# Patent

## Title
Semiconductor device and manufacturing method thereof

## Abstract

A TFT having a high threshold voltage is connected to the source electrode of each TFT that constitutes a CMOS circuit. In another aspect, pixel thin-film transistors are constructed such that a thin-film transistor more distant from a gate line drive circuit has a lower threshold voltage. In a further aspect, a control film that is removable in a later step is formed on the surface of the channel forming region of a TFT, and doping is performed from above the control film.

## Background
This is a continuation of U.S. application Ser. No. 08/908,281, filed Aug. 7, 1997 now U.S. Pat. No. 6,353,244, which is a file wrapper continuation of U.S. application Ser. No. 08/620,462, filed Mar. 22, 1996 now abandoned.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reduction of power consumption of a thin-film semiconductor circuit constituted of crystalline silicon. The invention also relates to reduction of power consumption of a drive circuit of an active matrix display device that is constituted of crystalline silicon.
The invention also relates to an ion doping technique for a semiconductor material, and a manufacturing method of a semiconductor and a semiconductor device using that ion doping technique.
Further, the invention relates to reduction of a leak current while a thin-film transistor (hereinafter abbreviated as “TFT”) is off.
2. Description of the Related Art
In recent years, extensive studies have been made of the active matrix display device using a liquid crystal. In the active matrix display device, a switching element is provided for each pixel and a signal coming from an image signal line is supplied to each pixel through the corresponding switching element.
Although previously TFTs using an amorphous silicon semiconductor were used as the switching element, in recent years TFTs have been developed which use a crystalline silicon semiconductor (i.e., a silicon semiconductor including crystal components) having a high operation speed.
However, in a TFT using a crystalline silicon semiconductor, the leak current (off-current) that flows when the gate electrode is reversely biased is larger than in a TFT using an amorphous silicon semiconductor.
This phenomenon, which is believed due to the existence of crystal grain boundaries, is the most serious problem, because it deteriorates the characteristics of a crystalline silicon-based circuit constituting an active matrix display device and increases its power consumption.
In the case of an N-channel TFT, when VGS (source-gate voltage of the TFT) is negative, a leak current is determined by currents flowing through PN junctions that are formed between a P-type layer that is induced in the surface of a semiconductor thin film and N-type layers of source and drain regions. Because of many traps existing in the semiconductor thin film (particularly in grain boundaries), these PN junctions are incomplete, likely causing a relatively large junction leak current.
The reason why the leak current increases as the gate electrode is negatively biased more deeply is that the carrier concentration of the P-type layer formed in the surface of the semiconductor thin film increases, which lowers the energy barrier height of the PN junction, which in turn causes an electric field concentration, resulting in increase of the junction leak current.
The leak current that is caused by the above mechanism strongly depends on the source-drain voltage; that is, the leak current rises sharply as the voltage applied between the source and the drain of a TFT is increased. For example, in some cases, a leak current with a source-drain voltage of 10 V is more than 10 times, rather than 2 times, larger than that with a source-drain voltage of 5 V.
The above nonlinearity also depends on the gate voltage. In general, the leak current difference between the above two cases is larger when the gate electrode is reversely biased more deeply (in an N-channel TFT, when a larger negative voltage is applied).
Typical examples of products using the active matrix display device include a notebook-type personal computer and a portable information terminal. However, in current models of these products, the active matrix display device is responsible for most of the total power consumption. Therefore, to satisfy the need of long-term driving by a battery, it is now desired to reduce the power consumption of the active matrix display device.
However, even where a peripheral drive circuit of an active matrix display device are constituted of CMOS (complementary metal-oxide-semiconductor) TFTs, large leak currents flow through the P-channel TFTs in an off state, making the power consumption of the entire circuit large.
In the case of a high-resolution, large-screen type active matrix display device, long gate lines of the screen cause a problem that there is a delay until the TFT of a selected pixel is turned on. A wiring resistance Rwire and a wiring capacitance Cwire of a gate line are approximated as

Rwire=ρ.L/(W.T)

Cwire=nox.W.L/H 

where

    
    
        ρ: resistivity of a wiring material
    
    


L: wiring length
W: wiring width
T: wiring film thickness
nox: dielectric constant of a field oxide film.
A delay twire, which is equal to a time constant Rwire.Cwire, is expressed as

twire=ρ.nox.L2/(T.H).

This equation indicates that the signal delay due to a wiring line is proportional to the square of the wiring length.
Conventionally, the signal delay due to a wiring line is prevented by reducing the wiring length of gate lines by providing a gate line drive circuit on both sides of the active matrix display device. However, this measure is not sufficient.
SUMMARY OF THE INVENTION
In view of the above problems, an object of the present invention is to provide a thin-film semiconductor integrated circuit formed on crystalline silicon and, in particular, a peripheral drive circuit of an active matrix liquid crystal display device which circuits are low in power consumption.
Another object of the invention is to provide a pixel-switching thin-film transistor of an active matrix liquid crystal device which transistor can prevent a signal delay due to a wiring line.
A further object of the invention is to provide a method for forming thin-film transistors having different threshold voltages (Vth) on the same substrate.
As described above, the leak current in a CMOS circuit as a thin-film semiconductor integrated circuit formed on crystalline silicon can be reduced by decreasing the source-drain voltage.
To this end, according to the invention, a P-channel TFT and an N-channel TFT are connected to the respective source electrodes of a P-channel TFT and an N-channel TFT of a CMOS circuit. The threshold voltages of the additional P-channel and N-channel TFTs are made higher than those of the TFTs of the CMOS circuit. Thus, by making the additional P-channel and N-channel TFTs in an off state, the CMOS circuit can be separated from a power supply when it is not in use.
Since the TFTs of the CMOS circuit are lower, the leak current of the CMOS circuit is reduced. Thus, the power consumption of an integrated circuit constituted of thin-film transistors can be reduced.
According to another aspect of the invention, pixel TFTs are constructed such that the threshold voltage of each TFT is determined in consideration of a voltage drop due to a wiring resistance of a gate line of an active matrix display device. That is, the TFTs are constructed such that a TFT more distant from a gate line drive circuit has a lower threshold voltage. As a result, the gate voltage of a pixel TFT that is about to be driven is lower than in the conventional case. Therefore, the wiring capacitance of a gate line can be shortened, so that pixel TFTs distant from the gate line drive circuit can be turned on in shorter time than in the conventional case.
A description will be made of a method for making the threshold voltages of a plurality of thin-film transistors formed on the same substrate different from one another, to realize the above-described configurations.
In recent years, it has been attempted to dope the channel region of a thin-film transistor (TFT). In the following, this technique is called “channel doping.”
The channel doping enables control of Vth (threshold voltage). Although inherently Vth should be 0 V (actually, ID (drain current) approximately has a minimum value when VG (gate voltage) is 0 V), it may deviate much from 0 V when a semiconductor material is processed to enhance its crystallinity or improve its uniformity. Other factors may also deviate Vth from 0 V. In any case, Vth can be restored to about 0 V by proper channel doping.
Whether an N-type dopant (a doped semiconductor exhibits N-type conductivity) or a P-type dopant (a doped semiconductor exhibits P-type conductivity) should be used depends on the direction (negative side or positive side) of the Vth deviation. The dose of channel doping is changed in accordance with the amount of Vth deviation.
That is, no matter how Vth deviates, it can be restored to about 0 V by adjusting the dose of channel doping. Further, fine adjustment of Vth can be done by changing the dose.
In general, the dose of channel doping is lower than that for source/drain implantation.
Annealing is performed after doping to repair lattice defects that have been produced by a large amount of dopant implanted into a semiconductor material.
A method for intentionally making Vth's of a plurality of thin-film transistors different from one another will be described below. It is possible to give different Vth's to respective transistors by changing doses in doping the channel regions of those transistors, as described above.
FIG. 2 shows a concentration profile of a dopant implanted into a silicon film by doping.
To control the concentration for an individual thin-film transistor, the nature of doping which exhibits the concentration profile of FIG. 2 in a doped object is utilized. In FIG. 2, the vertical axis represents the concentration and the horizontal axis represents the depth from the surface of a doped object.
As seen from FIG. 2, the concentration covers a range of about three orders in the depth direction.
This type of profile allows the following concentration adjustment. That is, in forming a thin-film transistor, a control film, for instance, a silicon oxide film, that can be stripped off in a later step is formed on the surface of the channel region of the thin-film transistor, and doping is performed from above the control film. In this manner, the concentration of an impurity introduced into the channel region can be adjusted.
In this method, a concentration profile is provided in a single substrate in doping it with a single dopant. However, this method can vary Vth only in one direction (i.e., positive or negative direction).
This problem can be solved by implanting different kinds of dopants into different regions.
Further, to produce both of thin-film transistors in which Vth is not shifted at all and those in which Vth is shifted, undoped regions and doped regions may be formed.
The above techniques of controlling Vth can be used to make Vth's of individual thin-film transistors constituting a circuit different from one another, but also to eliminate differences among Vth's of respective thin-film transistors, i.e., to make Vth's uniform.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general configuration of a doping apparatus;
FIG. 2 shows a concentration profile of a dopant implanted into a silicon film by doping;
FIG. 3 shows the concept of a laser annealing apparatus used in a second embodiment;
FIG. 4 is an optical path diagram inside an optics used in the laser annealing apparatus of FIG. 3;
FIG. 5 shows concentration profiles in a case where a silicon oxide film has two different thicknesses;
FIG. 6 shows a general configuration of a shift register circuit of an active matrix display device according to a third embodiment of the invention;
FIG. 7 is a circuit diagram of timing generation circuits that constitute the shift register circuit of FIG. 6;
FIG. 8 is a circuit diagram showing a voltage control circuit that constitutes the shift register of FIG. 6; and
FIG. 9 shows a general configuration of a gate line drive circuit and pixel TFTs of an active matrix display device according to a fourth embodiment of the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
In this embodiment, a channel doping step is inserted in a process of forming a plurality of thin-film transistors on the same substrate. In particular, this embodiment is directed to a manufacturing method of thin-film transistors having superior circuit characteristics.
More specifically, this embodiment relates to a technique of improving the characteristics of the entire circuit or reduce the power consumption of the entire circuit by making Vth values of transistors different from one anther by controlling Vth's by use of a channel doping technique.
In this embodiment, a description will be made of part of a process to a channel doping step. Transistors can be completed by an ordinary method by using a silicon film produced by this embodiment.
First, thin-film transistors in the midst of their manufacture, i.e., before being subjected to doping are prepared. Although the term “thin-film transistors” is used here, at this stage they merely include a 500-Å-thick silicon film formed on a glass substrate (Corning 7059) and a 1,200-Å-thick silicon oxide film formed thereon. Naturally the thicknesses of these films may be set as desired.
The silicon oxide film is a control film for controlling the dose of a dopant to be implanted into a channel portion. The control film may be a silicon nitride film. Details will be described later.
The reason why the channel doping is performed at this stage is that the channel portion of a planar thin-film transistor is usually located under the gate electrode and therefore doping should be performed before formation of the gate electrode. The silicon film as used here means a film mainly made of silicon, and has an amorphous structure or a crystalline structure having crystal components such as a microcrystalline or polycrystalline structure.
Next, a doping apparatus will be described briefly. FIG. 1 shows a general configuration of a doping apparatus.
Plasma is generated in a plasma source 501 (top portion of FIG. 1), and ions existing in the plasma are accelerated by an electric field developing in an ion acceleration region that is located under the plasma source 501. As shown in FIG. 1, three power supplies, i.e., a drawing power supply 502, an acceleration power supply 503, and a deceleration power supply 504 are provided to apply voltages to three respective electrodes arranged from the top. In an actual doping operation, voltages are applied to the three electrodes in the order of arrangement starting from the lowest one. A substrate holder 505 incorporates a heater to keep the substrate at an arbitrary temperature.
An actual doping operation is performed in the following manner. Boron is used as a dopant in the following example. (Other dopants may be used; that is, the dopant is changed in accordance with the purpose of a doping operation.)
In this embodiment, a 5%-dilution diborane gas is used. A diborane gas is introduced into the plasma source 501 of the doping apparatus, and a substrate having thin-film transistors in the midst of their manufacture, i.e., before doping is placed on the substrate holder 505 (see FIG. 1). At this time, the heater incorporated in the substrate holder 505 is so set as to provide a substrate temperature of 50 to 500° C. By keeping the substrate at a high temperature, the activation of a semiconductor substrate under doping can be accelerated.
Subsequently, the ion acceleration region is vacuumized, and the diborane gas is introduced therein from the plasma source 501. Immediately after application of a deceleration voltage of −1 kV from the deceleration power supply 504, an acceleration voltage of 27 kV is applied from the acceleration power supply 503 and this state is maintained for 5 seconds. Finally, a drawing voltage that is supplied from the drawing power supply 502 is continuously raised to 3 kV in 1 second.
As a result of the above operation, boron is implanted into the silicon oxide film (control film; including its surface region) and the silicon film with a profile shown in FIG. 2. FIG. 2 shows a concentration profile of boron that is implanted into the silicon oxide film and the silicon film, in which the horizontal axis represents the depth from the surface of the silicon oxide film and the vertical axis represents the boron concentration.
As seen from FIG. 2, a profile from a depth 1,200 Å to 1,700 Å is such that the dopant is implanted at 6×1016 to 6×1017 atoms/cm3. This profile is due to a light ion species BHx that mainly consists of one boron atom.
On the other hand, a profile from the surface to a depth 1,200 Å assumes a mountainous shape, and has a maximum dopant concentration of 7×1018 atoms/cm3.
This mountainous profile is due to a heavy ion species B2Hy that mainly consists of two boron atoms. This portion does not influence the characteristics of TFTs at all, because it corresponds to the silicon oxide film.
The reason why the silicon oxide film is formed is to dope the silicon film at a low concentration of about 1×1017 atoms/cm3. (It is difficult for currently available doping apparatuses to directly realize a low concentration of about 1×1017 atoms/cm3.)
The above concentration values are presented just for reference, and a dopant may be implanted at a desired dose in accordance with a purpose of doping. Experiments conducted by the inventors have revealed that in channel doping a dopant may be implanted at a lower dose than in source/drain doping, and that the threshold voltage control can be performed properly if the concentration falls within a range of 5×1015 to 5×1017 atoms/cm3.
By using the above-described doping technique, the concentration of a dopant implanted into a channel region can be made different from one transistor to another. Techniques necessary for that purpose will be described below.
1. A method for forming devices without channel doping and devices with channel doping on the same substrate.
This can be done simply by masking, during a doping operation, a region that should not be doped. Specifically, a resist film is formed on the above-mentioned silicon oxide film (control film), and then patterned. Since a region under the resist film is not doped, a portion to be doped and a portion not to be doped can be discriminated from each other. After the doping operation, the silicon oxide film and the resist film are removed. Any films other than a resist film can be used as long as they have the same function as the resist film.
2. A method for implanting the same kind of dopant into a plurality of regions at different concentrations.
This can be done by forming silicon oxide films (control films) for respective regions at different thicknesses, and conducting doping from above the thus-formed silicon oxide films. According to this method, the concentration of a dopant implanted into a silicon film that exists under a silicon oxide film can be made lower by making the silicon oxide film thicker. FIG. 5 shows a concentration profile of boron in which a silicon oxide film have two different thicknesses of 1,000 Å and 1,500 Å.
A doping step according to this method is performed under the same conditions as the above-described doping step. The doping apparatus shown in FIG. 1 is used, and a 5%-dilution diborane is used as a doping gas. The substrate temperature is kept at 50 to 500° C., and the diborane gas is introduced into the ion acceleration region that is being vacuumized. Immediately after a deceleration voltage of −1 kV is applied from the deceleration power supply 504, an acceleration voltage of 27 kV is applied from the acceleration power supply 503 and this state is maintained for 5 seconds. Finally, a drawing voltage that is supplied from the drawing power supply 502 is continuously raised to 3 kV in 1 second.
As a result of the above operation, boron is implanted into the silicon oxide film (control film; including its surface region) and the silicon film with a profile shown in FIG. 5. The boron concentration is about 1018 atoms/cm3 at surface A of the 1,000-Å-thick silicon oxide film and about 1017 atoms/cm3 at surface B of the 1,500-Å-thick silicon oxide film. In this manner, the dopant concentration in the silicon film can easily be controlled by varying the thickness of the silicon oxide film (control film).
To vary the thickness of the silicon oxide film, a resist film is formed on the silicon oxide film, patterned, and then etched to a desired thickness. The resist film is then removed, leaving a silicon oxide film having different thicknesses for respective regions.
3. A method for implanting different kinds of dopants into respective regions.
Method 1 is utilized. A simple procedure is to conduct method 1 two times, but it includes too many steps. Therefore, only a resist film is removed after the first doping operation, and a new resist film is formed. After the new resist film is patterned with a different pattern from the first one, and the second doping operation is performed.
Although the above three methods are intended to dope two regions differently, they may be used to dope three or more regions differently. These methods enable fine control of Vth's from one device to another.
After the doping step described above, thermal annealing is performed to repair lattice defects that have been produced by a large amount of dopant implanted into a semiconductor material.
Thereafter, a silicon film obtained in the above manner is processed to form gate insulating films, gate electrodes, source/drain electrodes, passivation films, pixel electrodes, etc. Thus, thin-film transistors having different threshold voltages can be formed on the same substrate.
Embodiment 2
A laser annealing step is inserted after the step of the first embodiment or performed instead of the thermal annealing, to further reduce lattice defects in the silicon oxide film, improve the crystallinity, and distribute the dopant uniformly over the entire substrate.
The silicon oxide film is removed by etching before the laser annealing.
First, a description will be made of a laser apparatus. FIG. 3 shows the concept of a laser annealing apparatus used in this embodiment. KrF excimer laser light (wavelength: 248 nm; pulse width 25 ns) is emitted from an oscillator 2. Apparently other excimer lasers and other types of lasers may also be used.
The laser light emitted from the oscillator 2 is reflected by full-reflection mirrors 5 and 6, amplified by an amplifier 3, reflected by full-reflection mirrors 7 and 8, and introduced into an optics 4. Immediately before entering the optics 4, the laser beam has a rectangular shape of 3×2 cm2. The laser beam is shaped by the optics 4 into a long and narrow beam (linear beam) of 10 to 30 cm in length and 0.1 to 1 cm in width. The laser beam as output from the optics 4 has a maximum energy of 1,000 mJ/shot.
The reason for shaping the laser beam into the long and narrow beam is to improve its processing ability. The linear laser beam output from the optics 4 is reflected by a full-reflection mirror 9, and applied to a sample 11. Since the laser beam is longer than the width of the sample, it can illuminate the entire sample 11 if the sample 11 is moved in one direction. Therefore, a sample stage/driving device 10 for the sample 11 is simple in structure and can be maintained easily. In addition, an alignment operation in setting the sample 11 can be performed easily.
The sample stage 10 which is to be illuminated with the laser beam is controlled by a computer, and is so designed as to move perpendicularly to the linear laser beam. The stage 10 is provided with a heater to keep the sample 11 at a prescribed temperature during laser beam illumination.
FIG. 4 is an optical path diagram inside the optics 4. The laser beam that has been input to the optics 4 passes through a cylindrical concave lens A, a cylindrical convex lens B, and horizontal and vertical flyeye lenses C and D, so that its original Gaussian distribution is converted into a rectangular distribution. Then, the laser beam is passed through cylindrical convex lenses E and F, reflected by a mirror G (which corresponds to the mirror 9 in FIG. 3), focused by a cylindrical lens H, and finally applied to the sample 11.
A specific laser beam illuminating operation is as follows. The laser beam is shaped into a rectangular form by the beam shape converting lenses to provide an illuminating beam area of 125 mm×1 mm. The sample 11, which is mounted on the stage 10, is moved at a rate of 2 mm/s to allow illumination of its entire surface.
As for the laser beam illumination conditions, two-step illumination consisting of preliminary illumination of 150 to 250 mJ/cm2 and main illumination of 200 to 380 mJ/cm2 and the pulse rate is set at 300 pulses/s. The reason for employing the two-step illumination is to minimize degradation of the uniformity of a film surface due to the laser beam illumination, to thereby produce a film having better crystallinity.
The substrate temperature is kept at 200° C. during the laser beam illumination to reduce the speed of increase and decrease of the substrate surface temperature due to the laser beam illumination. Although in this embodiment the substrate temperature is set at 200° C., in practice it is set at a temperature most suitable for laser annealing in a range of 100 to 600° C. No particular atmosphere control is performed; that is, the illumination is conducted in the air.
Instead of using laser light, strong light equivalent to laser light, for instance, infrared light may be applied by using an infrared lamp.
Embodiment 3
FIG. 6 shows a shift register circuit constituting a drive circuit of an active matrix display device which circuit can reduce leak current.
One output of the shift register is produced by one timing generation circuit and two voltage control circuit.
FFi−2 to FFi+2 are timing circuits, whose configuration is shown in FIG. 7. Each of the timing generation circuits FFi−2 to FFi+2 consists of one clocked inverter that is constituted of thin-film transistors having a low threshold voltage Vth and two inverters each constituted of thin-film transistors having a high threshold voltage Vth.
These thin-film transistors having different threshold voltages are ones that have been manufactured by the method of the first embodiment.
The timing generation circuits FFi−2 to FFi+2 generate scanning timing or image signal output timing of the active matrix display device.
In FIG. 7, clk is an operation clock signal for the timing generation circuit FFi, and *clk (logical inversion signal of clk) is an operation clock signal for the timing generation circuit FFi+1. Qi and Qi+1 are output signals of the timing generation circuits FFi and FFi+1, respectively, i.e., output signals of the shift register. *Qi and *Qi+1 are logical inversion signals of the signals Qi and Qi+1, respectively. The timing generation circuit FFi receives the signal Qi−1 and outputs the signals Qi and *Qi. The timing generation circuit FFi+1 receives the signal Qi and outputs the signals Qi+1 and *Qi+1.
VCi−2 to VCi+2 are voltage control circuits. FIG. 8 is a circuit diagram of the voltage control circuit VCi, which is an SR latch consisting of one two-input NAND having a high threshold voltage and one three-input NAND having a large threshold voltage. The voltage control circuit VCi controls separation from a power supply of the timing generation circuit FFi.
In FIG. 8, PONi is a signal for on/off-controlling a P-channel TFT that is connected to the timing generation circuit FFi, and NONi is a signal for on/off-controlling an N-channel TFT that is connected to the timing generation circuit FFi. *RESET is a signal that is input for a certain period after the power-on to prevent the voltage control circuit VCi (SR latch) from producing contradictory outputs and thereby fix the logic.
The operation of the above shift register circuit will be described below.
Upon reception of a pulse signal Qi−1, the ith-stage timing generation circuit FFi produces output signals Qi and *Qi that are clock-synchronized by the clocked inverter. The output signal Qi serves both as a timing signal for the active matrix display device and an input pulse for the timing generation circuit FFi+1 of the next stage, i.e., the (i+1)th stage.
The output signal *Qi is input to the voltage control circuit VCi+2, and makes output signals PONi+2 and NONi+2 active. As a result, the timing generation circuit FFi+2 is connected to the power supply.
At the same time, the output signal *Qi is input to the voltage control circuit VCi−2, which makes output signals PONi−2 and NONi−2 non-active. As a result, the timing generation circuit FFi−2, which has already produced an output signal, is separated from the power supply.
In this embodiment, where the shift register produces n-stage outputs, (n+2) timing circuits and (2n−4) voltage control circuits are needed for the following reason. To perform voltage control (separation from the power supply) of the (n−1)th-stage and nth-stage timing generation circuits, (n+1)th-stage and (n+2)th stage timing circuits are needed, which means (2n+4) voltage control circuits are necessary. However, to prevent oscillation of the shift register (timing control circuits) after the power-on, two voltage control circuits of each of the first-stage and second-stage (head portion) timing generation circuits and the (n+1)th-stage and (n+2)th-stage (tail portion) timing generation circuits should not be provided. Therefore, (2n−8), i.e., (2n+4) minus 8, voltage control circuits are sufficient.
In this embodiment, in the shift register circuit shown in FIG. 6, the TFTs of the voltage control circuits and the P-channel and N-channel TFTs connected to the voltage control circuits are constituted of TFTs having a high threshold voltage, and the timing control circuit (clocked inverter) is separated from the power supply by means of the thus-constructed voltage control circuits. Further, since the TFTs constituting the timing control circuit (clocked inverter) have a lower threshold voltage than the TFTs connected to the voltage control circuits, the leak current can be reduced from the conventional case. In addition, since the inverter of the timing generation circuit holds the output signal, no leak current flows through it, which contributes to reduction of the power consumption of the active matrix display device.
Embodiment 4
This embodiment is directed to an example of an arrangement of pixel TFTs having increased switching speed in an active matrix display device.
FIG. 9 shows a general configuration of a gate line drive circuit and pixel TFTs of an active matrix display device according to this embodiment.
A gate line drive circuit 901 is constituted of a series connection of inverters.
TFT1 to TFTn are N-channel pixel TFTs, which have been manufactured according to the method of the first embodiment so that threshold voltages Vth1 to Vthn of the respective TFTs satisfy a relationship Vth1≧Vth2≧ . . . ≧Vth(n−1)≧Vthn. That is, a pixel TFT more distant from the gate line drive circuit 901 has a lower threshold voltage.
With the above configuration, a TFT more distant from the gate line drive circuit 901 can be given a lower gate voltage. As a result, the charging time of the wiring capacitance of a gate line can be shortened, so that the pixel TFTs distant from the gate line drive circuit 901 can be turned on in shorter time than in the conventional case.
Thus, in the active matrix display device, the gate line drive circuit 901 can turn on the pixel TFTs in shorter time than in the conventional case.
As described above, according to the invention, it becomes possible to reduce leak current and therefore power consumption of a thin-film semiconductor integrated circuit made of crystalline silicon and, in particular, a peripheral drive circuit of an active matrix display device.
It becomes possible to form, on the same substrate, TFTs having two or more different threshold voltages.
Further, in pixel TFTs of the display portion of an active matrix display device, pixel TFTs distant from a gate line drive circuit can be turned on in shorter time than in the conventional case.
In summary, the invention makes it possible to improve the circuit characteristics by controlling the threshold voltages Vth of thin-film transistors made of crystalline silicon.

## Claims

1. A method of manufacturing a semiconductor device comprising the steps of:
forming a crystalline semiconductor film on an insulating surface;
forming an insulating film on said crystalline semiconductor film;
introducing a dopant impurity into said crystalline semiconductor film through said insulating film by an ion doping;
annealing said crystalline semiconductor film to repair lattice defects caused by the introduction of the dopant impurity;
forming a gate electrode over said insulating film, and
forming a channel region in the doped region of the crystalline semiconductor film,
wherein a peak of a concentration profile of said dopant impurity is located in said insulating film.

2. A method according to claim 1 wherein said insulating film comprises silicon oxide.

3. A method according to claim 1 wherein said dopant impurity is boron.

4. A method according to claim 1 wherein said crystalline semiconductor film comprises polycrystalline silicon.

5. A method according to claim 3 wherein said boron is supplied by diborane gas.

6. A method according to claim 1 further comprising a step of removing said insulating film.

7. A method according to claim 1 wherein said semiconductor device comprises an active matrix display device having thin-film transistors.

8. A method according to claim 1 wherein said semiconductor device comprises a shift register circuit having thin-film transistors.

9. A method according to claim 1 further comprising a step of irradiating a laser light to said crystalline semiconductor film.

10. A method according to claim 1 wherein forming said insulating film on said crystalline semiconductor film comprises controlling a thickness of said insulating film so that the peak of the concentration profile of the dopant impurity is located in said insulating film.