KR20010112417A - Method and device for treating thin layers of amorphous silicon - Google Patents

Abstract

The present invention relates to the field of treating the thin film by energy input in order to modify the selected physicochemical properties of the hydrogenated amorphous silicon thin film. The invention is characterized in that it comprises irradiating the thin film with a light source suitable for emitting radiation of wavelengths lying within the optical absorption band of amorphous silicon and capable of producing a processing pulse lasting from 50 ns to 500 ns. As a result, the electron mobility μ of the thin film may be increased by 1.5 to 10 times.

Description

Method and device for treating thin layers of amorphous silicon

In order to produce an electronic component using an amorphous silicon thin film and a transistor for a flat screen using an active liquid crystal matrix, in particular, depositing at least one amorphous silicon thin film, in particular a hydrogenated amorphous silicon thin film, on a substrate made of a predetermined material It is common for the first step to be performed. The deposited thin film has a crystal structure in which some silicon atoms share an electron bond with a hydrogen atom.

The disadvantage of this crystal structure is that its electron mobility is low. In particular in the aforementioned applications, it is recommended to increase the electron mobility, for example when trying to increase the size of a flat screen.

In order to increase the electron mobility, it is known to anneal the hydrogenated amorphous silicon thin film in the first step. During annealing, the substrate is heated to near 400 ° C. for about 2 to 5 hours, so that hydrogen desorbs from the thin film. This annealing step is followed by irradiating the thin film with a light source, typically a laser source, to quickly melt the amorphous silicon. The amorphous silicon then solidifies and partially crystallizes. Commonly used lasers emit pulses whose length does not exceed 50 ns.

However, in this known method, it is disadvantageous in that a material such as plastic, which is suitable as a substrate for flat screens, cannot withstand such a treatment because of its low cost.

Another disadvantage of the known method of using laser irradiation as described above is that the electron mobility is generally increased to near 50 times or more. This is because the treated thin film is crystallized considerably. Since this electron mobility causes leakage current, it is recommended to modify the structure of the transistor to reduce it. For example, it may be by a method of depositing an additional thin film that can limit such leakage current.

Other types of injecting energy to modify at least one selected physicochemical property of an amorphous silicon thin film are also known. This involves irradiating the thin film with one or more lamps that emit radiation in the selected wavelength range (ultraviolet rays for raising the thin film and infrared rays for raising the substrate). The pulse length from this ramp cannot actually be less than 500 ns. Thus, the temperature of the thin film is raised to approximately 1200 ° C. for several seconds. As pointed out above, most of the glass and plastic materials cannot withstand these temperatures, although they are preferred as substrate materials.

The present invention relates to the treatment of electronic components using amorphous silicon thin films. The present invention is generally applied to the production of electronic components using amorphous silicon thin films and especially flat screen transistors using an active liquid crystal matrix.

1A is a diagram showing the linear chain atomic arrangement of SiH ₂ bonds in untreated hydrogenated amorphous silicon.

FIG. 1B shows a planar atomic arrangement of SiH bonds in hydrogenated amorphous silicon treated according to the method of the present invention. FIG.

FIG. 2 is a graph showing the energy (x-axis) to be supplied to amorphous silicon in order to increase the electron mobility (y-axis) of the material, quantitatively indicating the size of the crystallized silicon grain in the amorphous continuum.

FIG. 2A shows the grain density and size in silicon crystallized in accordance with the present invention. FIG.

FIG. 2B shows the grain size and shape in crystalline silicon obtained by a known method of partially crystallizing amorphous silicon.

FIG. 3 is a diagram showing a structure of a transistor based on an amorphous silicon semiconductor thin film and integrated into an active flat panel matrix.

4 is a view showing a detailed structure of a transistor produced by applying the present invention.

5 shows an apparatus according to the invention for irradiating and treating an amorphous silicon thin film.

6 is a view showing an absorption curve of amorphous silicon.

7 shows a profile of a light pulse with respect to time, with solid lines from light sources belonging to the device according to the invention and dashed lines from laser sources belonging to known devices.

The present invention provides a solution to these problems.

The present invention comprises the steps of: a) obtaining at least one amorphous silicon thin film deposited on a substrate of a predetermined material and comprising selected elements other than silicon; And

b) processing the thin film thus obtained by applying energy to modify at least one selected physicochemical property of the thin film.

In a general definition of the invention, said processing step b) is a light source (b1) suitable for emitting radiation of wavelengths lying at least within an optical absorption band of amorphous silicon and capable of producing a processing pulse of length of at least 50 ns to 500 ns ( providing a light source; And

b2) illuminating the amorphous silicon thin film containing the element with the light source to substantially increase the electron mobility of the thin film by 1.5 to 10 times.

This treatment step can be advantageously carried out even if the thin film comprises elements other than silicon, such as for example hydrogen. The thin film thus treated has a crystal structure in which very little or no crystallization of silicon is carried out, and the electron mobility according to this structure is increased by about three times the electron mobility of the initial hydrogenated amorphous silicon. Is sufficient for the production of transistors for flat screens. In addition, there is an advantage that the annealing step included in the known production method is omitted. Thus, the amorphous silicon layer can be deposited and then processed as soon as it is separated from the deposition equipment.

According to an important feature of the invention, the profile of the treatment pulse with respect to time exhibits a half-height width of substantially 20 ns to 250 ns. It also has an increase time of substantially 5 ns to 70 ns and a decay time of substantially 15 ns to 200 ns.

Since the length of the processing pulse is, for example, at a level of 200 ns, even if the thin film still contains hydrogenated amorphous silicon before irradiation, the electron mobility of the thin film is increased by about three times.

In practice, the thickness of the thin film is substantially 50 nm to 150 nm, and the energy density of the light source is substantially 0.2 J / cm ² to 0.9 J / cm ² . The light source is a gas excimer laser using xenon and chlorine. The emission wavelength of the light source is close to 308 nm.

The invention also relates to the method described above in the manufacture of electronic components comprising at least one amorphous silicon thin film comprising selected elements other than silicon, for example hydrogen, and in particular for flat screen transistors using an active liquid crystal matrix. It is related to application.

The present invention is also directed to emitting radiation of a wavelength that lies within the optical absorption band of amorphous silicon with pulses of at least substantially 50 ns to 500 ns in length to substantially increase the electron mobility of the thin film by 1.5 to 10 times. It relates to a device comprising a light source suitable for.

Other advantages and features of the present invention will become apparent from the following detailed description and the accompanying drawings.

The following detailed description and the annexed drawings mainly cover those of defined nature. Thus, it not only makes the present invention better understood, but also helps define the present invention when necessary.

The quality of transistors is mobility μ (cm ² / Vs), the ratio of on-state current to off-state current I _on / I _off , threshold voltage V _th (V), loss under threshold S (V / decade) ) Are evaluated from the basic parameters.

Amorphous silicon transistors are generally regarded as inferior transistors. Its mobility is on the level of 0.3 to 0.6 cm ² / Vs, the threshold voltage is 1 to 2 V, and I _on / I _off is in the range of 10 ⁶ to 10 ⁸ . Despite these relatively inferior performance figures, flat screens using an active liquid crystal matrix having a diagonal of 10 to 15 inches (25.4 to 38.1 cm) can be manufactured without additional effort. This size matrix is compatible with models that conform to the video graphic array (VGA) standard with 400 to 480 x 640 x 3 pixels or the extended graphic array (XGA) standard with 1024 to 1066 x 768 x 3 pixels. In this type of screen the pitch of the pixels is generally close to 0.3 mm and each pitch increment consists of three pixels.

On the other hand, since these transistors have a low electron mobility μ, they need a large enough area (typically 10 μm × 10 μm) to maintain satisfactory electrical characteristics. For example, in order for such a screen to be introduced more widely, only the size of the pixels can be reduced. As a result, the ratio of useful area and total area of the pixel (referred to as 'aperture ratio') is reduced. The light transmitted through the screen is then substantially lower than a given level of energy entered.

Moreover, if the diagonal length of the screen is increased, it may not have the screen refresh rate of 30 Hz that can be obtained from a normal VGA or XGA screen. The mobility of 1 cm ² / Vs is sufficient for a 15 inch diagonal. However, for 20 inches, the mobility should be close to 2 cm ² / Vs. Currently known thin film transistors of hydrogenated amorphous silicon do not have this mobility.

However, it is known that the electron mobility of amorphous silicon is increased to about 50 times or more by converting amorphous silicon into polycrystalline silicon. Commonly used methods include thin filmed hydrogenated amorphous silicon in an oven at sufficiently high temperatures (near 400 ° C.) for 2-5 hours to desorb most of the hydrogen initially present in the material. The amorphous silicon thus obtained is injected with sufficient energy to crystallize part of the silicon. In practice, the material is irradiated with a laser source that provides a high energy relatively short pulse (near 50 ns). The material so irradiated melts and then partially crystallizes as it turns into a solid phase. Overall, it is seen as microcrystals of silicon in an amorphous continuum (FIG. 2A).

Due to the high mobility of the material obtained (at least 50 cm ² / Vs), I _on / I _off of the polycrystalline silicon thin film transistor is substantially increased. Because of the leakage current then it is necessary to provide additional steps in the method of manufacturing such transistors.

In particular, Applicants have a high mobility (preferably around 3 cm ² / Vs) hydrogenated amorphous, which can be deposited on a substrate made of glass or plastic material, but does not require additional insulating films for the desired electron mobility. Efforts have been made to obtain silicon.

Thin film silicon is typically deposited in the gas phase in hydrogen or another selected elemental atmosphere, such as, for example, carbon. However, to produce thin film transistors, good grades of amorphous silicon are generally obtained by deposition in a hydrogen atmosphere. The amorphous silicon thus deposited contains a negligible amount (10-15%) of hydrogen atoms, leaving dangling bonds (lattice cells not saturated with electrons) in the material. Hydrogenated amorphous silicon is obtained by breaking SiH ₄ bonds under selected pressure (and partial pressure of other elements) and temperature conditions. The thin film has a crystal structure in which a part of silicon atoms share an electron bond with a hydrogen atom (FIGS. 1A and 1B).

Locally, the unprocessed hydrogenated amorphous silicon thin film has an atomic arrangement of the type as shown in FIG. 1A. In Figure 1a we see a linear chain type of SiH ₂ bond. The energy input from the laser capable of providing a sufficiently long pulse (e.g., longer than 120 ns) in accordance with the invention can locally break the SiH ₄ bond. The hydrogen atom H then recombines with the silicon atom Si to form a SiH type bond, which produces a planar atomic arrangement as shown in FIG. 1B. The change from the arrangement of linear chain atoms to the arrangement of planar atoms in themselves increases the electron mobility of the material. Indeed, a first result according to the invention is obtained by irradiating a film to be treated with an excimer laser which usually emits energy of 0.2 to 0.9 J / cm ² . The increase in mobility is close to 1.5 times.

In addition, irradiating the hydrogenated amorphous silicon thin film with a light source providing a long pulse in accordance with the present invention can further increase the electron mobility of the material by encouraging its explosive crystallization. "Explosive crystallization" here means very localized crystallization of amorphous silicon. Applicants have, in fact, surprisingly noted that the use of light sources that provide pulses with long durations (eg 170 ns level or more) encourages explosive crystallization in amorphous silicon. This was also the case when amorphous silicon contained external atoms of hydrogen or carbon type.

Hereinafter, referring to FIG. 2, the behavior of an amorphous silicon thin film having a uniform thickness as a function of energy E supplied in the form of light will be described from a crystallographic point of view. In the graph, the x-axis is the light energy that must be supplied to cause the material to partially crystallize, and the y-axis is the logarithm of the electron mobility in the material.

While heating the thin film, the amorphous silicon is at least partially melted. Upon cooling, it solidifies to form grains of crystallized silicon embedded in the amorphous silicon bed. The electron mobility of the material increases with the proportion of recrystallized silicon. Quantitative information about the crystal structure of the material can be extracted from the density and size of the grain.

The curve indicated by the solid line shows the change in the logarithmic value of mobility as a function of the energy E supplied to the amorphous silicon thin film. The curve indicated by the dotted line shows the change in the logarithmic value of the electron mobility as a function of the energy E supplied to the hydrogenated amorphous silicon layer according to the invention.

To illustrate the change in the curve indicated by the solid line, the starting point would be a low energy equal to that of substantially hydrogenated amorphous silicon, which generally has a electron mobility of 0.3 to 0.6 cm ² / Vs. The material is hydrogen desorption at all stages. The higher the temperature of the thin film, the more energy supplied increases the rate of crystallization. The maximum ratio is obtained from the supplied energy E ₀ ^B , which makes it possible to obtain polycrystalline silicon by the method described above (irradiation with a laser source providing high power and pulses close to 50 ns). Microcrystals of crystallized silicon are formed whose size and proportion in the thin film gradually increase up to the E ₀ ^B energy level (FIG. 2B). The electron mobility usually measured in the silicon thin film treated at energy level E ₀ ^B is increased by approximately 100 times compared to the initial electron mobility of amorphous silicon.

The dashed curve shows the effect of irradiation with a source that provides a pulse of length close to 200 ns for the hydrogenated amorphous silicon thin film. These surveys are low on their own₀ ^BDespite Only a small portion of the thin film is allowed to crystallize. This crystallization is called "explosive" and occurs even when there is a negligible amount of hydrogen in the thin film. The grain size and density of the crystallized silicon is certainly smaller than that commonly found in polycrystalline silicon.

Referring to FIG. 2A, the overall structure of a hydrogenated amorphous silicon thin film treated by irradiation with a relatively long pulse in accordance with the present invention will be described. The film 1 is deposited in a hydrogen atmosphere on the substrate 2 by known techniques such as, for example, plasma enhanced chemical vapor deposition (PECVD).

Irradiation of the membrane according to the invention is carried out after some hydrogen desorption and triggers an explosive crystallization step of hydrogenated amorphous silicon. Indeed, amorphous silicon absorbs the light waves coming from the excimer laser and turns them into thermal waves. If the applied energy is sufficient, the entire thickness of the film is liquefied. Upon cooling, the silicone liquid solidifies. This solidification causes the formation of a number of crystallized grains (nanocrystals) whose size and density depend on the temperature and pressure conditions of the treatment.

The size and density of the grain 11 obtained according to the invention is much smaller than that obtained according to the conventional method. However, the electron mobility of the material so treated is increased to be sufficient for applications relating to the manufacture of electronic components and in particular transistors for flat screens using active liquid crystal matrices.

FIG. 2B is a view showing an amorphous silicon thin film processed by a known annealing method using a pulse having a length close to 50 ns. Prior to this treatment, hydrogen is desorbed by annealing in an oven at 400-450 ° C. for 2-5 hours. The irradiation step is optimal at energy E ₀ ^B leading to the formation of polycrystalline silicon.

The method according to the invention thus avoids the step of going through the oven at high temperatures, thereby preventing mechanical deformation of relatively weak substrates such as plastic materials. The substrate may be made of conventional silica-based glass. The quality of the interface 13 is advantageously sufficient for the foreseen application.

In this case, the long pulse length (170 ns or more) of the excimer laser encourages explosive crystallization of amorphous silicon. On the other hand, using high energy short pulses will desorb a significant amount of hydrogen and damage the interface.

Explosive crystallization increases the mobility of the material. The nanocrystals of silicon thus formed are actually better electron donors than dangling bonds in amorphous silicon even if the amorphous silicon is hydrogenated. The electron mobility of the material obtained by explosive crystallization is increased about five times that of the original material. This increase is sufficient to introduce this membrane into the active flat screen matrix. Such flat screens are formed on substrates of conventional silica-based glass or plastic materials.

Referring to Fig. 3, the structure of the transistor to be disposed in the active flat screen matrix will be described. This kind of transistor is formed on an amorphous silicon thin film (so called TFT (thin film transistor)).

This type of transistor structure (bottom gate) includes a substrate of glass underneath, as shown in FIG. An insulating film, for example SiO ₂ , is deposited on the substrate to form a thermal barrier. This film thermally blocks the substrate during irradiation with an excimer laser while forming a spacer to ensure good quality of the interface. An insulating film of the nitride film (Si ₃ N ₄ ) is also provided on the silica layer to act as a chemical barrier. A metal (chromium, titanium, tantalum, aluminum, molybdenum or alloy of such metal) thin film is deposited between the insulating film 3 and the substrate to form the bottom gate of the transistor. Two thin films 5a and 5b of n + doped amorphous silicon are deposited on the amorphous silicon thin film 1 forming the channel of the transistor to form a source and a drain of the transistor. The irradiation step according to the invention makes it possible to obtain low resistance and high mobility in the transistor, but does not cause any significant change in its structure. Laser irradiation can be performed after the upper portion of the channel is etched between the films 5a and 5b. With a pulse length of 120 to 400 ns, the electron mobility in the channel 1 (FIG. 3) is substantially increased by 1.5 to 10 times. Pulses approaching 200 ns are typically increased nearly three times.

And since the parasitic resistance between the n + layer and the channel is reduced, the Schottky barrier of such a transistor is advantageously less steep after the irradiation step.

It can be seen that the structure of the above-described transistor also includes a thin film 6 made of a silica type material in order to form an antireflection film. This film 6 allows the excimer laser beam (arrow L) to pass through better to crystallize the hydrogenated amorphous silicon thin film 1. The thickness of the anti-reflection film is selected to be the thickness of the anti-reflection film and the amorphous silicon film combined to minimize the reflection but maximize the transmission for the wavelength near 308 nm.

An apparatus according to the present invention for investigating a hydrogenated amorphous silicon thin film will be described with reference to FIG. 5. The device includes a xenon chlorine gas excimer laser 20 that emits radiation at a wavelength close to 308 nm. This laser is sold by the applicant under the name very extra-large excimer laser (VEL).

For example, the emitted beam 21 is reflected by a mirror 22 inclined at 45 ° and passes through a lens 23 selected to make the beam 21 substantially uniform. The thin film 1 deposited on the substrate 2 is seated on the sample-holder 24. The sample-holder is advantageously seated on a plate that can translate on three orthogonal coordinate axes XYZ. The translation of the plate can be controlled by a computer (not shown) that is connected to at least one motor on the plate and that is servo-controlled.

The excimer laser used emits as much as 0.9 J / cm ² of radiant energy. And, the length of the pulse to be emitted is advantageously 120 ns or more (the curve indicated by the solid line in FIG. 7).

These pulse lengths combine with this energy to encourage the formation of silicon microcrystals by explosive crystallization of amorphous silicon. Most of the emitted light has a wavelength close to 308 nm, which is absorbed by the thin film 1. The optical absorption spectrum of amorphous silicon actually has a maximum near 300 nm (FIG. 6). The absorbed light generates thermal wavelengths to melt the material of the thin film 1 over at least a large part of its thickness e. This thickness is usually 50 nm to 150 nm. The excimer laser may emit an energy density of substantially 0.2 J / cm ² to 0.9 J / cm ² to increase mobility over the thickness of the thin film.

The present invention is, of course, not limited to the above described embodiments. It extends to many other forms.

The method according to the invention can be applied to the manufacture of any component comprising an amorphous silicon thin film and is not limited to the manufacture of the transistors mentioned above.

The thin film 1 may be deposited by other methods such as PECVD or low pressure CVD (LPCVD). This deposition technique allows the amorphous silicon thin film to contain a small amount of hydrogen.

The irradiation step can also be applied to carbonized amorphous silicon thin films. In this case, the pulse length will be close to but in the range of 120 ns to 200 ns. The excimer laser used in accordance with the present invention allows pulse lengths on the order of 500 ns.

Claims (17)

As a method of processing an electronic component using an amorphous silicon thin film and a flat screen transistor using an active liquid crystal matrix, in particular, a) obtaining at least one amorphous silicon thin film deposited on a substrate of a predetermined material and comprising selected elements other than silicon; And b) processing said thin film so obtained by applying energy to modify at least one selected physicochemical property of said thin film, The processing step b) b1) providing a light source suitable for emitting radiation of wavelengths lying at least within the optical absorption band of amorphous silicon and capable of producing a processing pulse of length of at least 50 ns to 500 ns; And b2) treating the amorphous silicon thin film comprising illuminating the amorphous silicon thin film containing the element with the light source to substantially increase the electron mobility of the thin film by 1.5 to 10 times. Way. 2. The method of claim 1, wherein the profile of the processing pulse over time exhibits a half-height width of substantially 20 ns to 250 ns. A method according to claim 1 or 2, characterized in that the profile of the treatment pulse with respect to time has an increase time of substantially 5 ns to 70 ns. 4. A method according to any one of the preceding claims, wherein the profile of the processing pulse with respect to time has a decay time of substantially 15 ns to 200 ns. The thin film (1) according to any one of claims 1 to 4, wherein the thin film (1) comprises hydrogenated amorphous silicon, and the wavelength of the processing pulse is close to 200 ns, which is about three times the electron mobility of the thin film. A method for processing an amorphous silicon thin film, characterized in that the increase. The energy density of the light source 20 is substantially 0.2 J / cm ² to 0.9 J / cm ² , and the thickness e of the thin film 1 is substantially 50 nm to 150 nm. A method for processing an amorphous silicon thin film, characterized by the above-mentioned. 7. The method of claim 6, wherein the light source is a gas excimer laser (20) comprising xenon and chlorine, and the wavelength is close to 308 nm. The process according to any one of claims 1 to 7, in the manufacture of an electronic component comprising at least one amorphous silicon thin film containing selected elements other than silicon, and in particular a flat screen transistor using an active liquid crystal matrix. Processing method applying method. The method of claim 8, wherein after step b), And depositing a thin antireflective film (6) on the amorphous silicon thin film (1) so as to enhance light penetration into the amorphous silicon thin film (1). 10. Process according to claim 8 or 9, characterized in that the material of the substrate (2) is silica glass. 10. Process according to claim 8 or 9, characterized in that the material of the substrate (2) is a plastic material. The processing method according to any one of claims 8 to 11, wherein the amorphous silicon thin film is suitable for forming a channel of a thin film transistor. An apparatus for processing an electronic component comprising at least one amorphous silicon thin film containing selected elements other than silicon, and particularly a flat screen transistor using an active liquid crystal matrix, wherein at least one selected physicochemical property of the thin film 1 A means suitable for supplying energy capable of modifying the thin film to the thin film, wherein the energy supply means is at least substantially 50 ns to 500 to increase the electron mobility of the thin film substantially 1.5 to 10 times. Apparatus for processing an amorphous silicon thin film, characterized in that it comprises a light source (20) suitable for emitting radiation (21) of wavelength that lies at least within the optical absorption band of amorphous silicon with a pulse of length ns. 14. The amorphous silicon of claim 13, wherein said energy supply means comprises a xenon chlorine gas excimer laser 20 that emits light radiation having a pulse length of substantially 50 ns to 500 ns at a wavelength close to 308 nm. Thin film processing device. The thickness e of the thin film 1 is substantially 50 nm to 200 nm, and the excimer laser 20 has a density of 0.2 J / cm ² to 0.9 in the thin film 1. A device for processing an amorphous silicon thin film, characterized in that it transmits radiant energy of J / cm ² . 16. An amorphous silicon thin film according to any one of claims 13 to 15, comprising a lens 23 selected to supply substantially uniform light 21 on the thin film 1. Processing unit. 17. The apparatus of claim 16, comprising a sample holder (24) seated on an aligned plate for translational translation at (XYZ) and means for adjusting the movement of the plate.