Method of manufacturing a liquid crystal display

The present invention provides a liquid crystal display apparatus with high reliability in which accuracy of a width of a comb-shaped electrode is improved in the plane, particularly, at a boundary of divisional exposure, display unevenness of the boundary portion of divisional exposure is reduced in a lateral direction electric field method, and provides a manufacturing method thereof. In the method of manufacturing the liquid crystal display apparatus of the present invention wherein the liquid crystal display apparatus is manufactured so that a counter electrode opposite to a pixel electrode is provided and an electric field whose direction is horizontal to the surface of a substrate is applied between the pixel electrode and the counter electrode and thus liquid crystal is driven, when a pixel section is exposed, the exposure is carried out repeatedly by using one mask so that the pattern is formed. Moreover, according to this manufacturing method, a liquid crystal display apparatus having high reliability, in which display unevenness at the boundary portion of divisional exposure is reduced, is obtained.

BACKGROUND OF THE INVENTION
 The present invention relates to an active matrix liquid crystal display
 apparatus and a manufacturing method thereof.
 In an active matrix liquid crystal display apparatus, a method in which a
 direction of an electric field applied to the liquid crystal is parallel
 with a substrate (hereinafter, "lateral direction electric field method")
 is used mainly as a method of obtaining a wide viewing angle (for example,
 Japanese Unexamined Patent Publication No. 254712/1996). When adopting
 this method, it is clear that a change in contrast when the viewing angle
 direction is changed and inversion of gray scale level can be small (for
 example, M.Oh-e and the others, Asia Display '95.pp.577-580).
 FIG. 9 schematically shows an arrangement of one pixel of a substrate of a
 TFT (thin film transistor) array which is an essential element of a
 conventional active matrix liquid crystal display apparatus using this
 method. An image signal is supplied from a signal line 2 to a pixel
 electrode 6 via a TFT 4 switched by a gate line 1, and an electric field
 whose direction is parallel with the substrate is formed between the pixel
 electrode 6 and a counter electrode 5 so that liquid crystal is driven.
 The counter electrode 5 is connected with a common line 3. The substrate
 of the TFT integrate apparatus is composed of a pixel section 7, the pixel
 of which are arranged in a matrix form, and a terminal section 8 for
 inputting a signal from a circuit (FIG. 10). A counter substrate is
 laminated onto the pixel section 7 with the liquid crystal being
 sandwiched therebetween, and a circuit for transmitting an image signal to
 the gate line and the signal line is mounted to the terminal section 8 so
 that the liquid crystal display apparatus is manufactured.
 The following will describe a method of manufacturing the substrate of the
 TFT array which is a component of the active matrix liquid crystal display
 apparatus on reference to a sectional view showing the process in FIG.
 11(a) to 11(d). The counter electrode 5 and the common line 3 are formed
 on a glass substrate 10 simultaneously with the gate line 1 (FIG. 11(a)).
 The gate line 1 serves also as a gate electrode of the TFT. Next, after a
 gate insulating film 11 is deposited on the whole surface, amorphous
 silicon 12 and amorphous silicon 13 with which impurity was doped are
 formed (FIG. 11(b)). The signal line 2 and the pixel electrode 6 are
 formed at the same time when a source/drain area 14 of the TFT is formed.
 Thereafter, the amorphous silicon 13 with which impurity was doped is
 removed by dry etching or the like using the source/drain area as a mask
 (FIG. 11(c)). Finally, a passivation film 9 is formed on the whole surface
 by a transparent insulating film made of silicon nitride, silicon oxide or
 the like (FIG. 11(d)). The respective layers are formed by the processes
 of deposition, photolithography and etching. The photolithography process
 is a method such that coating, exposure and development of a photoresist
 are carried out so that the photoresist is formed into a desired shape.
 The exposure is the core process of the all, and in the manufacturing of
 the active matrix liquid crystal display apparatus, one of the stepper
 method and the mirror projection method is mainly adopted. In the stepper
 method, the liquid crystal display apparatus is divided into two or more
 areas, and while the stage is being moved, the exposure is carried out
 with a mask being exchanged in every area. In the mirror projection
 method, the liquid crystal display apparatus is not divided, and one large
 mask and a glass substrate are scanned integrally and the exposure is
 carried out collectively. In the stepper method, since accuracy of
 lamination between the respective layers is high in the whole area of the
 screen, the characteristic, capacity and the like of the TFT become
 uniform in the plane, so a DC voltage component which is generated due to
 the non-uniformity of the characteristic and capacity in the plane can be
 small, and thus there is an advantage that the liquid crystal display
 apparatus, in which a liquid crystal material is not easily deteriorated
 and which has high reliability, can be manufactured. Meanwhile, in the
 mirror projection method, since the exposure is carried out collectively,
 there is an advantage such that that throughput can be improved. FIG. 3
 shows a conventional divisional exposing method in the case of using the
 stepper method. The pixel section 7 and the terminal section 8 shown in
 FIG. 10 are divided into some areas (in the drawing, divided into four
 areas), and the areas are exposed respectively by using different masks.
 In the case where a liquid crystal display apparatus using the lateral
 direction electric field method is manufactured by the stepper method, as
 mentioned above, the liquid crystal display apparatus with high
 reliability can be manufactured, but there arises a problem that a
 boundary where the divisional exposure is carried out is detected as
 display unevenness. Also in a liquid crystal display apparatus adopting
 the TN method using a longitudinal direction electric field, a boundary is
 occasionally detected as display unevenness in a portion where the
 lamination between the respective layers is shifted greatly, but in the
 lateral direction electric field method, a boundary is detected as display
 unevenness even when the lamination is not shifted, and the boundary is
 detected more easily. It is an object of the present invention to provide
 a liquid crystal display apparatus, in which accuracy in a width of a
 comb-shaped electrode is improved in the plane (particularly in a boundary
 of divisional exposure), and in a lateral direction electric field method,
 display unevenness in the boundary portion of the divisional exposure is
 reduced and simultaneously high reliability is obtained, and to provide a
 manufacturing method thereof.
 FIG. 12 is a result of obtaining a relationship between a changing amount
 of an electrode width and a changing rate of luminance according to an
 experiment. As a result, it was clear that in the lateral direction
 electric field method, a change in the electrode width caused a change in
 luminance. From FIG. 12, it is found that in order to suppress the
 changing rate of luminance to not more than 6%, for example, scattering of
 the electrode width (permissible value of the electrode width) should be
 suppressed to not more than about 0.3 .mu.m. In this case, the electrode
 width is 9 .mu.m. When the changing rate of luminance in the boundary is
 not less than about 6%, the boundary portion of divisional exposure is
 detected clearly. Therefore, in this case, it is necessary to suppress a
 difference between the electrode widths in the boundary of the divisional
 exposure to not more than about 0.3 .mu.m in the liquid crystal display
 apparatus using the lateral direction electric field method. The intensity
 of the lateral direction electric field is considered to be proportional
 to the electrode width, and the above-mentioned permissible value is also
 considered to be proportional to the electrode width. Therefore, it is
 found that the following (please see formula (1)) relation between a limit
 value (.DELTA.W) of the permissible value of the electrode width and a
 design value (S) of the electrode width in the boundary of divisional
 exposure should be satisfied.
EQU .DELTA.W&lt;0.3.times.S/9=S/30 (1)
 Also in conventional TFT-LCD using the TN method, a change in luminance in
 a boundary of exposure is a problem, and it is caused mainly by parasitic
 capacity of TFT or the like. As a countermeasure against this problem, as
 disclosed in Japanese Unexamined Patent Publication No. 305651/1992,
 accuracy of lamination is improved, and storage capacitance is increased,
 a boundary is made to be zigzag so as to be unclear, and a boundary of
 exposure between a gate electrode and a source/drain electrode are in
 different positions.
 SUMMARY OF THE INVENTION
 In the aforementioned publication, in order to arrange the characteristic
 of the TFT and the parasitic capacitance of the TFT in respective exposed
 areas, a pixel portion is exposed repeatedly by using one mask in a layer
 which is a component of the TFT. The problem at this time is an error of
 the position (an error of lamination of the gate electrode, source/drain
 electrode and the like composing the thin film transistor), and as
 described in the aforementioned publication, the error is about 1 to 2
 .mu.m. On the contrary, the problem of the present invention is the width
 of the comb-shaped electrode, and it is an object of the present invention
 to suppress the change in the electrode width at a boundary of the
 divisional exposure to not more than 0.2 to 0.5 .mu.m and to reduce the
 change in luminance. This is a technical problem which is peculiar to the
 lateral direction electric field method.
 It is considered that main causes for the change in the electrode width at
 the boundary portion of divisional exposure are a difference in a
 dimension between masks, in-plane distribution exposure amount of an
 exposing apparatus (stepper), and deviation of a shutter speed of the
 exposing apparatus. The present invention suppresses these influences so
 as to suppress the display unevenness of the boundary portion of
 divisional exposure.
 According to a first aspect, in order to reduce a difference in dimension
 of masks, different masks are not used on both sides of the boundary
 portion, but the same mask is used so that patterning is carried out. As a
 result, an influence due to a difference in dimension between masks is
 eliminated, and only a difference in dimension of one mask (difference in
 right and left sides or upper and lower sides) causes a change in an
 electrode width due to the mask. However, when this method is used, a
 pixel section and a terminal section are divided so as to be exposed, so a
 number of times of the exposure is generally increased, and thus the
 throughput is deteriorated. Therefore, this method may be used only for
 layers where a pixel electrode and a counter electrode are formed.
 Moreover, when the dry etching method is used for producing a mask, a
 difference in dimension (in particular, difference in right and left sides
 and difference in upper and lower sides) of the mask is reduced.
 According to a second aspect, in order to suppress an influence of an
 exposing amount distribution of an exposing apparatus, an area on which
 exposure is carried out once is reduced as small as possible. However,
 this increases a number of times of divisional exposure, and the
 throughput is deteriorated. Therefore, an exposed area of only the layers
 where a pixel electrode and a counter electrode are formed is reduced.
 Further, when the pixel electrode and the counter electrode are formed on
 different layers, the boundary of divisional exposure should be in
 different positions of the layers.
 Further, the layers where the pixel electrode and the counter electrode are
 formed are formed by the mirror projection method, and the layer where the
 thin film transistor is formed is formed by the stepper method.
 In addition, a thick photoresist, which is used for forming the layers
 where the pixel electrode and the counter electrode are formed, is used
 and the exposing time is set longer.
 Furthermore, a phase shift mask is used as a mask for the exposure on the
 layers where the pixel electrode and the counter electrode are formed. In
 addition, as a mask for the exposure on the layers where the pixel
 electrode and the counter electrode are formed, a mask, whose accuracy is
 higher than that used on the other layers whose in-plane tolerance is not
 more than .+-.S/60 .mu.m, is used.

DETAILED DESCRIPTION
 EMBODIMENT 1
 The following will describe a substrate of a TFT array according to
 EMBODIMENT 1 of the present invention and a method of manufacturing a
 liquid crystal display apparatus having the substrate. First, a gate line
 1 is formed by Al, Cr, Mo or W, or alloy mainly containing Al, Cr and Mo,
 or a laminated film made of them. At this time, the gate line 1 and a
 counter electrode 5 are formed simultaneously. At this time, in order to
 obtain reliability of a liquid crystal display apparatus, the stepper
 method is used as exposure in the photolithography process. At this time,
 in order to prevent scattering of an electrode width due to a difference
 in dimension between masks from occurring, a pixel section 7 is exposed
 repeatedly by using one mask, and a terminal section 8 is exposed by using
 another mask. FIGS. 1 and 2(a) to 2(b) show this exposing method. FIG. 1
 shows positions on the substrate of the TFT array where the exposure is
 carried out, and FIGS. 2(a) to 2(b) schematically show positions of masks
 21 and 22 where the exposure is carried out. As for the pixel section
 shown in FIG. 1, the portions A shown in FIG. 2(a) are exposed repeatedly,
 for example, four times. As for the terminal section, the portions shown
 by portion B through I in FIG. 2(a) and FIG. 2(b), which are represented
 by portion B through I in FIG. 1, are exposed. After a gate insulating
 film is formed on the whole surface, amorphous silicon 12 and amorphous
 silicon 13 including impurity are formed simultaneously. When the
 amorphous silicon 12 and the amorphous silicon 13 with which impurity was
 doped are formed, exposure may be carried out by the method shown in FIGS.
 1 and 2, but one panel including the terminal section is divided into not
 less than two, for example, four portions as shown in FIG. 3, and the
 respective patterns may be exposed by using different masks. In the method
 shown in FIGS. 1 and 2(a) to 2(b), sixteen times of exposure is required,
 but when the exposure is carried out in a manner shown in FIG. 3, a number
 of times of the exposure is only four, so throughput can be prevented from
 being lowered greatly. Next, a signal line 2 and a pixel electrode 6 are
 formed by Al, Cr, Mo or W, or alloy mainly containing them, or a laminated
 film made of them by using the exposing method shown in FIGS. 1 and 2(a)
 to 2(b) at the same time when a source/drain electrode of the TFT is
 formed. After a passivation film made of silicon nitride or silicon oxide
 is formed on the whole surface, the passivation film on the terminal is
 removed so that the substrate of the TFT array is produced. A counter
 substrate is laminated on the pixel section of the substrate of the TFT
 array with liquid crystal being sandwiched therebetween, a circuit for
 transmitting image signals to the gate line and the signal line is mounted
 to the terminal section 8, and a back light is mounted to a rear surface
 of the substrate of the TFT array so that the liquid crystal display
 apparatus is manufactured.
 EMBODIMENT 2
 In the EMBODIMENT 1, the counter electrode 5 which is formed simultaneously
 with the gate line 1, and the pixel electrode 6 which is formed
 simultaneously with the signal line 2 were exposed by using the method
 shown in FIGS. 1 and 2(a) to 2(b), but exposure may be carried out so that
 the boundary is in a different position on the pixel section. For example,
 the gate line 1 and the counter electrode 5 are exposed by the method
 shown in FIGS. 1 and 2(a) to 2(b), and the signal line 2 and the pixel
 electrode 6 are exposed by using masks 23 and 24 shown in FIGS. 4 and 5(a)
 to 5(c) in a manner that the pixel section is divided into more portions.
 As a result, positions where widths of the pixel electrode 6 and the
 counter electrode 5 are changed can be in different positions, so
 unevenness on the boundary of exposure is hardly seen. Moreover, when the
 pixel electrode 6 is exposed, a portion used for an exposed area by the
 exposing apparatus is small, so distribution of exposing energy of the
 exposing apparatus can be small, and thus a change in the electrode widths
 due to the distribution can be small.
 EMBODIMENT 3
 The EMBODIMENTS 1 and 2 refer to the case where the counter electrode 5 and
 the pixel electrode 6 are formed on different layers, but they may be
 formed on the same layer. FIG. 6 shows an example of a plan view of one
 pixel. The pixel electrode 6 and the counter electrode 5 are connected
 respectively with the source drain electrode 14 and the common line 3 via
 contact holes 15. The following will describe an example of the
 manufacturing method according to the sectional view showing the process
 in FIGS. 11(a) to 11(d). First, the gate line 1 is formed by Al, Cr, Mo or
 W, or alloy mainly containing them, or a laminated film made of them (FIG.
 7(a)). Next, after the gate insulating film is formed on the whole
 surface, the amorphous silicon 12 and the amorphous silicon 13 with which
 impurity was doped are formed (FIG. 7(b)). Further, the signal line 2 is
 formed by Al, Cr, Mo or W, or alloy mainly containing them or a laminated
 film made of them simultaneously with the source/drain electrode 14 of the
 TFT. Thereafter, the amorphous silicon 13 with which impurity was doped is
 removed by dry etching or the like by using the source/drain electrode 14
 as a mask (FIG. 7(c)). After a passivation film made of silicon nitride or
 silicon oxide is formed on the whole surface, the contact holes 15 are
 formed on the source/drain electrode 14 and the common line 3(FIG. 8(a)).
 In the above process, the stepper method is used, and, for example,
 exposure is carried out by the method shown in FIG. 3. Next, the pixel
 electrode 6 and the counter electrode 5 are formed simultaneously by Al,
 Cr, Mo or W, or alloy mainly containing them, or a laminated film made of
 them (FIG. 8(b)). As the exposing method at this time, the method shown in
 FIGS. 1 and 2(a) and 2(b) is used, and a change in the electrode widths at
 a joint of the divisional exposure is made to be small. A counter
 substrate is laminated on the pixel section of the substrate of the TFT
 array with liquid crystal being sandwiched therebetween, a circuit for
 transmitting image signals to the gate line and the signal line is mounted
 to the terminal section 8, and a back light is mounted so that the liquid
 crystal display apparatus is manufactured. As a result, a layer, which is
 necessary to be formed so that accuracy of the electrode width becomes
 high, can be arranged as one layer.
 EMBODIMENT 4
 In the EMBODIMENT 3, when the pixel electrode 6 and the counter electrode 5
 are formed, the exposure was carried out by using the method shown in
 FIGS. 1 and 2(a) to 2(b), but collective exposure may be carried out by
 the mirror projection method. Namely, the gate line 1, the amorphous
 silicon 4, the signal line 2 and the contact holes are formed by the
 stepper method, and the pixel electrode 6 and the counter electrode 5 are
 formed by the exposure using the mirror projection method. As a result,
 the characteristic and capacitance of the TFT are not scattered in the
 plane and thus the liquid crystal display apparatus with high reliability
 can be manufactured, but since the layers where the pixel electrode and
 the counter electrode are formed are not divisionally exposed,
 satisfactory display in which the boundary of the divisional exposure is
 not visible can be obtained.
 EMBODIMENT 5
 In the EMBODIMENTS 1 through 3, when a thickness of a photoresist at the
 time of forming the pixel electrode 6 or the counter electrode 5 is set to
 be larger, the exposing time becomes longer, and thus the influence
 exerted on the electrode widths due to the scattering of the shutter speed
 of the exposing apparatus can be small. If this method is used on the
 other layers, a cost of a material of the photoresist becomes high, and
 the throughput is lowered due to extension of the exposing time, so only
 the photoresist, which forms the pixel electrode 6 or the counter
 electrode 5 or both of them, is made to be thicker.
 EMBODIMENT 6
 In the EMBODIMENTS 1 through 3, as the photoresist used for the pixel
 electrode 6 or the counter electrode 5, photoresist, in which a change in
 its pattern width is smaller with respect to a change in the exposing
 energy, is used. As a result, an influence exerted on the electrode widths
 due to distribution of the energy and the scattering of the shutter speed
 of the exposing apparatus can be small. In general, since photoresist, in
 which a change in its pattern width is smaller with respect to a change in
 the exposing energy, is expensive, such photoresist is used only for
 forming the pixel electrode 6, or the counter electrode 5 or both of them,
 and thus an increase in the cost is prevented.
 EMBODIMENT 7
 In the EMBODIMENTS 1 through 3, when a phase shifting mask is used for
 forming the pixel electrode 6 or the counter electrode 5, the influence
 exerted on the electrode widths due to the distribution of the exposing
 energy and the scattering of the shutter speed of the exposing apparatus
 can be small. The phase shifting mask is such that a phase shifter is
 added to a chromium mask so that high resolution and high density of a
 transfer pattern are achieved. A phase shifting mask is generally
 expensive, and phase shift masks other than those having a slit shape is
 hardly produced, so such a mask may be used for forming the pixel
 electrode 6, the counter electrode 5 or both of them.
 EMBODIMENT 8
 In the EMBODIMENTS 1 through 3, when an in-plane dimension tolerance of a
 mask used for forming the pixel electrode 6 or the counter electrode 5 is
 made to be small, an influence exerted on the electrode widths due to the
 in-plane tolerance of the dimension on the mask can be small. In general,
 the dimension tolerance of the mask used for manufacturing a liquid
 crystal display apparatus is .+-.0.2 to 0.5 .mu.m, but as mentioned above,
 in the liquid crystal display apparatus adopting the lateral direction
 electric field method, it is necessary to suppress a change in the
 electrode widths within about S/30 .mu.m. Therefore, it is necessary to
 suppress the in-plane dimension tolerance of the mask used for forming the
 pixel electrode 6 or the counter electrode 5 within at least .+-.S/60
 .mu.m. That is, the in-plane dimension tolerance is about .+-.0.17 .mu.m
 in the case where the electrode width is 10 .mu.m. The higher precision
 mask is preferably used, because the scattering or fluctuation of the
 electrode width caused by in-plane distribution of exposed light energy
 produced in an exposing apparatus. However, since the mask which was
 manufactured with high accuracy is expensive, this mask may be used only
 for forming the pixel electrode 6, the counter electrode 5 or both of
 them. Moreover, such a mask is generally formed by processing chromium and
 chromium oxide according to the wet etching method, but the in-plane
 dimension tolerance of the mask can be small by processing it according to
 the dry etching method.
 As mentioned above, according to the present invention, the liquid crystal
 display apparatus, which adopts the lateral electric field method and has
 a wide viewing angle, can be manufactured with the reliability being
 maintained by using the stepper method and unevenness at the boundary of
 the divisional exposure being reduced. Moreover, according to the method
 of the EMBODIMENT 4, the liquid crystal display can be realized without a
 change in the electrode widths at the boundaries of the divisional
 exposure and with few luminance unevenness at the boundaries of the
 divisional exposure.
 In the method of manufacturing a liquid crystal display apparatus according
 to the invention, wherein a liquid crystal display apparatus is
 manufactured so that a counter electrode opposed to a pixel electrode is
 included and an electric field whose direction is horizontal to the
 surface of a substrate is applied between the pixel electrode and the
 counter electrode and thus liquid crystal is driven, when a pixel section
 is exposed repeatedly through one mask so as to be patterned. Accordingly,
 since the exposure is carried out repeatedly by using one mask, this
 method produces an effect that scattering of the electrode widths does not
 occur.
 In a method of manufacturing a liquid crystal display apparatus according
 to the invention, wherein the liquid crystal display apparatus is
 manufactured so that a counter electrode opposed to a pixel electrode is
 provided and an electric field whose direction is horizontal to the
 surface of a substrate is applied between the pixel electrode and the
 counter electrode and thus liquid crystal is driven, when a pixel section
 on at least one of layers where the pixel electrode and the counter
 electrode are formed is exposed, its exposed area is smaller than the
 other layers. Therefore, this method produces an effect that unevenness at
 the boundaries of the exposure is hardly seen.
 In a method of manufacturing a liquid crystal display apparatus according
 to the invention, wherein the liquid crystal display apparatus is
 manufactured so that a counter electrode countered to a pixel electrode is
 provided and an electric field whose direction is horizontal to the
 surface of a substrate is applied between the pixel electrode and the
 counter electrode and thus liquid crystal is driven, when a pixel section
 on at least one of layers where the pixel electrode and the counter
 electrode are formed is exposed, the exposure is carried out repeatedly by
 using one mask so that patterning is carried out, the other layers are
 exposed and patterned by using masks obtained by dividing one panel
 including the terminal section into two or more. Therefore, this method
 produces an effect that the throughput can be prevented from being lowered
 greatly.
 In a liquid crystal display apparatus according to the invention, wherein
 the liquid crystal display apparatus includes a counter electrode opposed
 to a pixel electrode is provided and an electric field whose direction is
 horizontal to the surface of a substrate is applied between the pixel
 electrode and the counter electrode, such that liquid crystal is driven,
 since a difference between widths of the pixel electrode and the counter
 electrode at the boundary of divisional exposure is not more than S/30
 .mu.m maximumly. Therefore, this method produces an effect that a liquid
 crystal display apparatus which adopts the lateral direction electric
 field method and has a wide viewing angle can be manufactured with its
 reliability being maintained by using the stepper method and unevenness at
 the boundary of divisional exposure being reduced.