(12) United States Patent
`Kubota et ai.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006300927Bl
`US 6,300,927 BI
`Oct. 9,2001
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) DISPLAY DEVICE
`
`(75)
`
`Inventors: Yasushi Kubota, Nara; Kenichi Katoh,
`Hiroshima; Jun Koyama, Kanagawa,
`all of (JP)
`
`(73) Assignees: Semiconductor Energy Laboratory
`Co., Ltd., Kanagawa-ken; Sharp
`Kabushiki Kaisha, Osaka, both of (lP)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.c. 154(b) by 0 days.
`
`(21) Appl. No.: 08/932,246
`
`(22) Filed:
`
`Sep. 17, 1997
`
`(30)
`
`Foreign Application Priority Data
`
`Sep. 20, 1996
`
`(JP) ................................................... 8-271614
`
`Int. CI? ....................................................... G09G 3/36
`(51)
`(52) U.S. CI. .............................. 345/92; 345/98; 345/100;
`345/206
`(58) Field of Search ................................ 345/92, 98, 100,
`345/80, 206; 257/57, 59; 438/157
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,694,347 * 9/1987 Ito ........................................ 348/794
`5,459,483 * 10/1995 Edwards ................................. 345/98
`
`5,680,149 * 10/1997 Koyama ................................. 345/98
`5,693,549 * 12/1997 Kim ..................................... 438/157
`5,734,366 * 3/1998 Kubota et al. ....................... 345/100
`5,818,068 * 10/1998 Sasaki .................................... 257/59
`5,841,317 * 11/1998 Ohmori et al.
`...................... 327/563
`5,854,494 * 12/1998 Yamasaki et al.
`..................... 257/57
`* cited by examiner
`
`Primary Examiner~teven Saras
`Assistant Examiner~ritz Alphonse
`(74) Attorney, Agent, or Firm~ish & Richardson P.c.
`
`(57)
`
`ABSTRACT
`
`There is disclosed an active matrix liquid crystal display that
`receives input signals having small amplitudes and needs no
`externally attached level-shifting circuit. The active matrix
`liquid crystal display has a level-shifting circuit for ampli(cid:173)
`fying various input signals, such as control signals, a clock
`signal, and a start pulse signal, and supplying the amplified
`signals to peripheral driver circuits. These peripheral driver
`circuits drive an active matrix circuit. The level-shifting
`circuit is made of a differential amplifier consisting of
`polysilicon TFTs. This amplifier is an analog amplifier. This
`level-shifting circuit is fabricated on the same substrate as
`the active matrix circuit and the peripheral driver circuits. If
`the amplitudes of the input signals are small, level-shifting
`operations can be performed well. Since any externally
`attached level-shifting circuit is dispensed with, the cost can
`be curtailed.
`
`22 Claims, 7 Drawing Sheets
`
`---I
`I
`I
`I
`I
`I
`I
`I
`I
`---!
`
`~+-----+-----+-O~PUT
`
`VDD
`
`r---
`I
`I
`I
`I
`I
`I
`I
`I t ___ _
`
`INPUT 1 ---1
`
`INPUT 2
`
`GND
`
`701
`
`SHARP EXHIBIT 1004
`
`Page 1 of 12
`
`

`
`u.s. Patent
`
`Oct. 9,2001
`
`Sheet 1 of 7
`
`US 6,300,927 BI
`
`106
`
`-
`
`T
`
`,105
`/
`/
`
`1
`
`r 101
`
`/
`
`;' 100
`
`/
`
`r 104
`r 1
`
`/
`
`102J
`
`/
`
`,~
`103
`
`FIG. 1
`
`SCANNING 201
`LINE
`LIQUID
`CRYSTAL
`MATERIAL
`
`FIG. 2
`(Prior Art)
`
`POTENTIAL AT
`COUNTER ELECTRODE
`
`Page 2 of 12
`
`

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`u.s. Patent
`
`Oct. 9,2001
`
`Sheet 2 of 7
`
`US 6,300,927 BI
`
`301
`
`302
`
`303
`
`303
`
`303
`
`FIG.3A
`(Prior Art)
`
`FIG. 38
`(Prior Art)
`
`Page 3 of 12
`
`

`
`u.s. Patent
`
`Oct. 9,2001
`
`Sheet 3 of 7
`
`US 6,300,927 BI
`
`r 401 SIGNAL LINE CIRCUIT
`
`/
`
`~400
`
`I
`
`~
`
`ACTIVE MATRIX CIRCUIT
`
`~
`i'-i'--403
`
`~
`~
`402 SCANNING LINE DRIVER
`FIG. 4
`(Prior Art)
`
`VDD
`
`~-+--r--OUTPUT1
`.....--------t---- OUTPUT 2
`INPUT 2---+------+---.
`
`INPUT1--i
`"'- 502
`501 J
`-~----~- GND
`
`FIG. 5
`(Prior Art)
`
`Page 4 of 12
`
`

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`u.s. Patent
`
`Oct. 9,2001
`
`Sheet 4 of 7
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`US 6,300,927 BI
`
`604
`
`601
`
`600
`
`605
`
`604
`
`602
`
`FIG. 6
`(Prior Art)
`
`603
`
`VDD -........--"'r-........-----..-....."..::----..,.----
`r--(cid:173)
`I
`I
`I
`I
`I
`I
`I
`I
`I
`
`~---- ----- ------ --~-- -----
`
`I
`
`~~----~----~O~PUT
`
`INPUT 1 ---1
`
`INPUT 2 -------+--------'
`
`GND
`
`701
`
`FIG. 7
`
`Page 5 of 12
`
`

`
`u.s. Patent
`
`Oct. 9, 2001
`
`Sheet 5 of 7
`
`US 6,300,927 BI
`
`805
`VDD ----..--r-----+--------+--
`~806
`
`r-- ------- --------- ---1
`I
`1
`1
`I
`1
`I
`I
`1
`I
`I
`I
`I 1___ _________________ _ __
`
`INPUT 1-1
`
`mp~2------~----~
`
`801
`
`802
`
`GND-------+---------~-----
`
`FIG. 8
`
`Page 6 of 12
`
`

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`u.s. Patent
`
`Oct. 9,2001
`
`Sheet 6 of 7
`
`US 6,300,927 BI
`
`910
`
`911
`
`FIG.9A
`
`901
`
`912
`
`91
`
`FIG. 98
`
`PIONS
`+ + + + + + + + + + + + + + +
`
`BIONS
`+ + + + +
`
`FIG.9C
`
`91
`
`FIG. 90
`
`Page 7 of 12
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`

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`u.s. Patent
`
`Oct. 9,2001
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`Sheet 7 of 7
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`US 6,300,927 BI
`
`.,....
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`0')
`
`~
`.,.....
`ci
`Li::
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`
`Page 8 of 12
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`

`
`US 6,300,927 Bl
`
`1
`DISPLAY DEVICE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to a display device such as
`an active matrix liquid crystal display device and, more
`particularly, to a monolithic active matrix liquid crystal
`display device comprising a substrate that has driver circuits
`as well as an active matrix circuit.
`2. Description of the Related Art
`An active matrix display is shown in FIG. 2 and has
`scanning lines 201 and signal lines 202 meeting at intersec(cid:173)
`tions. A liquid crystal material 204 is disposed as a pixel at
`each intersection. Every pixel is equipped with a switching 15
`device 203. Information about the pixels is represented by
`turning on and off the switching devices. For example, a
`liquid crystal material is used as a display medium in such
`a display device. In the present invention, thin-film transis(cid:173)
`tors (TFTs) each having three terminals, i.e., gate, source, 20
`and drain, are used as switching devices.
`In the present specification, a row of a matrix construction
`means scanning lines (gate lines) which extend parallel to
`the row and are connected with the gate electrodes of the
`TFTs in the row. A column of the matrix construction means
`signal lines (source lines) which run parallel to the column
`and are connected with the source (or drain) electrodes of the
`TFTs in the column. A circuit for driving the scanning lines
`is referred to as a scanning line driver circuit. A circuit for
`driving the signal lines is referred to as a signal line driver 30
`circuit. A thin-film transistor is referred to as a TFT.
`FIGS. 3(a) and 3(b) show one conventional active matrix
`liquid crystal display. This liquid crystal display uses TFTs
`made of amorphous silicon to build an active matrix circuit. 35
`A scanning line driver circuit and a signal line driver circuit
`are each made of an integrated circuit using a single-crystal
`silicon substrate. Typically, a single-crystal silicon driver
`circuit IC 301 is mounted to the outer periphery of an active
`matrix circuit 302 made up of amorphous silicon TFTs by 40
`TAB techniques (FIG. 3(a)). Alternatively, a single-crystal
`silicon driver circuit IC chip 303 is mounted by COG
`(chip-on-glass) techniques (FIG. 3(b)).
`This conventional liquid crystal display suffers from the
`following problems. First, the reliability poses problems, 45
`because the scanning and signal line driver circuits are
`connected with the scanning lines and the signal lines,
`respectively, of the active matrix circuit by TAB or wire
`bonding.
`For example, where the display device is built in accor- 50
`dance with the video graphics array (VGA) standards, there
`exist 1920 signal lines and 480 scanning lines. As the
`resolution is enhanced every year, their numbers tend to be
`increased. Where a viewfinder for use in a video camera or
`a projector using a liquid crystal display is manufactured, it 55
`is necessary to make the whole display device compact. A
`liquid crystal display manufactured using TAB is unsuited
`for such applications because this display needs a large
`space.
`TFTs built, using polysilicon, have been developed to 60
`fabricate an active matrix liquid crystal display free of the
`foregoing problem. One example is shown in FIG. 4, where
`an insulating substrate 400 is made of glass or the like. On
`this substrate 400, a signal line driver circuit 401 and a
`scanning line driver circuit 402 are constructed from poly- 65
`silicon TFTs simultaneously with TFTs forming an active
`matrix circuit 403. Polysilicon TFTs can be fabricated by
`
`2
`high-temperature polysilicon processes. That is, high(cid:173)
`temperature polysilicon TFTs are formed on a quartz sub(cid:173)
`strate at a temperature higher than 10000 C. Also, low(cid:173)
`temperature polysilicon TFTs can be manufactured on a
`5 glass substrate by low-temperature processes at tempera(cid:173)
`tures below 650 0 C.
`The mobilities of amorphous silicon TFTs are approxi(cid:173)
`mately 0.5 cm2N sec. On the other hand, the mobilities of
`polysilicon TFTs can be set 30 cm2N or higher sec and thus
`10 they can be operated with signals having frequencies only on
`the order of MHz.
`Both digital and analog driver circuits are available to
`drive active matrix liquid crystal displays. Since a circuit of
`digital construction has much more devices than a circuit of
`analog construction, it is customary to use an analog driver
`circuit where polysilicon TFTs are employed. In one type of
`scanning line driver circuit and signal line driver circuit,
`shift registers are utilized. In another type, decoders are
`used. Driver circuits using low-temperature polysilicon
`TFTs as described above have the following disadvantages.
`The low-temperature polysilicon TFTs process have
`smaller mobilities and larger threshold values than single(cid:173)
`crystal silicon transistors. Therefore, assuming that the TFTs
`are driven at more than 1 MHz to sample the input video
`25 signal, it is necessary to set the power-supply voltage to
`about 15-18 V, for example.
`However, a circuit for controlling a driver circuit of a
`liquid crystal display is normally made of an integrated
`circuit using single-crystal silicon. The operating voltage is
`approximately 5 V. Therefore, the output signal is also
`approximately 5 V. Under this condition, it is difficult to
`control the driver circuit consisting of low-temperature
`polysilicon TFTs.
`In recent years, ICs made of single-crystal silicon tend to
`be driven by decreasing power-supply voltages. If the
`power-supply voltage for a control circuit is 5 V, a level(cid:173)
`shifting circuit operating digitally and consisting of
`N-channel TFTs 501, 502 and P-channel TFTs 503, 504 can
`be constructed, as shown in FIG. 5, by suppressing the
`threshold value to about 2 V. This level-shifting circuit is
`employed in an ordinary CMOS circuit at low frequencies.
`However, where the power-supply voltage for the IC is 3 V,
`it is difficult to operate the above-described level-shifting
`circuit even at decreased frequencies unless the threshold
`value is set less than 1 V.
`Accordingly, it is common practice to insert a level(cid:173)
`shifting circuit 604 made of a proprietary single-crystal IC
`or externally attached transistors between a control circuit
`605 made of single-crystal silicon and each of a signal line
`driver circuit 601 and a scanning line driver circuit 602, as
`shown in FIG. 6, to drive an active matrix circuit 603
`fabricated on an insulating substrate 600 made of glass or the
`like. The signal line driver circuit 601 and the scanning line
`driver circuit 602 are fabricated from low-temperature poly(cid:173)
`silicon TFTs on the substrate 600. Where these level-shifting
`circuits are attached externally, if more signals are treated,
`the number of externally attached level-shifting circuits are
`increased accordingly. This leads to an increase in the cost.
`
`SUMMARY OF THE INVENTION
`The foregoing problems are solved by an active matrix
`liquid crystal display comprising an insulating substrate, an
`active matrix circuit using TFTs as switching devices, a
`signal line driver circuit and a scanning line driver circuit for
`driving the active matrix circuit, and at least one differential
`amplifier. All of the active matrix circuit, the signal line
`
`Page 9 of 12
`
`

`
`3
`driver circuit, the scanning line driver circuit, and the
`differential amplifier are fabricated on the insulating sub(cid:173)
`strate. The differential amplifier is made up of TFTs and
`amplifies non-inverted and inverted input signals entered
`from a pair of input terminals. The amplified signals are sent
`to the signal line driver circuit or to the scanning line driver
`circuit.
`In one feature of the invention, the amplitudes of the input
`signals described above are less than 5 V.
`In another feature of the invention, the differential ampli- 10
`fier described above comprises a differential circuit made up
`of plural TFTs whose sources are connected together. A
`constant-current source is connected with the differential
`circuit.
`In a further feature of the invention, the differential 15
`amplifier described above is an analog amplifier made up of
`plural TFTs whose sources are connected together. A
`constant-current source is connected with the differential
`amplifier. The output from the differential amplifier is ampli(cid:173)
`fied by a source-grounded amplifier circuit.
`In a still other feature of the invention, the differential
`amplifier described above is made up of low-temperature
`polysilicon TFTs fabricated by a low-temperature process
`having a maximum process temperature lower than 650 0 C.
`In the structure described above, the input signals mean 25
`control signals, clock signals, etc. for controlling the scan(cid:173)
`ning line driver circuit and the signal line driver circuit. Each
`of the scanning and signal line driver circuits mainly con(cid:173)
`sists of shift registers or decoders.
`An active matrix circuit in accordance with the present
`invention is driven by peripheral driver circuits comprising
`level-shifting circuits for amplifying input signals such as
`control signals, a clock signal, and a start pulse signal. Each
`level-shifting circuit is fabricated from a differential ampli(cid:173)
`fier consisting of TFTs of polysilicon, or polycrystalline 35
`silicon. These level-shifting circuits are operated in the
`analog mode rather than in the conventional digital mode.
`These level-shifting circuits are fabricated on the same
`substrate as the active matrix circuit and the peripheral
`driver circuits. As a result, where the signals input to the 40
`liquid crystal display have small amplitudes (i.e., the power(cid:173)
`supply voltage for the control circuit is 5 V or less),
`level-shifting operations can be performed well. Also, any
`externally attached level-shifting circuit is dispensed with.
`In consequence, the cost can be curtailed.
`Other objects and features of the invention will appear in
`the course of the description thereof, which follows.
`
`45
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram of a liquid crystal display in
`accordance with the present invention;
`FIG. 2 is a fragmentary circuit diagram of an active matrix
`circuit using TFTs;
`FIGS. 3(a) and 3(b) are top plan views of conventional
`active matrix displays using amorphous silicon TFTs;
`FIG. 4 is a top plan view of the prior art active matrix
`liquid crystal display using polysilicon TFTs;
`FIG. 5 is a circuit diagram of the prior art level-shifting
`circuit;
`FIG. 6 is a block diagram of the prior art liquid crystal
`display;
`FIG. 7 is a circuit diagram of a differential amplifier used
`in a liquid crystal display in accordance with the invention;
`FIG. 8 is a circuit diagram of another differential amplifier 65
`used in a liquid crystal display in accordance with the
`invention;
`
`55
`
`US 6,300,927 Bl
`
`5
`
`4
`FIGS. 9(A)-9(D) are cross-sectional views illustrating
`low-temperature polysilicon process steps for fabricating a
`liquid crystal display in accordance with the invention; and
`FIGS. 10(A) and 10(B) are cross-sectional views illus-
`trating other low-temperature polysilicon process steps for
`fabricating a liquid crystal display in accordance with the
`invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`EXAMPLE 1
`
`Referring to FIG. 1, there is shown an active matrix liquid
`crystal display having driver circuits and embodying the
`concept of the present invention. This liquid crystal display
`comprises an insulating substrate 100 on which an active
`matrix circuit 103, a signal line driver circuit 101, a scanning
`line driver circuit 102, a signal line differential amplifier
`104, and a scanning line differential amplifier 105 are
`20 fabricated. The signal line driver circuit 101 and the scan(cid:173)
`ning line driver circuit 102 drive the active matrix circuit
`103. The signal line differential amplifier 104 and the
`scanning line differential amplifier 105 are located between
`the driver circuits 101 and 102.
`An external control circuit 106 is made of a single-crystal
`IC and produces signals for driving the signal line driver
`circuit 101 and the scanning line driver circuit 102. These
`signals are supplied to a display device. Normally, the
`amplitudes of the signals are lower than the power-supply
`30 voltage inside the liquid crystal display, usually approxi(cid:173)
`mately between 3 V and 5.5 V.
`An example of the differential amplifiers described above
`is shown in FIG. 7. The illustrated differential amplifier
`consists of TFTs whose sources are connected together to
`form a differential circuit that is operated with a constant
`current. Therefore, at least one of the TFTs of the differential
`circuit is ON at all times and operates in an analog manner.
`Consequently, the circuit can cope with smaller input signals
`than heretofore, e.g., the amplitudes of the input signals are
`less than about 3 V. This differential amplifier operates in the
`manner described below.
`In FIG. 7, the differential circuit is made of N-channel
`TFTs 702 and 703 and has a pair of input terminals, or inputs
`1 and 2, to which non-inverted and inverted input signals
`such as driving signals, a clock signal, and a start pulse, are
`applied. In particular, when a non-inverted signal is applied
`to the gate (input 1) of the TFT 702 and an inverted signal
`is applied to the gate (input 2) of the TFT 703, the voltage
`50 across the gate and source of the TFT 702 increases but the
`voltage across the gate and source of the TFT 703 decreases.
`The sources of the TFTs 702 and 703 are connected
`together and operated with a constant-current source 701.
`The current flowing through the TFT 703 decreases by an
`amount equal to the increase in the current flowing through
`the TFT 702.
`The drains of the TFTs 702 and 703 are connected with
`current mirror circuits 705 and 706, respectively. The cur(cid:173)
`rents flowing through these drains are inverted at the same
`60 magnitude. The output from the current mirror circuit 706 is
`coupled to a current mirror circuit 707 and inverted simi(cid:173)
`larly.
`The outputs of the current mirror circuits 705 and 707 are
`connected together and tied to the input of the signal line
`driver circuit or the scanning line driver circuit directly or
`via a buffer. If the current flowing through the TFT 702
`increases, the current flowing through the current mirror
`
`Page 10 of 12
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`

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`US 6,300,927 Bl
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`5
`circuit 705 increases but the current flowing through the
`current mirror circuit 706 decreases. Therefore, the current
`flowing through the current mirror circuit 707 decreases.
`A capacitance is attached to the input terminal of the
`driver circuit or the input terminal of the buffer connected
`with the outputs of the current mirror circuits 705 and 707.
`This capacitance is charged according to the difference
`between the currents flowing through the current mirror
`circuits 705 and 706, respectively. As a result, the potential
`at the capacitance reaches the potential at the positive 10
`terminal ( +) of the power supply. If the signals applied to the
`TFTs 702 and 703 have an opposite relation to the above(cid:173)
`described case, a reverse operation takes place. The potential
`at the input terminal of the driver circuit or the input terminal
`of the buffer circuit decreases down to the potential at the 15
`negative terminal (-) of the power supply. In this way, a
`large input signal to be supplied into the driver circuit can be
`formed from an input signal having a small amplitude. In the
`present embodiment, each differential circuit is built from
`N-channel TFTs. The differential circuits may also be 20
`constructed, using P-channel TFTs.
`
`6
`silicon oxide is formed to a thickness of 500 to 2000 A by
`sputtering in an oxygen ambient. The gate insulator film may
`be formed by plasma CVD. Where the silicon oxide film is
`formed by plasma CVD, it is desired to use nitrogen
`5 monoxide (N20) as a gaseous raw material. Alternatively,
`oxygen (02) and mono silane (SiH4) may be employed. After
`the formation of the gate insulator film, thermal annealing
`may be carried out 650° C. or less.
`Subsequently, an aluminum layer having a thickness of
`2000 to 6000 A is formed by sputtering over the whole
`surface of the laminate. The aluminum may contain silicon,
`scandium, palladium, or other material to prevent generation
`of hillocks in later thermal processing steps. This aluminum
`layer is etched to form gate electrodes 907, 908, and 909
`(FIG. 9(A)).
`Thereafter, the aluminum layer is anodized to form alu(cid:173)
`minum oxide, 910, 911, and 912, on the surface of the
`aluminum layer. These aluminum regions act as an insulator
`(FIG. 9(B)).
`Then, a photoresist mask 913 which covers the active
`layer 903 of the P-channel TFTs is formed. Phosphorus ions
`are introduced by ion doping while using phosphine as a
`dopant gas. The dose is 1x1012 to 5xlO13 atoms/cm2. As a
`result, heavily doped N-type regions, or source 914 and
`25 drain 915, are formed (FIG. 9(C)).
`Thereafter, a photoresist mask 916 for covering both
`active layer 904 for the N-channel TFTs and active layer 905
`for the pixel TFTs is formed. Boron ions are introduced
`30 again by ion doping, using diborane (B2H6) as a dopant gas.
`The dose is 5x1014 to 8x1016 atoms/cm2. As a result, P-type
`regions 917 are formed. Because of the doping steps
`described thus far, heavily doped N-type regions (source 914
`and drain 915) and heavily doped P-type regions (source and
`35 drain 917) are formed (FIG. 9(D)).
`Then, the laminate is thermally annealed at 450-850° C.
`for 0.5 to 3 hours to repair the damage created by the doping.
`In this way, the dopants are activated. At the same time, the
`crystallinity of the silicon is recovered. Thereafter, a silicon
`40 oxide film having a thickness of 3000 to 6000 A is formed
`as an interlayer dielectric 918 over the whole surface by
`plasma CVD. This may be a silicon nitride film or a
`multilayer film of silicon oxide layers and silicon nitride
`layers. The interlayer dielectric 918 is etched by a wet
`45 etching process or a dry etching process to form contact
`holes in the source/drain regions.
`Then, an aluminum film or a multilayer film of titanium
`and aluminum is formed to a thickness of 2000 to 6000 A by
`sputtering techniques. This film is etched so as to create
`50 electrodes/interconnects, 919, 920, and 921, for peripheral
`circuits and pixels/interconnects, 922 and 923, for pixel
`TFTs (FIG. 10(A)).
`Subsequently, a silicon nitride film 924 is formed as a
`passivation film having a thickness of 1000 to 3000 A by
`plasma CVD. This silicon nitride film is etched to create
`contact holes extending to the electrodes 923 of the pixel
`TFTs. An ITO (indium-tin oxide) film having a thickness of
`500 to 1500 A is formed by sputtering. Finally, the ITO film
`is etched to form pixel electrodes 925. In this manner, under
`the thermal annealing at 650° C. or less, the differential
`amplifier circuits, the peripheral driver circuits, and the
`active matrix circuit are formed simultaneously on the same
`substrate (FIG. 10(B)).
`As described thus far, in a liquid crystal display device in
`accordance with the present invention, an analog differential
`amplifier made of polysilicon TFTs is formed on a same
`substrate, together with an active matrix circuit and periph-
`
`EXAMPLE 2
`
`Referring next to FIG. 8, there is shown another example
`of differential amplifier. A differential circuit is constructed
`from a constant-current source 801, TFTs 803, 804, and a
`current mirror circuit 805. This differential circuit is com(cid:173)
`bined with a source-grounded amplifier circuit consisting of
`a constant-current source 802 and a P-channel TFT 806. This
`differential amplifier is inferior to a differential circuit alone
`in frequency characteristics. However, a greater amplifica(cid:173)
`tion factor is obtained. Hence, this amplifier is beneficial
`where input signals have small amplitudes.
`
`EXAMPLE 3
`
`A method of fabricating a liquid crystal display using a
`monolithic active matrix circuit in accordance with the
`invention is next described by referring to FIGS. 9(A)-9(D)
`and 10(A)-10(B). The method is a low-temperature poly(cid:173)
`silicon fabrication process. The process sequence for fabri(cid:173)
`cating TFTs forming differential amplifiers and peripheral
`driver circuits is shown to the left of FIGS. 9(A)-9(D). The
`process sequence for fabricating TFTs forming the active
`matrix circuit is shown to the right.
`First, a silicon oxide film is formed as a buffer layer 902
`on a glass substrate 901 to a thickness of 1000 to 3000 A.
`This silicon oxide film may be formed in an oxygen ambient
`by sputtering or plasma CVD.
`Then, an amorphous silicon film is formed to a thickness
`of 300 to 1500 A, preferably 500 to 1000 A, by plasma CVD
`or LPCVD. The amorphous film is thermally annealed at a
`temperature 500° C. or higher, preferably 500-600° c., to
`crystallize the amorphous silicon film or to enhance the
`crystallinity. After the crystallization making use of thermal 55
`annealing, the crystallinity may be further enhanced by
`carrying out photo-annealing making use of laser light.
`Furthermore, during the crystallization making use of ther(cid:173)
`mal annealing, an element (or, a catalytic element) such as
`nickel for promoting crystallization of silicon may be added, 60
`as described in Unexamined Published Japanese Patent
`Applications Nos. 6-244103 and 6-244104.
`Then, the silicon film is etched to form islands of an active
`layer 903 for P-channel TFTs forming a driver circuit,
`islands of an active layer 904 for N-channel TFTs, and 65
`islands of an active layer 905 for pixel TFTs forming a
`matrix circuit. Furthermore, a gate insulator film 906 of
`
`Page 11 of 12
`
`

`
`US 6,300,927 Bl
`
`7
`eral driver circuits for driving the active matrix circuit.
`Where the amplitudes of input signals are 5 V or less,
`level-shifting operations can be performed well. This can
`dispense with any externally attached level-shifting circuit.
`Consequently, the number of the components and the cost 5
`can be reduced.
`While the present invention has been described with
`reference to the preferred embodiments, the scope of the
`invention should not be limited to those embodiments. Many
`modifications may be made without departing from the 10
`scope of the appended claims.
`What is claimed is:
`1. A display device comprising:
`as active matrix circuit comprising thin-film transistors
`(TFTs) as switching devices and formed over an insu- 15
`lating substrate;
`a signal line driver circuit and a scanning line driver
`circuit for driving said active matrix circuit, said signal
`line driver circuit and said scanning line driver circuit 20
`being formed over said insulating substrate; and
`at least one differential amplifier formed over said insu-
`1ating substrate and comprising TFTs, said differential
`amplifier having a pair of input terminals for a non(cid:173)
`inverted input signal an inverted signal of said non(cid:173)
`inverted input signal to amplify said non-inverted and
`inverted input signals, wherein the signal amplified by
`said differential amplifier is sent to said signal line
`driver circuit or to said scanning line driver circuit.
`2. A device according to claim 1, wherein said input 30
`signals have amplitudes less than 5 V.
`3. A device according to claim 1, wherein said differential
`amplifier comprises a differential circuit comprising TFTs
`whose sources are connected together, and wherein a
`constant-current source is connected with said differential 35
`circuit.
`4. A device according to claim 1, wherein
`(A) said differential amplifier is an analog amplifier
`comprising a differential circuit which comprises TFTs
`whose sources are connected together;
`(B) a constant-current source is connected with said
`differential circuit; and
`(C) a source-grounded differential amplifier amplifies an
`output signal from said differential amplifier.
`5. A device according to claim 1, wherein said differential 45
`amplifier comprises low-temperature polysilicon TFTs fab(cid:173)
`ricated by a low-temperature process having a maximum
`process temperature of 650 0 C. or less.
`6. A device according to claim 1, wherein said device is
`an active matrix liquid crystal device.
`7. A display device according to claim 1 wherein said
`non-inverted input signals are at least one of a control signal,
`a clock signal and a start pulse signal and said inverted input
`signals are inverted signals of said non-inverted input sig-
`~
`8. A display device according to claim 1 wherein said
`display device is a liquid crystal display device.
`9. A display device comprising:
`an active matrix circuit comprising a plurality of first thin 60
`film transistors formed over a substrate;
`a signal line driver circuit and a scanning line driver
`circuit for driving said active matrix circuit, both of
`said signal line driver circuit and said scanning line
`driver circuit comprising second polysilicon thin film
`transistors formed over said substrate;
`
`8
`a single crystal semiconductor integrated circuit for gen(cid:173)
`erating signals for driving said signal line driver circuit
`and said scanning line driver circuit; and
`an analog amplifying circuit for amplifying the signals
`from said integrated circuit and supplying the amplified
`signals to at least one of the signal line driver circuit
`and the scanning line driver circuit, wherein said sig(cid:173)
`nals are at least one of control signals, clock signals,
`and start pulse signals,
`wherein said amplifying circuit comprises third polysili(cid:173)
`con thin film transistors formed over said substrate.
`10. A device according to claim 9 wherein said second and
`third polysilicon thin film transistors have substantially a
`same structure.
`11. A device according to claim 9 wherein said analog
`amplifying circuit comprises a differential amplifier.
`12. A device according to claim 7 wherein the signals
`from said integrated circuit and inverted signals of said
`signals are input to said analog amplifying circuit.
`13. A display device according to claim 12 wherein said
`signals are at least a control signal.
`14. A display device according to claim 12 wherein said
`signals are at least a clock signal.
`15. A display device according to claim 12 wherein said
`25 signals are at least a start pulse signal.
`16. A display device according to claim 9 wherein said
`display device is a liquid crystal display device.
`17. A display device comprising:
`an active matrix circuit comprising a plurality of first thin
`film transistors formed over a substrate;
`a signal line driver circuit and a scanning line driver
`circuit for driving said active matrix circuit, both of
`said signal line driver circuit and said scanning line
`driver circuit comprising second thin film transistors
`formed over said substrate;
`at least one differential amplifier operationally connected
`to one of said signal line driver circuit and said scan(cid:173)
`ning line driver circuit, said amplifier comprising at
`least one differential circuit and at least one current
`mirror circuit,
`wherein each of said differential circuit and said current
`mirror circuit comprises third thin film transistors
`formed over said substrate,
`wherein said differential amplifier has a pair of input
`terminals for an input signal and an inverted signal of
`said input signal, and said input signals are amplified by
`said differential amplifier, and the amplified signal is
`sent to one of said signal line driver circuit and said
`scanning line driver circuit.
`18. A device according to claim 17 wherein each of said
`f