JPH0992836A - Polysilicon thin film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the separation of a gate electrode and a gate insulating film in a laser-activating process by computing the transmittance of laser light reaching a polysilicon layer at the time of being laser-activated to find the film thickness conditions of a gate insulating film and a layer insulating film on the basis thereof. SOLUTION: For the analysis of a laser-activating process, the transmittance of laser light is computed by using the data of the complex refractive index of a monolithic crystal silicon as the value of the complex refractive index of a polysilicon layer on the basis of a model in which the interference effect of a thin film is taken into consideration. In the laser-activating process, the total film thickness (d) of a gate insulating film and a layer insulating film in which the transmittance of the laser light reaching the polysilicon layer through the layer insulating film and the gate insulating film exhibits the maximum width is found from a formula, where the wavelength of laser light is represented by λ, the complex refractive index of a monolithic crystal silicon is represented by n1 +ik1 and the refractive index of the gate insulating film and the layer insulating film is represented by n2 . Therefore, the mobility of a polysilicon thin film transistor can be ensured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a top gate type polysilicon thin film transistor used for a switching element of a liquid crystal display panel.

[0002]

^2. Description of the Related Art A polysilicon thin film transistor is used as a switching element or a driving circuit element in a pixel portion of an active matrix liquid crystal display panel because it has a high mobility of several tens to several hundreds cm ² / Vs. .

FIG. 6 shows a typical cross-sectional shape of a polysilicon thin film transistor having a top gate structure. The polysilicon thin film transistor includes an undercoat layer 12, a channel polysilicon layer 13, a source / drain region 14, a gate insulating film 15, a gate electrode 16, an interlayer insulating film 17, a source formed on the surface of a glass substrate 11. It is formed by sequentially stacking the drain wiring layer 18 and the like.

As a method of manufacturing a polysilicon thin film transistor, a laser annealing method as a method of forming the channel polysilicon layer 13 or a solid phase growth method using heat,
Known methods of forming the source / drain regions 14 include a method of activating laser after ion implantation, and a method of thermally activating after ion implantation. The method of using a laser for forming the channel polysilicon layer 13 and the source / drain regions is a low temperature process, and since an inexpensive glass substrate can be used, a liquid crystal display panel using a polysilicon thin film transistor is manufactured. This is a very effective technique.

FIG. 7 shows a manufacturing process of a top gate type polysilicon thin film transistor using a laser annealing method and a laser activation method. The polysilicon thin film transistor is formed by the following steps. (1) An undercoat layer 12 of SiO ₂ is formed on a glass substrate 11, and an amorphous Si film is formed on the undercoat layer 12.
Then, the amorphous Si film is crystallized by laser annealing to be changed into a polysilicon film (FIG. 7A). (2) After patterning the polysilicon film to form the channel polysilicon layer 13, the gate insulating film 15 is formed of SiO ₂ , and then the gate electrode 16 is formed (FIG. 7B). (3) Ion implantation is performed using the gate electrode 16 as a mask to form the source / drain regions 14 in the channel polysilicon layer 13 (FIG. 7C). As the ion implantation, there are an ion implantation method with mass separation and an ion doping method without mass separation. (4) Form the SiO ₂ interlayer insulating film 17 (see FIG. 7).
(D)). (5) From above the gate insulating film 15 and the interlayer insulating film 17,
Laser activation of the source / drain regions 14 is performed (FIG. 7).
(E)). Note that the steps (4) and (5) may be exchanged in order to perform laser activation from above the gate insulating film 15 before forming the interlayer insulating film 17. (6) After opening the contact hole, the source / drain wiring layer 18 is formed (FIG. 7F).

Through the above steps, a polysilicon thin film transistor is formed. In the conventional polysilicon thin film transistor, the gate insulating film made of SiO ₂ has a thickness of about 200 [nm] or less, and the interlayer insulating film made of SiO ₂ has a thickness of about 100 to 1000 [nm]. This value has been set so as to satisfy the requirements for the design of the display panel, such as the auxiliary capacitance and the capacitance coupling between the signal line and the gate line. For example,
As a typical value, the thickness of the gate insulating film is 100 [nm],
The thickness of the interlayer insulating film was 400 [nm].

In this case, since the reflection from the gate insulating film and the interlayer insulating film on the polysilicon layer is large during laser activation, it is necessary to increase the laser irradiation power.
Thermal damage to the gate electrode was large, and peeling easily occurred between the gate insulating film and the gate electrode. It is effective to thicken the gate electrode to prevent peeling, but this causes a problem because the aperture ratio of the pixel portion is reduced. In addition, the damage to the susceptor due to the light transmitted through the substrate during the laser activation is large, and there is a problem that impurities are mixed from the susceptor or the life of the susceptor is shortened. In terms of transistor characteristics, there is a high probability that defects with low ON current will appear, and even good products will have a mobility of 10 to 100 cm ² / V.
However, the performance as a drive circuit element was not sufficient. The above problems directly cause a reduction in the yield of liquid crystal display panels.

[0008]

As described above, in the conventional polysilicon thin film transistor, peeling easily occurs between the gate insulating film and the gate electrode at the time of laser activation, and in addition, in terms of characteristics of the transistor. There are various problems such as a high probability that a defect in which the ON current becomes low appears, and that there is a large variation in characteristics even with a good product.

According to the present invention, the transmittance of laser light that reaches the polysilicon layer through the interlayer insulating film and the gate insulating film at the time of laser activation is calculated using a model considering the interference effect of the thin film, Based on the result, a condition for the film thickness of the gate insulating film and the interlayer insulating film that does not impair the adhesion between the gate insulating film and the gate electrode is obtained, and a high-yield and high-performance polysilicon thin film transistor is provided. With the goal.

[0010]

A polysilicon thin film transistor according to the present invention comprises a channel polysilicon layer on an insulating substrate, source / drain regions formed in the channel polysilicon layer, a gate insulating film, a gate electrode, and an interlayer insulating film. And a source / drain wiring layer in order, and the source / drain region is formed by ion implantation after formation of the gate electrode and activation by laser irradiation after formation of the interlayer insulating film. In the silicon thin film transistor, the laser wavelength in the activation step is λ [nm], the complex refractive index of single crystal silicon at the wavelength is n ₁ + ik ₁ , and the refractive index of the gate insulating film and the interlayer insulating film is n ₂ [nm]. Then, the total film thickness d [nm] of the gate insulating film and the interlayer insulating film is represented by Formula 4. .

[0011]

[Equation 4]

However, L is an integer of 0 or more, | Δ | ≦ 30,
i represents an imaginary unit, Arg (Z) is a complex number Z = r ·
Arg (Z) = θ, 0 ≦ θ <with respect to exp (iθ)
2π, which is a function.

As the complex refractive index of the above single crystal silicon, Aspne, D. E. FIG. Data (Phys.
Rev. B, Vol.27, No.2, 1983, 985). Also,
By setting | Δ | ≦ 10 in Expression 4, a larger mobility can be secured for the polysilicon thin film transistor.

The absolute value of the total film thickness d of the gate insulating film and the interlayer insulating film is 300 [nm] or more and 1500
[Nm] or less is suitable. In addition, the gate insulating film and the interlayer insulating film are made of SiO ₂ , and the wavelength λ =
When a 308 [nm] XeCl excimer laser is used, Formula 1 can be represented by d = (47 + 104L) ± Δ; where | Δ | ≦ 30 and L represents an integer of 3 or more and 14 or less; .

Further, by setting | Δ | ≦ 10 in the above equation, a larger value can be secured as the mobility of the polysilicon thin film transistor. The above conditions indicate the optimum range of the total film thickness d of the insulating film in the step of activating the laser irradiation from above the interlayer insulating film and the gate insulating film.

On the other hand, in the case of the step of performing activation by laser irradiation from above the gate insulating film before the formation of the interlayer insulating film, the film thickness t [nm] of the gate insulating film is expressed by the equation 5. The value.

[0017]

(Equation 5)

However, L is an integer of 0 or more and | Δ | ≦ 10. In this case, the thickness t [nm] of the gate insulating film
The absolute value of is preferably 40 [nm] or more and 300 [nm] or less.

The gate insulating film and the interlayer insulating film are made of Si.
O ₂ and wavelength λ = 308 [nm] in the activation step
In the case of using the XeCl excimer laser of, the formula 5 is: t = (47 + 104L) ± Δ; where | Δ | ≦ 10 and L represents an integer of 0 or more and 2 or less;
Can be represented by

The meanings of equation 4 (equation 1 and equation 2) and equation 5 (equation 3) will be described below. When calculating the transmittance of laser light based on the model considering the interference effect of the thin film, if the data of the complex index of refraction of single crystal silicon is used as the value of the complex index of refraction of the polysilicon layer, the interlayer activation in the laser activation process will be performed. The film thickness d at which the transmittance of the laser light reaching the polysilicon layer through the insulating film and the gate insulating film shows the maximum value
Is represented by Equation 4.

Since there is a clear correspondence between the transmittance of the laser light and the mobility of the polysilicon thin film transistor as will be shown later, the total value of the film thicknesses of the interlayer insulating film and the gate insulating film is taken as the laser light. By setting the transmittance of 1 to the maximum value, it is possible to secure sufficient mobility of the polysilicon thin film transistor with a relatively small laser irradiation power. Therefore, the source / drain regions can be activated without lowering the yield due to peeling between the gate insulating film and the gate electrode.

Further, in the case where the laser activation is performed from above the gate insulating film before the formation of the interlayer insulating film, when the film thickness t of the gate insulating film is under the condition expressed by the equation 5, The transmittance shows the maximum value. Therefore, by setting the film thickness of the gate insulating film in the vicinity of the value expressed by Equation 5, it becomes possible to secure sufficient mobility of the polysilicon thin film transistor even with a relatively small laser irradiation power. .

The upper and lower limits of the thickness of the gate insulating film are
The transistor performance is determined by the range satisfying the predetermined specifications, and the range of 40 [nm] to 300 [nm] is suitable. Further, the upper and lower limit values of the thickness of the interlayer insulating film are determined by a range in which stable film formation can be performed without defects in the process, and about 260 [nm] to 1200 [nm] is suitable. Therefore, the range of the total value of the film thicknesses of the interlayer insulating film and the gate insulating film is preferably about 300 to 1500 [nm].

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a structural cross-sectional view of a polysilicon thin film transistor according to the present invention. Glass substrate 1
A polysilicon layer for forming a channel polysilicon layer 13 and a source / drain region 14 is laminated on the surface of 1 through an undercoat layer 12, and a gate insulating film 15 is laminated on this polysilicon layer to form a gate insulating film. A gate electrode 16 is formed on the film 15. The source / drain region 14 in the polysilicon layer is the gate electrode 1
6 is formed so as to match with 6. An interlayer insulating film 17 is stacked on the gate electrode 16, and the source / drain wiring layer 18 is connected to the source / drain region 14 through a contact hole formed in the interlayer insulating film 17 and the gate insulating film 15. are doing. In the figure, d represents the total film thickness of the gate insulating film 15 and the interlayer insulating film 17, and t represents the film thickness of the gate insulating film 15.

FIG. 2 shows an example of a sectional view of a liquid crystal display panel manufactured by using the polysilicon thin film transistor according to the present invention. The circuit driving unit 21 is configured by coupling n-type 22 and p-type 23 transistors to each other by the source / drain wiring 18, and the pixel unit 24 couples the pixel electrode 19 and the n-type polysilicon thin film transistor 25. At the same time, the auxiliary capacitance line 26 is arranged at the coupling portion of both.

Since the manufacturing process itself of the polysilicon thin film transistor according to the present invention is the same as that used in the description of the prior art (FIG. 7), its description is omitted. The feature of the present invention is that the film thickness conditions of the gate insulating layer and the interlayer insulating layer that enable laser activation with a relatively small laser irradiation power in the laser activation step shown in FIG. Where it is.

In the polysilicon thin film transistor according to the present invention, the laser wavelength at the time of laser activation is λ [nm], and the complex refractive index of single crystal silicon with respect to this wavelength is n ₁ + i.
When k ₁ and the refractive index of the gate insulating film and the interlayer insulating film are n ₂ , the total value d of the film thicknesses of the gate insulating film and the interlayer insulating film
[Nm] satisfies the condition of Expression 4 above.

Equation 4 is the complex index data (Aspne. DE; Phys. Rev. B, Vol. 27, No. 2, 1983, data of complex index of refraction for single crystal silicon as the value of the complex index of the polysilicon layer.
985) is used to calculate the transmittance of laser light based on a model that considers the interference effect of the thin film, the interlayer insulating film and the gate insulating film are formed in the laser activation step (FIG. 7 (e)). It represents the value of the film thickness at which the transmittance of the laser light reaching the polysilicon layer through reaches its maximum value.

Next, there is a clear correspondence between the calculation result of the transmittance of the laser light considering the interference effect under the assumption of the complex refractive index and the measured value of the mobility of the polysilicon thin film transistor. Explain that.

In FIG. 3, the data of the complex refractive index of the above-mentioned single crystal silicon is used as the value of the complex refractive index of the polysilicon layer, and Si is used as the gate insulating film and the interlayer insulating film.
The result of calculation of the relationship between the film thickness of the insulating film and the transmittance of laser light when O ₂ is used and a XeCl excimer laser (λ = 308 [nm]) is used in the laser activation step is shown. As shown in FIG. 3, the transmittance of laser light fluctuates greatly periodically with respect to the film thickness of the insulating film. In FIG.
The value of the total film thickness of the gate insulating film and the interlayer insulating film that maximizes the transmittance of laser light is d = (47 + 104L) [n
m]; provided that L represents an integer of 0 or more.

The above formula is obtained by substituting the refractive index of SiO ₂ and the wavelength of the laser beam into the general formula represented by Formula 4. As shown in FIG. 4, a SiO ₂ film is used as a gate insulating film and an interlayer insulating film, and a XeCl excimer laser (λ = 308 [n
m]) is used, the measurement result of the dependence of the mobility of the polysilicon thin film transistor on the total film thickness of the insulating film is shown. The laser irradiation power used here is 175 mJ.
/ Cm ² . The laser irradiation power is 175 mJ
If it exceeds / cm ² , the peeling of the gate wiring becomes remarkable, so that a laser irradiation power exceeding this value is not practical.

As shown in FIG. 4, the mobility of the polysilicon thin film transistor is such that the total film thickness of the insulating film is d = (47+
104L) [nm]; provided that L represents an integer of 0 or more;
In the above, the maximum value is shown, and it is understood that there is a clear correspondence between the calculation result of the transmittance of the laser light based on the above assumption of the complex refractive index and the actually measured value of the mobility of the polysilicon thin film transistor.

By using a film thickness within the range of ± 10 [nm] centering on the film thickness corresponding to the maximum value of the transmittance of the laser light, a mobility of 100 cm ² / Vs or more can be obtained, and , By using a film thickness in the range of ± 30 [nm], a mobility of 30 cm ² / Vs or more can be obtained.

Depending on the driving method, the mobility is 30 cm ² /
If the value is Vs or more, NT, which is one of the standard television systems
The mobility is sufficient for creating a display compatible with SC (National Television System Committee). Further, if the mobility is a value of 100 cm ² / Vs or more, the mobility is sufficient for producing a display compatible with high-definition television (HDTV).

The above is an example of the case where laser activation is performed from above the interlayer insulating film. Next, in the case where laser activation is performed from above the gate insulating film before the formation of the interlayer insulating film. An example of conditions for the optimum value of the film thickness of the interlayer insulating film will be described.

In the case of a polysilicon thin film transistor manufactured by this process, the laser wavelength at the time of laser activation is λ [n
m], the complex index of refraction of single crystal silicon for this wavelength is n ₁ + ik ₁ , and the index of refraction of the gate insulating film and the interlayer insulating film is n ₂ , the film thickness t [nm] of the gate insulating film is given by
Set to satisfy the condition of.

In FIG. 5, when the SiO ₂ film is used as the gate insulating film and the interlayer insulating film, and the XeCl excimer laser (λ = 308 [nm]) is used in the laser activation step, the film thickness t of the gate insulating film is t. The measurement result of the dependence of the mobility of the polysilicon thin film transistor with respect to is shown. The laser irradiation power used here is 125 mJ / cm ² . When the laser irradiation power exceeds 125 mJ / cm ² , the peeling of the gate wiring becomes remarkable, so that the laser irradiation power exceeding this value is not practical.

As shown in FIG. 5, the mobility of this polysilicon thin film transistor is such that the thickness of the gate insulating film is t =
(47 + 104 L) [nm], where L represents an integer of 0 or more; and also in this case, the calculation result of the transmittance of laser light based on the above assumption of complex refractive index and the polysilicon thin film transistor It can be seen that there is a clear correspondence with the measured value of the mobility of.

A mobility of 30 cm ² / Vs is obtained by using a film thickness within a range of ± 10 [nm] centering on the film thickness corresponding to the maximum value of the laser light transmittance represented by this formula.
The above values are obtained.

[0040]

In the present invention, first, in order to analyze the laser activation process, the transmittance of laser light is calculated based on a model considering the interference effect of the thin film, and at this time, the complex refraction of the polysilicon layer is calculated. The data of the complex refractive index of single crystal silicon was used as the value of the index. Based on this assumption, in the laser activation process, the film thickness at which the transmittance of the laser light reaching the polysilicon layer through the interlayer insulating film and the gate insulating film shows the maximum value was obtained. Next, it was confirmed that there is a clear correspondence between the transmittance of the laser light calculated in this way and the measured value of the mobility of the polysilicon thin film transistor.

Based on this result, the total value of the film thicknesses of the gate insulating film and the interlayer insulating film was made to coincide with the range in which the transmittance of the laser light shows the maximum value, so that the laser irradiation power is relatively small. However, it has become possible to secure sufficient mobility of the polysilicon thin film transistor. As a result, the source / drain regions can be activated without lowering the yield due to peeling between the gate insulating film and the gate electrode, and at the same time, high-performance polysilicon with small variation in characteristics can be obtained. It has become possible to design thin film transistors.

When performing laser activation from above the gate insulating film before forming the interlayer insulating film, the film thickness of the gate insulating film should be made to coincide with the range where the transmittance of the laser light shows the maximum value. Thereby, the same effect can be achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a structural cross-sectional view of a polysilicon thin film transistor according to the present invention.

FIG. 2 is a diagram showing an example of a cross-sectional view of a liquid crystal display panel manufactured using a polysilicon thin film transistor according to the present invention.

FIG. 3 is a diagram showing an analysis result of a relationship between a film thickness of an insulating film and a transmittance of laser light.

FIG. 4 is a diagram showing a measurement result of dependence of mobility of a polysilicon thin film transistor on a total film thickness of an interlayer insulating film and a gate insulating film.

FIG. 5 is a diagram showing a measurement result of dependence of mobility of a polysilicon thin film transistor on a film thickness of a gate insulating film.

FIG. 6 is a diagram showing an example of a structural cross-sectional view of a conventional polysilicon thin film transistor.

FIG. 7 is a diagram illustrating a manufacturing process of a polysilicon thin film transistor. (A)-(f) represents the order of a process.

[Explanation of symbols]

11 ... Glass substrate, 12 ... Undercoat layer,
13 ... Channel polysilicon layer, 14 ... Source / drain region, 15 ... Gate insulating film, 16 ...
Gate electrode, 17 ... Interlayer insulating film, 18 ... Source / drain wiring layer, 19 ... Pixel electrode, 20 ... Protective film, 21 ... Drive circuit section, 22 ... N-type transistor , 23 ... P-type transistor, 24 ... Pixel portion, 25 ... N-type transistor, 26 ... Auxiliary capacitance line.

Claims (8)

[Claims] 1. A channel polysilicon layer, a source / drain region formed in the channel polysilicon layer, a gate insulating film, a gate electrode, an interlayer insulating film, and a source / drain wiring layer are sequentially provided on an insulating substrate, and The source / drain regions are formed in the top gate type polysilicon thin film transistor formed by ion implantation after formation of the gate electrode and activation by laser irradiation after formation of the interlayer insulating film. Is λ [nm], and the complex refractive index of single crystal silicon at the wavelength is n ₁ + ik
₁ , the refractive index of the gate insulating film and the interlayer insulating film is n ₂ [n
m], the total film thickness d [nm] of the gate insulating film and the interlayer insulating film is represented by Formula 1. [Equation 1] However, L is an integer of 0 or more, | Δ | ≦ 30, i represents an imaginary unit, and Arg (Z) is a complex number Z = r · exp (i.
.theta.), Arg (Z) =. theta., 0.ltoreq..theta. <2.pi. 2. The polysilicon thin film transistor according to claim 1, wherein a total film thickness d [nm] of the gate insulating film and the interlayer insulating film is represented by Formula 2. [Equation 2] However, L is an integer of 0 or more, | Δ | ≦ 10, i represents an imaginary unit, and Arg (Z) is a complex number Z = r · exp (i.
.theta.), Arg (Z) =. theta., 0.ltoreq..theta. <2.pi. 3. The polysilicon thin film transistor according to claim 1, wherein the total film thickness d of the gate insulating film and the interlayer insulating film is 300 [nm] or more and 1500 [nm] or less. 4. The gate insulating film and the interlayer insulating film are SiO ₂
In the activation step, X of wavelength λ = 308 [nm]
Using an eCl excimer laser, the total film thickness d [nm] of the gate insulating film and the interlayer insulating film is d = (47 + 104L) ± Δ; where | Δ | ≦ 30 and L represents an integer of 3 or more and 14 or less; The polysilicon thin film transistor according to claim 1, which is represented by the following formula. 5. The gate insulating film and the interlayer insulating film are SiO ₂
In the activation step, X of wavelength λ = 308 [nm]
Using an eCl excimer laser, the total film thickness d [nm] of the gate insulating film and the interlayer insulating film is d = (47 + 104L) ± Δ; where | Δ | ≦ 10 and L represents an integer of 3 or more and 14 or less; The polysilicon thin film transistor according to claim 2, which is represented by: 6. A channel polysilicon layer, a source / drain region formed in the channel polysilicon layer, a gate insulating film, a gate electrode, an interlayer insulating film, and a source / drain wiring layer are sequentially provided on an insulating substrate, and The source / drain regions are formed in the top gate type polysilicon thin film transistor formed by ion implantation after formation of the gate electrode and activation by laser irradiation. The laser wavelength in the activation step is λ [nm], The complex index of refraction of single crystal silicon of n ₁ + ik
₁ , the refractive index of the gate insulating film and the interlayer insulating film is n ₂ [n
m], a film thickness t [nm] of the gate insulating film is represented by Formula 3. (Equation 3) However, L is an integer of 0 or more, | Δ | ≦ 10, i represents an imaginary unit, and Arg (Z) is a complex number Z = r · exp (i.
θ), a function that satisfies Arg (Z) = θ and 0 ≦ θ <2π. 7. The film thickness t [nm] of the gate insulating film is 40.
7. The polysilicon thin film transistor according to claim 6, wherein the thickness is not less than [nm] and not more than 300 [nm]. 8. The gate insulating film is SiO ₂ , and a XeCl excimer laser having a wavelength λ = 308 [nm] is used in the activation step, and the film thickness t [nm] of the gate insulating film is t = (47 + 104 L). ± Δ; where | Δ | ≦ 10 and L represents an integer of 0 or more and 2 or less;
7. The polysilicon thin film transistor according to claim 6, which is represented by: