Border masking structures for liquid crystal display

A display may have a thin-film transistor layer and color filter layer. The display may have an active area and an inactive border area. Light blocking structures in the inactive area may prevent stray backlight from a backlight light guide plate from leaking out of the display. The thin-film transistor layer may have a clear substrate, a patterned black masking layer on the clear substrate, a clear planarization layer on the black masking layer, and a layer of thin-film transistor circuitry on the clear planarization layer. The black masking layer may be formed from black photoimageable polyimide. The clear planarization layer may be formed from spin-on glass. The light blocking structures may include a first layer formed from a portion of the black masking layer and a second layer such as a layer of black tape on the underside of the color filter layer.

BACKGROUND

Electronic devices often include displays. For example, cellular telephones, computers, and televisions have displays.

A display such as a liquid crystal display has an active area filled with an array of display pixels. The active area is surrounded by an inactive border area. It may be desirable to minimize or eliminate the use of unsightly bezel structures in the inactive border area. In displays with small bezels or no bezels, there is a risk that backlight can leak through the inactive border area. If care is not taken, stray backlight will undesirably lighten the inactive area.

It would therefore be desirable to be able to provide improved light blocking structures for inactive border regions in displays such as liquid crystal displays.

SUMMARY

An electronic device may be provided with a display such as a liquid crystal display. The liquid crystal display may have an upper polarizer and a lower polarizer. A layer of liquid crystal material may be interposed between a thin-film transistor layer and a color filter layer. The thin-film transistor layer may be interposed between the liquid crystal layer and the upper polarizer. The color filter layer may be interposed between the liquid crystal layer and the lower polarizer.

The thin-film transistor layer and color filter layer may have an associated array of display pixels that define an active area for the display. The display pixels of the active area may be used to display images for a user. An inactive border area in the display may run along the periphery of the active area. Light blocking structures in the inactive area may prevent stray backlight from a backlight light guide plate from leaking out of the display.

The thin-film transistor layer may have a clear substrate, a patterned black masking layer on the clear substrate, a clear planarization layer on the black masking layer, and a layer of thin-film transistor circuitry on the clear planarization layer. The black masking layer may be formed from black photoimageable polyimide. The clear planarization layer may be formed from spin-on glass. The light blocking structures may include a first layer formed from a portion of the black masking layer in the inactive area and may include a second layer such as a layer of black tape on the underside of the color filter layer adjacent to the lower polarizer.

DETAILED DESCRIPTION

Illustrative electronic devices of the types that may be provided with displays are shown inFIGS. 1, 2, 3, and 4.

Electronic device10ofFIG. 1has the shape of a laptop computer and has upper housing12A and lower housing12B with components such as keyboard16and touchpad18. Device10has hinge structures20(sometimes referred to as a clutch barrel) to allow upper housing12A to rotate in directions22about rotational axis24relative to lower housing12B. Display14is mounted in housing12A. Upper housing12A, which may sometimes be referred to as a display housing or lid, is placed in a closed position by rotating upper housing12A towards lower housing12B about rotational axis24.

FIG. 2shows an illustrative configuration for electronic device10based on a handheld device such as a cellular telephone, music player, gaming device, navigation unit, or other compact device. In this type of configuration for device10, housing12has opposing front and rear surfaces. Display14is mounted on a front face of housing12. Display14may have an exterior layer that includes openings for components such as button26and speaker port28. Device10may, if desired, be a compact device such as a wrist-mounted device or pendant device (as examples).

In the example ofFIG. 3, electronic device10is a tablet computer. In electronic device10ofFIG. 3, housing12has opposing planar front and rear surfaces. Display14is mounted on the front surface of housing12. As shown inFIG. 3, display14has an opening to accommodate button26.

FIG. 4shows an illustrative configuration for electronic device10in which device10is a computer display, a computer that has an integrated computer display, or a television. Display14is mounted on a front face of housing12. With this type of arrangement, housing12for device10may be mounted on a wall or may have an optional structure such as support stand30to support device10on a flat surface such as a table or desk.

Display14may be a liquid crystal display or a display formed using other suitable display technologies. A cross-sectional side view of an illustrative configuration for display14of device10(e.g., a liquid crystal display for the devices ofFIG. 1,FIG. 2,FIG. 3,FIG. 4or other suitable electronic devices) is shown inFIG. 5. As shown inFIG. 5, display14may include backlight structures such as backlight unit42for producing backlight44. During operation, backlight44travels outwards (vertically upwards in dimension Z in the orientation ofFIG. 5) and passes through display pixel structures in display layers46. This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight44may illuminate images on display layers46that are being viewed by viewer48in direction50.

Display layers46may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing12or display layers46may be mounted directly in housing12(e.g., by stacking display layers46into a recessed portion of housing12).

Display layers46may include a liquid crystal layer such a liquid crystal layer52. Liquid crystal layer52may be sandwiched between display layers such as display layers58and56. Layers56and58may be interposed between lower (innermost) polarizer layer60and upper (outermost) polarizer layer54.

Layers58and56may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers56and58may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers58and56(e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers58and56and/or touch sensor electrodes may be formed on other substrates.

With one illustrative configuration, outer substrate layer56may be a thin-film transistor layer that includes an array of thin-film transistors and associated electrodes (display pixel electrodes) for applying electric fields to liquid crystal layer52and thereby displaying images on display14. Inner substrate layer58may be a color filter layer that includes an array of color filter elements for providing display14with the ability to display color images.

Backlight structures42may include a light guide plate such as light guide plate78. Light guide plate78may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures42, a light source such as light source72may generate light74. Light source72may be, for example, an array of light-emitting diodes.

Light74from light source72may be coupled into edge surface76of light guide plate78and may be distributed in dimensions X and Y throughout light guide plate78due to the principal of total internal reflection. Light guide plate78may include light-scattering features such as pits or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate78.

Light74that scatters upwards in direction Z from light guide plate78may serve as backlight44for display14. Light74that scatters downwards may be reflected back in the upwards direction by reflector80. Reflector80may be formed from a reflective material such as a layer of white plastic or other shiny materials.

To enhance backlight performance for backlight structures42, backlight structures42may include optical films70. Optical films70may include diffuser layers for helping to homogenize backlight44and thereby reduce hotspots, compensation films for enhancing off-axis viewing, and brightness enhancement films (also sometimes referred to as turning films) for collimating backlight44. Optical films70may overlap the other structures in backlight unit42such as light guide plate78and reflector80. For example, if light guide plate78has a rectangular footprint in the X-Y plane ofFIG. 5, optical films70and reflector80may have a matching rectangular footprint.

Display14may have an array of display pixels (e.g., a rectangular array having rows and columns) for displaying images to a viewer. Vertical signal lines called data lines may be used to carry display data to respective columns of display pixels. Horizontal signal lines called gate lines may be used to carry gate line signals (sometimes referred to as gate control signals or gate signals) to respective rows of display pixels. The outline of the array of display pixels in display14defines an active area for display14. The active area may have a rectangular shape and may be surrounded by an inactive border region. An inactive border area may, for example, run along one edge, two edges, three edges, or all four edges of the active area.

A cross-sectional side view of an illustrative electronic device having a display such as display14ofFIG. 5is shown inFIG. 6. As shown inFIG. 6, images may be displayed on central active area AA of display14. Inactive area IA may have a rectangular ring shape that runs around the rectangular periphery of active area AA. To avoid unsightly bezel structures in device10, it may be desirable to keep inactive area IA free of overlapping housing structures, bezels, or other potentially unattractive border structures.

To avoid light leakage in inactive area IA (e.g., to prevent stray light from escaping in the absence of a bezel or other overlapping structure), display14may be provided with border masking structures in inactive area IA. The border masking structures may help block stray backlight from backlight unit42and thereby ensure that border IA does not allow excess light to escape. Backlight from backlight unit42will therefore be confined to active area AA.

To provide satisfactory light blocking capabilities in inactive area IA, light blocking structures can be formed in two parts (e.g., two layers). A first part of the light blocking structures may be formed from a black masking layer on the underside of thin-film transistor layer56. In active area AA, the black masking layer may be patterned to form a black mask. The black mask is a grid-shaped series of intersecting black lines that define a rectangular array of clear display pixel openings in the thin-film transistor layer. Each of the openings in the black mask is aligned with a respective color filter element in a corresponding array of color filter elements on color filter layer58. The grid-shaped black mask on the thin-film transistor layer may sometimes be referred to as a black matrix. In inactive area IA, the black mask may form the first part of the light blocking structures. The second part of the light blocking structures may be formed from opaque structures on the underside of color filter layer58such as a layer of black tape in inactive area IA.

FIG. 7is a cross-sectional side view of a portion of thin-film transistor layer56showing layers of structures that may be formed on thin-film transistor layer56. As shown inFIG. 7, thin-film transistor layer56may include a transparent thin-film transistor substrate such as substrate100. Substrate100may be formed from a clear planar structure such as a sheet of transparent plastic, transparent glass, or other clear substrate layer. Black masking layer102may be patterned to form a black matrix in active area AA of display14and may be patterned to form part of a light-blocking black mask border in inactive area IA. The portion of display14that is shown inFIG. 7corresponds to a display pixel in the array of display pixels in inactive area AA. As shown inFIG. 7, black masking layer102may be patterned to form display pixel openings such as opening104that are aligned with patterned display pixel electrodes110. Electrodes110may be separated from common electrode (Vcom) trace112by dielectric layer114. Clear overcoat layer116may be formed on top of thin-film transistor124from a photoimageable polymer or other dielectric. Patterned metal118may be used to form transistor terminals such as source S, drain D, and gate G. Gate insulator120may be formed from dielectric materials such as silicon nitride and/or silicon oxide and may separate gate G from semiconductor region122. Semiconductor region122, which is used in forming the channel region for thin-film transistor124, may be formed from semiconductor materials such as amorphous silicon, polysilicon, indium gallium zinc oxide, or other semiconductors. Passivation layer126may be formed on top of gate insulator120.

Black masking material102may be formed from a photoimageable material such as black photoresist. The black photoresist may be formed from a polymer such as polyimide. To withstand the elevated temperatures involved in subsequent thin-film transistor fabrication steps, the polymer that is used in forming black masking material102preferably can withstand elevated temperatures (e.g., temperatures of 350° C. or higher or other suitable elevated temperatures). Opaque filler materials such as carbon black and/or titanium black may be incorporated into the polyimide or other polymer of layer102, so that layer102is opaque and is able to block at least part of the stray light in inactive area IA.

Planarization layer106is used to planarize black masking layer102so that thin-film transistor circuitry such as transistor124can be formed on black masking layer102(i.e., so that thin-film transistors can overlap black mask102as shown inFIG. 7). With one suitable arrangement, planarization layer106is formed from a black mask compatible material having a low dielectric constant such as a spin-on glass. For example, planarization layer106may be formed from a spin-on glass such as a silicon oxide based spin-on glass (e.g., a silicate spin-on glass). During thin-film transistor formation, the structures ofFIG. 7may be subjected to elevated processing temperatures (e.g., temperatures of 350° C. or higher). Polyimide black mask layer102and spin-on glass planarization layer106are preferably able to withstand processing at these elevated temperatures (i.e., spin-on glass layer106will not experience diminished transparency and polyimide layer102will not degrade).

In some embodiments, a buffer layer such as inorganic buffer layer107may be formed at the interface between planarization layer106and TFT layers108. Buffer layer107may be a thin layer of silicon nitride, silicon oxide, and/or other inorganic materials having a thickness of 250-3000 angstroms (as an example). Formed in this way, inorganic buffer layer107may serve to prevent chemicals such as etching solution from being injected into spin-on glass planarization layer106during formation of the TFT circuitry in layers108.

It is desirable to limit the amount of opaque filler in material102, as too much filler material may cause the resistivity of layer102to drop to an undesirably low level, potentially interfering with satisfactory operation of the thin-film transistor circuitry formed on thin-film transistor layer56. When the amount of opaque filler is limited, the opacity for the black mask layer in inactive border IA will also be limited. The thickness T1 of black masking layer102can be increased somewhat to increase optical density (opacity) for layer102, but excessive thicknesses for black masking layer102should generally be avoided. If black masking layer102is too thick, it may be difficult to planarize black masking layer102satisfactorily. In addition, excessive thickness T2 in the associated planarization layer may create an undesired color cast in the active area of display14and/or may reduce light transmittance in the active area of display14. Excessive values for thicknesses T1 and T2 may also lead to cracking in layers102and/or106(e.g., cracks may develop due to imperfect matching between the coefficients of thermal expansion for the materials of layers102and106).

In view of these constraints, it may be desirable to limit the thickness T1 of black mask layer102to a small value (e.g., about 1.5 microns, less than 2 microns, 1-2 microns, less than 3 microns, or other suitable value). Thickness T2 may then be limited to a comparably small thickness value. For example, thickness T2 of planarization layer106may be about 3 microns, less than 5 microns, 2-5 microns, less than 4 microns, less than 3 microns, or other suitable value).

In configurations for display14in which thickness T1 of black masking layer102is relatively small and in which the amount of opaque filler in layer102is limited, the black mask border formed from black masking layer102in inactive area IA may not be sufficiently opaque to serve as the exclusive light blocking structure for the border of display14. Accordingly, one or more additional layers of light blocking structures may be formed in inactive area IA to supplement the masking function performed by black masking layer102. An illustrative configuration of this type is shown inFIG. 8.

As shown inFIG. 8, display14may have an active area AA (e.g., a central rectangular active area filled with display pixels) and may have an inactive area IA that runs along the periphery of active area AA. The left-hand edge of the inactive area border region IA is shown in the illustrative portion of display14that is depicted inFIG. 8.

Thin-film transistor layer56is located above color filter layer58. Thin-film transistor layer56includes substrate100, black masking layer102, spin-on glass planarization layer106, and thin-film transistor circuitry such as thin-film transistor circuitry layer108. Liquid crystal material52is interposed between thin-film transistor layer56and color filter layer58. Sealant136(e.g., a rectangular ring of epoxy or other adhesive that runs around the rectangular periphery of display14) may be used to seal liquid crystal material52within display14. Color filter layer58has a transparent substrate such as substrate130. Substrate130may be formed from a planar layer of clear glass, a transparent plastic layer, or other transparent substrate material. An array of color filter elements134may be formed on the surface of substrate130. Color filter elements134may include red color filter elements R, blue color filter elements B, and green color filter elements G. Color filter elements134may be formed from colored photoimageable polymers. A layer of opaque masking material such as black photoimageable polymer layer132may form a black matrix in active area AA. The black matrix may have a grid shape with an array of rectangular openings. A respective color filter element134may be formed in each opening in the black matrix formed from opaque masking layer132on color filter substrate130. Each color filter element134in the array of color filter elements on color filter layer58may be laterally aligned with a respective opening104in the array of openings in the black matrix formed from layer102on the inner surface of thin-film transistor substrate layer100(i.e., each display pixel in display14may have an opening104, an associated display pixel electrode in layer108, and an associated aligned color filter element134through which backlight44passes). As shown inFIG. 8, some of black masking layer132on substrate130may extend into inactive area IA and may help to block stray light from backlight42.

Additional light blocking in inactive area IA may be provided by light blocking structures on the lower (outermost) surface of color filter layer substrate130(i.e., on the lower surface of color filter layer58). As shown inFIG. 8, for example, opaque tape such as black tape138may be laminated to the lower surface of color filter layer substrate130in inactive area IA. Black tape138may have an opaque carrier such as carrier142and may have an adhesive layer such as adhesive layer140.

Opaque carrier142may be formed from a flexible polymer layer such as a layer of triacetate cellulose, a layer of acrylic, a layer of polyethyleneterephthalate (PET), a layer formed from one or more other polymers, a fabric carrier, a conductive fabric carrier (e.g., a fabric tape substrate formed from conductive fibers such as metal fibers or metal-coated polymer fibers, a combination of conductive fibers and non-conductive fibers, etc.), a tape carrier having both a solid polymer layer and fibers, or other suitable tape layer or layers that serve as a carrier for adhesive such as adhesive layer140. The materials of carrier142may be rendered opaque by incorporating opaque filler material (e.g., carbon black, titanium black, etc.) into the polymer materials of carrier142and/or may be rendered opaque by coating one or both surfaces of carrier142with an opaque material such as black ink. If desired, opaque material (e.g. carbon black, titanium black, etc.) may be incorporated into adhesive layer140(i.e., adhesive layer140may be formed from an opaque material such as black adhesive). Adhesive layer140may be a pressure sensitive adhesive or other adhesive and may be formed from a polymer such as acrylic or other suitable material. Adhesive layer140may, if desired, be formed using a conductive material.

With one suitable arrangement, black tape138may have an optical density of about 5.7 (e.g., 4 or more, 5 or more, 4-7, or other suitable optical density), may have a total thickness of about 0.045 mm (e.g., 0.03-0.07 mm, more than 0.02 mm, less than 0.1 mm, etc.), and may be formed from a conductive fabric carrier coated with a layer of black conductive acrylic adhesive. Conductive tape may be used to provide radio-frequency interference shielding and/or electrical grounding in addition to serving as light shielding. Tape138may be die cut to form a desired shape (e.g., a rectangular ring), may be formed in elongated strips, or may be otherwise shaped into a desired configuration for serving as an additional light blocking layer for inactive area IA of display14. Tape138may be applied manually and/or using computer-controlled tape dispensing equipment.

As shown inFIG. 8, backlight44from backlight unit42may pass through polarizer60and the other layers of display14to serve as backlight in active area AA. In inactive area IA, it is desirable to block stray backlight such as illustrative stray backlight ray44′ inFIG. 8. This is accomplished using at least two light blocking structures in inactive area IA: tape138and the black border formed by black masking layer102on thin-film transistor layer56. Layer132on color filter layer58may also assist in blocking stray light in inactive area IA.

Illustrative steps involved in forming a display such as display14ofFIG. 8are shown inFIG. 9. As shown inFIG. 9, during the fabrication of thin-film transistor layer56, black masking layer102may be patterned on the lower surface of thin-film transistor layer substrate100(e.g., using photolithography). In active area AA, patterned black masking layer structures102may form a grid shaped black matrix defining an array of display pixel openings104. In inactive area IA, black masking layer structures formed from layer102may form a black border layer that serves as a light blocking structure. At step202, spin-on glass planarization layer106may be deposited on top of layer102to planarize layer102(e.g., by spinning on layer106using spin deposition techniques or using other suitable deposition techniques such as spraying techniques). In general, any suitable polymer, glass, or other clear material may be used in forming polarization layer106. An advantage of using silicate based spin-on glass materials is that this type of material is compatible with dry etch processes used in patterning metal traces in thin-film transistor circuitry layer108.

At step204, display layers46(FIG. 5), films70, and backlight42may be assembled to form display14. In particular, liquid crystal layer52may be formed between color filter layer58and thin-film transistor layer56, polarizer layers54and60may be laminated to the upper and lower surfaces of display14, respectively, and other display assembly operations may be performed.

At step206, black tape138may be attached to the lower surface of color filter layer58in inactive area IA (i.e., tape138may be applied to the lower surface of substrate130adjacent to polarizer60). If desired, opaque masking structures such as black ink (e.g., polymer with black filler), metal tape, ink containing metal particles (i.e., metal ink), a layer of metal, other opaque materials, or combinations of two or more of these structures may be used in addition to or instead of black tape138.

At step208, device assembly operations may be completed and device10may be used to display images for a user. During operation, backlight structures42may produce backlight44. In active area AA, backlight44is allowed to pass through color filter elements134on color filter layer58and associated openings104in the black matrix formed in the thin-film transistor layer56. In inactive area IA, stray backlight from backlight structures42(see, e.g., stray backlight44′ ofFIG. 8) is blocked by stray light blocking structures that include at least two stray light blocking layers. The innermost light blocking layer is formed form black tape138. The outermost light blocking layer is formed from the border portion of black masking layer102on the lower surface of thin-film transistor layer substrate100. Layer132on the upper surface of color filter layer58may also block some stray light in inactive area IA. Because tape138helps to block stray light, it is possible to form light blocking layer102from a thinner layer of black masking material than would otherwise be possible, ensuring that the black masking layer102and associated planarization layer106are not too thick.