area between the pixel electrodes
 32.
 In the above-described conventional structure, there are several problems.
 First, light incident on the edges of the pixel electrodes is scattered.
 The scattered light does not effectively act as light that constitutes a
 pixel, and causes a loss in reflected light.
 In addition, the regions between the pixel electrodes do not act as an
 effective light reflector. When adjacent pixel electrodes are driven
 simultaneously, an electric field similar to that applied across the
 liquid crystal on the pixel electrodes is also applied across the liquid
 crystal between the pixel electrodes, and the liquid crystal between the
 pixel electrodes behaves optically in a similar manner to the liquid
 crystal on the pixel electrodes.
 That is, the liquid crystal regions on the adjacent pixel electrodes and
 the liquid crystal regions between the adjacent pixel electrodes behave as
 if they are continuous. Therefore, if the light reflector were continuous,
 the reflected light from the regions between the pixel electrodes could
 contribute to an increase in the light output. However, since the area
 between the pixel electrodes cannot act as an effective light reflector,
 the efficiency of light utilization is reduced.
 Further, since the light reflective plane provided by the array of the
 pixel electrodes is not planar, the planarity of the alignment film
 deposited over the pixel electrodes is deteriorated. As a result, the
 alignment film is not evenly polished, which may cause poor liquid crystal
 alignment.
 A possible method for resolving the above problems is to fill the gap
 regions between pixel electrodes with an insulating material, and
 planarize the surface by chemical-mechanical polishing (CMP) process.
 However, when the CMP process is used, the center portions of the
 relatively large pixel electrodes become recessed like a dish (e.g.
 "dishing"), and the reflectivity is reduced. Therefore, this is not a
 preferable method. Further, even if a dielectric light reflective film is
 formed to cover the array of the pixel electrodes, it is difficult to
 obtain a completely planar surface.
 SUMMARY OF THE INVENTION
 In view of the foregoing and other problems of the conventional systems and
 methods, an object of the present invention is to provide a reflection
 type liquid crystal device whose light reflective face is planar and does
 not include optical discontinuity and which causes less reflected light
 loss.
 It is another object of the present invention to provide a method for
 manufacturing such a reflection type liquid crystal device.
 It is an additional object of the present invention to provide a projection
 display system that employs such a reflection type liquid crystal device
 as a light valve.
 In a first aspect of the invention a reflection type liquid crystal display
 according to the present invention is formed so that each pixel electrode
 includes an array of small electrode studs, i.e., an array of divided
 electrode elements. The studs have substantially uniform dimensions and
 are arranged on the pixel electrode at uniform spacing. The regions
 between the studs are filled with an insulating material so that the
 surface of the insulating material is substantially flush with the top
 surfaces of the studs. Chemical-mechanical polishing (CMP) process as used
 in a semiconductor damascene process is utilized to form the stud array
 having a substantially completely planar surface. A dielectric light
 reflective film is formed on the planar surface provided by the studs and
 the insulating material.
 In other aspects, the present invention provides a method for forming such
 a reflection type liquid crystal device and a projection display system
 using such a reflection type liquid crystal device.
 According to the present invention, the reflective film and the alignment
 film deposited thereon are substantially quite planar, and there is
 substantially no optical discontinuity between pixel electrodes.
 Therefore, the above-described light scattering at the edges of the pixel
 electrodes can be prevented, and the reflection of light from the regions
 between the pixel electrodes can be effectively used.
 In addition, poor liquid crystal alignment due to uneven rubbing
 (polishing) can be prevented. As a result, the reflection efficiency and
 the quality of a displayed image can be enhanced.
 Further, since the reflective film covers the entire surface, the light
 absorption layer may be omitted when the transmission of light through the
 reflective film is negligible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
 With reference to FIGS. 4 to 6D, a reflective liquid crystal device and a
 manufacturing method therefor will now be described. It should be noted
 that the components in the drawings are not shown to scale. A feature of
 the liquid crystal device of the present invention resides in the
 structure of the pixel electrodes. The structure of other portions of the
 device may be similar to that for the conventional liquid crystal device
 described with reference to FIG. 1.
 According to the present invention, as shown in FIG. 4, each of the pixel
 electrodes 32 is formed to have an array of small pillar-shaped electrode
 studs, i.e., divided electrode elements 68. In this example, the studs 68
 of each pixel electrode are arranged in an 8.times.8 array. The studs 68,
 all of which have substantially uniform dimensions or size, are located at
 substantially uniform spacing.
 In this example, 20 .mu.m.times.20 .mu.m pixel electrodes 32 are arranged
 at spaces of about 0.8 .mu.m. In this case, for example, the top surface
 of each of the studs 68 is about 1.5 .mu.m.times.1.5 .mu.m and its height
 is about 0.6 .mu.m, and the space between the studs 68 is about 1.0 .mu.m.
 For example, the pixel electrodes 32 are made of aluminum and the
 electrode studs 68 are made of tungsten, and the regions between the studs
 68 are filled with silicon dioxide 64. The surfaces of the studs 68 and
 the silicon dioxide 64 are planarized by CMP.
 FIGS. 5A-5G show a process according to the present invention for forming
 an array of metal electrode studs 68 on each of the pixel electrodes 32.
 In FIG. 5(A), a light absorption layer 26, a silicon nitride layer 28 and
 interconnecting studs 30 correspond to those in FIG. 1. The process steps
 to the point where the pixel electrodes 32 are formed are similar to those
 in the conventional process. The pixel electrodes 32 are made, for
 example, of aluminum about 1500 .ANG. thick.
 After an aluminum layer is deposited and etched by RIE to form an array of
 the pixel electrodes 32, a silicon dioxide layer 60 is deposited on the
 pixel electrodes 32 by a CVD process, as shown in FIG. 5(B). The silicon
 dioxide layer 60 is deposited so that it completely fills the grooves or
 recessed portions between the pixel electrodes 32 and so that the
 thickness of portions above the surface level of the pixel electrodes 32,
 (i.e., the thickness as measured from the surface level of the pixel
 electrodes) is equal to or greater than a desired height for the studs 68.
 Then, as shown in FIG. 5(C), the silicon dioxide layer 60 is planarized by
 CMP so that the thickness of the portions above the surface level of the
 pixel electrodes 32 substantially equals the desired height for the studs
 68. CMP can be performed by using a basic polishing slurry including
 silica particles and a potassium hydroxide solution (a pH of approximately
 11 to 11.5).
 Then, as shown in FIG. 5(D), an array of openings 62 is formed in the
 silicon dioxide layer 60 by photolithographic masking and RIE etching. The
 openings 62 are provided at positions where the studs 68 are to be formed.
 Other than the portions at the openings 62, the silicon dioxide layer 64
 is not removed.
 Next, as shown in FIG. 5(E), a tungsten layer 66 that is thick enough to
 fill the openings 62 is deposited by a CVD process. In this case,
 preferably an underlayer of titanium (Ti), a barrier layer of titanium
 nitride (TiN) and the tungsten layer 66 are formed in this order. The
 thickness of the underlayer is, for example, about 1600 .ANG., the
 thickness of the barrier layer is about 40 .ANG., and the thickness of the
 tungsten layer 66 is about 1.1 .mu.m. The underlayer provides an improved
 adhesion to the silicon dioxide layer 64, and forms ohmic contact to the
 aluminum layer 32. The barrier layer protects the titanium and the
 aluminum layers from WF6 (a material gas) and HF (a generated gas) while
 the CVD process is being performed to deposit the tungsten layer 66, and
 provides enhanced adhesion to the tungsten layer 66.
 Then, as shown in FIG. 5(F), the tungsten layer 66 is planarized by CMP. As
 a result, an array of the studs 68 is formed that is connected to the
 pixel electrodes 32 and that has a surface flush with the surface of the
 silicon dioxide layer 64. Acid polishing slurry that contains alumina
 particles and ferric nitrate solution (a pH of approximately 3 to 4), for
 example, can be used for the CMP of tungsten. The regions between the
 studs 68 are substantially completely filled with silicon dioxide 64.
 The height of the studs 68 may be arbitrary so long as a substantially
 planar surface can be obtained, and may be about 0.3 to 2 .mu.m. In this
 example, the height of the studs 68 is about 0.6 .mu.m. As the number of
 the studs is reduced, the distribution of electric fields across the
 liquid crystal on the pixel electrodes tends to become uneven. As the area
 of the top surfaces of studs 68 grows larger and the number of the studs
 68 is increased, dents (dishing) tend to occur in the top surfaces of the
 studs 68 during CMP. Generally, it is preferable that the area of the top
 surface of a stud 68 be equal to or smaller than about 4 .mu.m.sup.2 (2.0
 .mu.m.times.2.0 .mu.m), and that a stud density given by a ratio of the
 sum of the areas of the top surfaces of the studs for one pixel electrode
 to the area of the top surface of the pixel electrode is about 50% or
 less.
 Thereafter, as shown in FIG. 5(G), a dielectric light reflective film 70 is
 formed on the planarized stud array. Any well known material can be used
 for the dielectric reflective film 70. In this example, a silicon dioxide
 layer (a refractive index of 1.465) and a silicon nitride layer (a
 refractive index of 1.983) are alternately laminated to form a six-layer
 reflective film. The thickness of the first layer of silicon dioxide is
 about 80.25 .ANG., the thickness of the second layer of silicon nitride is
 about 64.62 .ANG., the thickness of the third layer of silicon dioxide is
 about 94.73 .ANG., the thickness of the fourth layer of silicon nitride is
 about 61.74 .ANG., the thickness of the fifth layer of silicon dioxide is
 about 20.63 .ANG., and the thickness of the sixth layer of silicon nitride
 is about 32.66 .ANG.. The thicknesses of these layers are based on
 theoretical values. In actuality, during the manufacturing process,
 thickness errors may occur within a range of several percent but an
 adequate reflection function still can be provided.
 Then, a liquid crystal molecule alignment layer (not shown), such as a
 polyimide layer, is formed on the dielectric light reflective film 70
 according to well known processes, and rubbing (polishing) for liquid
 crystal molecule alignment is performed.
 FIGS. 6A-6D show an alternative method for forming an array of studs on the
 pixel electrode. In this example, etching is used to form electrode studs
 that are integrally formed with the pixel electrode. Aluminum, for
 example, is used to make thick pixel electrodes 32', as shown in FIG.
 6(A).
 To form studs 68' as shown in FIG. 6(B), the pixel electrodes 32' are
 etched using photolithographic masking and RIE etching until the cuts
 reach a depth corresponding to the height specified for studs 68'.
 Then, in step 6(C) a CVD process is used to deposit a silicon dioxide layer
 74 on the pixel electrodes 32' that completely fills the regions between
 the pixel electrodes and between the studs. Finally, in step 6(D) the
 silicon dioxide layer 74 is polished by CMP and planarized by using a
 basic polishing slurry including silica particles and a potassium
 hydroxide solution (a pH of approximately 11 to 11.5). In this fashion, a
 stud array with a planarized top surface can be obtained in which regions
 between the studs 68' are filled with a silicon dioxide 64' with its top
 surface being flush with the top surfaces of the studs 68'.
 An explanation and illustration have been given in FIGS. 5A-5G for the
 liquid crystal device that includes the light absorption layer 26.
 According to the present invention, however, the dielectric light
 reflective film 70 covers the entire surface. Thus, if light transmission
 through the reflective layer 70 is of negligible order, then the light
 absorption layer 26 can be eliminated, and the manufacturing process and
 the structure can be simplified.
 FIG. 7 shows a projection display system that uses the reflection type
 liquid crystal device according to the present invention as a light valve
 or as a light modulation device.
 As shown, linearly polarized light emitted from a light source 42 is
 reflected by a polarization beam splitter 44 and is transmitted to a
 dichroic prism 46. The beam splitter 44 transmits or reflects incoming
 light in accordance with its polarization direction.
 The light is separated by the prism 46 into the three primary colors: red,
 blue and green. The colored light elements are guided to corresponding
 reflective liquid crystal devices 48, 50 and 52, which selectively perform
 light modulation and rotate the polarization directions of the respective
 light elements. The liquid crystal, for example, is a birefringent TN
 liquid crystal having a twist angle of about 45.degree.. The polarization
 direction is rotated up to 90.degree. depending upon a voltage across the
 liquid crystal. The three light elements from the liquid crystal devices
 are recombined by the dichroic prism 46, and the obtained light is input
 to the polarizing beam splitter 44. The light is transmitted through the
 polarizing beam splitter 44 in accordance with rotational angle, and is
 projected in enlarged form onto a screen 58 by a projection lens system
 54.
 In FIG. 7, the dichroic prism 46 is employed as means for separating light
 into three primary colors and for recombining the separated light
 elements. However, obviously the dichroic prism 46 could be replaced by
 other well known means, such as dichroic mirrors.
 While the invention has been described in terms of several preferred
 embodiments, those skilled in the art will recognize that the invention
 can be practiced with modification within the spirit and scope of the
 appended claims.
 For example, while aluminum is used for pixel electrodes in the above
 embodiments, other metals, such as copper or tungsten, could be used.
 Further, while tungsten is used for the studs in the embodiment shown in
 FIGS. 4 and 5A-5G, other metals, such as copper, could also be used.
 Furthermore, the present invention will be applicable to any reflective
 liquid crystal device with a light reflection member built therein.