Semiconductor apparatus and manufacturing method of same

Disclosed is a thin-film transistor (10) manufacturing method that includes a process for forming a nitrate film (12x) that includes residual nickel (22) on a surface thereof, by bringing a nitric acid solution into contact with a polysilicon layer (11x); and a process for removing the nitrate film (12x) that includes residual nickel (22) from the polysilicon layer (11x) surface. With this surface treatment process, a polysilicon layer (11) with reduced concentration of a surface residual nickel (22) is provided, and a thin-film transistor (10) having excellent surface smoothness is attained.

TECHNICAL FIELD

The present invention relates to a semiconductor apparatus and manufacturing method of the same, more particularly, to a semiconductor apparatus having a surface-treated semiconductor substrate and manufacturing method of the same.

BACKGROUND ART

Cleaning the surface of a semiconductor is indispensable in manufacturing a thin-film transistor (TFT) and the like from a semiconductor. As cleaning methods, in the case of silicon (Si), for example, in addition to a processing method for semiconductor substrates that employs a hydrogen peroxide solution (hereinafter referred to as the RCA processing method) developed by W. Kern et al. in 1970, chemical etching that processes the surfaces with diluted hydrofluoric acid (HF/H2O) and aqueous solution of ammonium fluoride (NH4F/HF/H2O) are used as known techniques.

However, while etching the silicon surface with ammonium fluoride (NH4F) aqueous solution is effective for removing natural oxide films, further improvement is required in removal of contaminants or in particle elimination measures. Also, while progress from a step is the dominant approach in etching of a plane direction (111), etching also progresses from some terraces such that the terrace portion affected by this etching becomes rough. That is, due to the plane direction dependency of conventional chemical etching techniques, obtaining a smooth surface at the atomic level (few nm level) is difficult, and therefore a uniform and flat surface cannot be expected from a polysilicon and an amorphous silicon with non-uniform crystal surfaces.

As art relating to the present invention, the present inventors have found that a highly-uniform quality silicon dioxide (SiO2) based oxide coating can be formed on a substrate surface using only a so-called chemical action of chemicals by immersing the silicon substrate in an oxidizing solution, for example, azeotropic nitric acid (concentration of 68%, boiling point of 120.7° C.), and have proposed to use this technique for gate insulators of TFT and the like (for example, the patent document 1).

Further, the present inventors have proposed a technique in which an oxide coating is formed on the surface of a polysilicon layer having a projection of as long as 100 nm in an azeotropic nitric acid and then the oxide coating is etched away such that the projection is reduced (patent document 2).

RELATED ART

Patent Documents

DISCLOSURE OF THE INVENTION

Object of the Invention

In the field of TFT, it is a major object to remove contaminants such as metal from the surface of the semiconductor to sufficiently clean the surface. However, as mentioned above, contaminants on a semiconductor surface cannot be sufficiently removed using the conventional chemical etching technique. Further, while a surface smoothness at the nm level is required from polysilicon and amorphous silicon with non-uniform crystal surfaces, flatness of such a level cannot be attained by a conventional surface treatment technique.

An object of the present invention is to provide a method for sufficiently removing metal catalyst and contaminants present at and near the semiconductor surface, and to provide a high performance semiconductor apparatus that has been removed of metal catalyst and the like. Further, another object of the present invention is to provide a method for uniformly smoothing the semiconductor surface at the level of several nm. Means below contribute to at least one of the objects mentioned above.

Means for Solving the Object

The inventors have found that removal of metal catalysts and contaminants is possible in addition to further improvement of semiconductor surface smoothness by forming an oxide coating on the semiconductor surface by chemical processing (oxidizing solution processing) most preferably at room temperature or around room temperature (less than 100° C.) and then removing the oxide coating (removal by chemical etching). Based on this knowledge, a semiconductor surface treatment method and a manufacturing method for making a semiconductor apparatus adopting the method is provided.

A method for manufacturing a semiconductor apparatus according to the present invention includes the steps of; preparing a semiconductor substrate containing at least a metal catalyst or a contaminant; and bringing the semiconductor substrate into contact with an oxidizing solution or gas to form an oxide film including at least a metal catalyst or a contaminant on the surface thereof.

The manufacturing method of semiconductor apparatus according to the present invention is particularly useful when polysilicon including nickel metal catalyst is used as the semiconductor substrate. By bringing the polysilicon including nickel into contact with oxidizing solution or gas to form an oxide film including nickel at the surface of the polysilicon and then removing this film, nickel can be efficiently removed from the surface and near the surface.

In the manufacturing method of semiconductor apparatus according to the present invention, the oxidizing solution or gas is preferably a nitric acid solution or gas generated from nitric acid. Further, the nitric acid concentration is preferably 68% by mass or more and 99% by mass or less. For example, an oxide film mainly made of silicon dioxide can be formed by chemical formation at a thickness of approximately 0.3 to 5 nm, for example, by immersing the semiconductor substrate in nitric acid so as to be in contact therewith for a short period of time of approximately 10 minutes at room temperature.

The manufacturing method of semiconductor apparatus according to the present invention includes a surface treatment process including the steps of: bringing a semiconductor substrate into contact with an oxidizing solution or gas to form an oxide film; and removing the oxide film from the surface of the semiconductor substrate; wherein the surface treatment process is repeated once or more such that a root-mean-square of the semiconductor substrate surface becomes at least four fifths or less of that before the surface treatment.

The manufacturing method of semiconductor apparatus according to the present invention may include the steps of preparing an oxide film including at least a metal catalyst or a contaminant on the semiconductor substrate in advance and bringing the semiconductor substrate from which the oxide film has been removed into contact with an oxidizing solution or gas to form a gate oxide film on the surface thereof; forming a gate electrode on the gate oxide film; and forming a source and a drain on the semiconductor substrate. Further, a step for forming an insulating film (for example, CVD) may be added such that an insulating film consisting of an oxide film formed by bringing the substrate into contact with an oxidizing solution or gas and an insulating film formed on this oxide film by CVD is made to function as a gate insulating film.

As mentioned above, according to the manufacturing method of semiconductor apparatus according to the present invention, in the case of a semiconductor substrate made of polysilicon and the like including at least a metal catalyst or a contaminant, forming an oxide film including the metal catalyst and the contaminant by bringing the substrate into contact with an oxidizing solution or gas of nitric acid and the like and then removing this oxide film by a surface treatment will make it possible to remove metal catalyst and contaminants such as nickel from the surface and near the surface of the semiconductor. Further, this surface treatment improves surface smoothness of the semiconductor substrate.

The above-mentioned removal of metal catalyst and contaminants and smoothing of the surface by surface treatment are considered to follow the mechanism as follows. With semiconductor substrates to be surface treated, in particular, polysilicon substrates formed by crystallization of silicon by laser annealing and the like, minute projections tend to form on the surface due to collisions occurring at the crystal grain boundary. In addition, metal catalysts and contaminants included in the semiconductor substrates, nickel, in particular, are considered to have a tendency to collect at the projection portion formed at the crystal grain boundary. Also, since this projection is minute ranging in the order of several nm to several tens of nm, oxidizing solution or gas tend to easily penetrate the projection portion and therefore oxidation of the projection portion tends to progress quickly. Thus, removal of the oxide film that has been formed not only reduces concentration of metal catalysts and contaminants on and near the semiconductor substrate but also smoothes the surface thereof.

The semiconductor apparatus according to the present invention includes a semiconductor substrate obtained by first forming on the surface thereof an oxide film including a metal catalyst or the like by bringing the semiconductor substrate containing at least a metal catalyst or a contaminant into contact with an oxidizing solution or a gas and then removing this oxide film such that a surface concentration of a metal catalyst or the like has been reduced; a gate insulating film formed on the semiconductor substrate with reduced surface concentration of a metal catalyst and the like; a gate electrode formed on the gate oxide film; and a source and a drain formed on the semiconductor substrate. The present semiconductor apparatus has reduced amount of metal catalyst and contaminants at the surface of the semiconductor substrate and therefore surface smoothness is improved. Accordingly, higher performance such as improvement of mobility and reduction of leakage current density, and reduction of voltage (for example, applied voltage between source and drain, reduction of gate drive voltage) can be expected.

The manufacturing method of a semiconductor apparatus according to the present invention may include the steps of bringing the semiconductor substrate containing at least a metal catalyst or a contaminant into contact with an oxidizing solution or gas to form an oxide film that functions as a gate oxide film on the surface thereof; forming a gate electrode on the gate oxide film; and forming a source and a drain on the semiconductor substrate. The semiconductor apparatus according to the present invention may include a semiconductor substrate formed thereon a gate oxide film including a contaminant by bringing the semiconductor substrate containing at least a metal catalyst or a contaminant into contact with an oxidizing solution or a gas; a gate electrode formed on the gate oxide film; and a source and a drain formed on the semiconductor apparatus.

The manufacturing method of semiconductor apparatus according to the present invention may include the steps of bringing the semiconductor substrate into contact with an oxidizing solution or gas to form a gate oxide film on the surface thereof, forming a gate electrode on the gate oxide film; and forming a source and a drain on the semiconductor substrate. Also, the semiconductor apparatus according to the present invention may include a semiconductor substrate having a gate oxide film on a surface thereof formed by bringing the semiconductor substrate into contact with an oxidizing solution or gas; a gate electrode formed on the gate oxide film; and a source and a drain formed on the semiconductor substrate. That is, the semiconductor apparatus and manufacturing method thereof may be applied when using semiconductor substrates not including a metal catalyst or a contaminant.

Effect of the Invention

With the manufacturing method according to the present invention, metal catalysts and contaminants present at and near the surface of the semiconductor substrate can be sufficiently removed, and at the same time minute projections present on the semiconductor substrate can be reduced and the surface of the semiconductor substrate can be uniformly smoothed in the order of several nm level.

Also, according to the semiconductor apparatus of the present invention, the amount of metal catalysts (nickel and the like) and contaminants at and near the surface of the semiconductor substrate is low and the surface smoothness is high. Therefore, surface level of the semiconductor substrate can be reduced to improve mobility of the charge and the performance of the semiconductor apparatus such as a TFT can be significantly improved.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, modes for carrying out the semiconductor apparatus according to the present invention and the manufacturing method thereof will be described in detail with reference to the drawings. Description below will be given with reference to a polysilicon substrate (layer) containing nickel as a metal catalyst and thin-film transistor having the polysilicon substrate (layer), but the present invention is not limited thereto.

First Embodiment

Polysilicon substrate9and the manufacturing method thereof with reduced surface nickel concentration, which is a first embodiment, more particularly the surface treatment method for a polysilicon layer11x, will be described in detail below with reference toFIGS. 1 through 3. Here, the polysilicon layer11x(polysilicon substrate9x) refers to a layer having a lot of residual nickel before being subjected to the surface treatment (hereafter, layers having a lot of residual nickel22in the surface shall be denoted by an “x”). Further, residual nickel22is assumed to include nickel compounds connected with silicon and nickel compounds weakly connected with or not connected with silicon.

First, configuration of a polysilicon substrate9will be described with reference toFIG. 1(c). As shown inFIG. 1(c), the polysilicon substrate9is a glass substrate20on which the polysilicon layer11has been formed. Ordinarily, a back coat layer21, which is a laminated film consisting of silicon dioxide (SiO2) and silicon nitride (SiN), is provided between the polysilicon layer11, which is semiconductor substrate, and the glass substrate20. This back coat layer21has the function to prevent impurities such as sodium (Na) from entering the polysilicon layer11from the glass substrate20during the laser annealing process and the like described hereinafter.

The polysilicon layer11, which is a semiconductor substrate, is obtained by depositing amorphous silicon or microcrystalline silicon on the glass substrate20by vapor deposition (plasma CVD) and applying nickel (Ni) thereon as a metal catalyst, and then instantaneously heating locally by laser annealing method and the like to polycrystallize the silicon. When conducting this laser annealing, using nickel as the metal catalyst will yield a polysilicon layer11with large grains. Further, the nickel catalyst that has been applied becomes residual nickel22with the surface concentration of approximately 1012(atoms/cm2), for example. Surface nickel concentration of the polysilicon layer11will be described hereinafter.

Now, the surface treatment method of the polysilicon layer11xwill be described in detail with reference toFIGS. 1(a) through (c).FIG. 1(a) illustrates the polysilicon layer11xformed on the glass substrate20. First, the surface of this polysilicon layer11xis cleaned with an appropriate known cleaning method (for example, an RCA processing method). Then, the substrate is immersed in a low-concentration diluted hydrofluoric acid solution for approximately 1 minute (hereinafter referred to as HF processing) to remove a natural oxide film that is assumed to be formed on the surface of the polysilicon layer11x. Lastly, it is rinsed (cleaned) with ultrapure water for 5 minutes.

Next, the glass substrate20including the polysilicon layer11xis immersed, for example, in a nitric acid aqueous solution with 70% by mass of nitric acid, for 10 minutes, for example, to form a nitrate film12xon the surface of the polysilicon layer11xas shown inFIG. 1(b) (hereinafter referred to as the nitric acid oxidation method). Here, while the concentration of the nitric acid aqueous solution may be arbitrarily changed, it should preferably be 68% by mass or more and 99% by mass or less. The temperature of the nitric acid aqueous solution should preferably be between room temperature and boiling temperature (for example, approximately 120° C.). Also, at least one oxidizing solution or a gas thereof selected from the group consisting of perchloric acid, sulfuric acid, and aqua regia may be arbitrarily used in place of nitric acid or in addition to nitric acid.

Nitrate film12xis an SiO2film (chemically generated oxide film) with a thickness of an order of several nm with low leakage current and a low interface level. Further, nitrate film12xcontains residual nickel22present at and near the surface of the polysilicon layer11x. The reason why the residual nickel22is easily included in the nitrate film12xis that the residual nickel22has a tendency to collect at the projection portion on the surface, as mentioned above, and also assumed to be because the oxide film forms at a low temperature at which the residual nickel22does not move during the nitric acid oxidation method.

Then, the HF processing is performed again to etch away the nitrate film12xincluding the residual nickel22, as shown inFIG. 1(c). InFIG. 1, the surface of the polysilicon layer11xhas been shown flat for a schematic purpose, but in reality, there are minute projections around which the residual nickel22tend to collect.

As mentioned above, by forming the nitrate film12xincluding the residual nickel22at and near the polysilicon layer11xand performing a surface treatment to remove this nitrate film12x, a portion of the residual nickel22can be effectively removed from the surface and near the surface of the polysilicon layer11x. Accordingly, with the above-mentioned method, the polysilicon substrate9having reduced concentration of surface nickel can be obtained.

Now, it will be shown that the residual nickel22is drastically reduced by the above-mentioned surface treatment, with reference toFIG. 2.FIG. 2shows the surface nickel concentration on the polysilicon layer11xquantitatively determined using total reflection X-ray fluorescence (TXRF). Surface nickel concentration was compared using one unprocessed sample that has not gone through cleaning and HF processing, another sample, or a once-processed sample, that has been immersed in a low-concentration diluted hydrofluoric acid solution for approximately one minute to remove natural oxide film (HF processing) and on which the nitrate film12xwas then formed to be further subjected to a surface treatment to etch away the nitrate film12x, and then still another sample, or a twice-processed sample, that has been subjected to a second surface treatment.

As shown inFIG. 2, while the surface nickel concentration of the unprocessed sample was 185 (×1010atm/cm2), just one processing reduced the surface nickel concentration to 100 (×1010atm/cm2), and it can be seen that the residual nickel22on the surface has been reduced to half.

Further, as a result of observing the surface condition of the above-mentioned once-processed sample with an atomic force microscope (AFM), it was found that the surface smoothness had been significantly improved.FIG. 3shows the surface condition measured by the AFM.FIG. 3also shows the above-mentioned unprocessed sample and the HF processed sample obtained by subjecting the unprocessed sample to HF processing for comparison.

As shown inFIG. 3, with the surface roughness of the unprocessed sample observed by the AFM expressed as a mean using the root-mean-square method being a root-mean-square roughness (RMS value) of 2.12 [nm], whereas the RMS value for the HF processed sample was 1.96 [nm], showing only a small change, the RMS value of the once-processed sample was significantly reduced to 1.42 [nm], showing that the surface has definitely been smoothed. The improvement in the RMS value differs to a certain degree depending on the initial roughness level, but it may improve down to ⅘ to ⅗ of the value before surface treatment by performing one surface treatment.

Second Embodiment

Referring toFIG. 4andFIG. 5, a thin film transistor10(hereinafter referred to as the TFT10), which is a second embodiment, will be described in detail hereinafter. The same components will be denoted by the same reference numbers and overlapping description thereof will be omitted (the same holds true for the third embodiment).

As shown inFIG. 4, the TFT10includes a polysilicon layer11formed on a glass substrate20via a back coat layer21, that is, a polysilicon substrate9, and a nitrate film12and a CVD insulating film13formed on the polysilicon layer11. Here, the nitrate film12is, as mentioned above, an SIO2film formed by bringing the polysilicon layer11into contact with nitric acid aqueous solution, which is an oxidizing solution, and the CVD insulating film13is an insulating film such as the SIO2film formed by a common plasma CVD method.

The TFT10uses the nitrate film12and the CVD insulating film13as gate insulating films, and is provided with a source region14and a drain region15formed on the polysilicon layer11, a gate electrode16formed on the CVD insulating film13, an interlayer17, a source electrode18and a drain electrode19provided in contact holes formed in the gate insulating film. In other words, the TFT10is a MOS type transistor.

Now, a manufacturing method of the TFT10will be described with reference toFIG. 5. Though not shown inFIG. 5, first, the polysilicon layer11xformed on the glass substrate20is surface treated to prepare the polysilicon substrate9having a polysilicon layer11with reduced surface nickel concentration (refer toFIG. 1).

As shown inFIG. 5(a), the nitrate film12is formed on the surface of the polysilicon layer11by immersing thus obtained polysilicon substrate9again in an aqueous solution with a concentration of 70% by mass of nitric acid for 10 minutes. The newly formed nitrate film12here is an oxide film mainly composed of SiO2that is dense with a thickness of the order of several nm and that realizes low leakage current and a low interface level.

Next, as shown inFIG. 5(b), the CVD insulating film13is formed on the nitrate film12by a common plasma CVD method to obtain the gate insulating film having a thin nitrate film12and a thick CVD insulating film13laminated. Thickness of the CVD insulating film13is, for example, approximately 40 nm. Since the nitrate film12is a dense film with low leakage current and interface level, as mentioned above, the thickness of the gate insulating film which is ordinarily necessary to be as thick as 80 nm, it can be made as thin as approximately 40 nm. Further, a predetermined amount of impurities are doped at a low concentration to the channel portion of the polysilicon layer11, as necessary, for threshold voltage control.

Next, after forming a conductive film on the gate insulating film by sputtering and the like, photolithography processing is performed to eliminate unnecessary portions, the process consisting of application of photoresist, exposing the pattern, development, and etching to form the gate electrode as shown inFIG. 5(c). Then, the gate electrode16that has been formed is used as a mask to dope such donors as phosphorus and arsenic by self alignment to form a source region14and a drain region15in the polysilicon layer11, as shown inFIG. 5(d).

Next, as shown inFIG. 5(e), after forming an interlayer17that covers the gate electrode16, a portion of the interlayer17and the gate insulating film is etched away to form a contact hole as shown inFIG. 5(f). Then, after forming a conductive layer by sputtering and the like, unnecessary portions are etched away by photolithography process to form a source electrode18and a drain electrode19in the contact holes to obtain the TFT10which is an N-channel MOS type semiconductor apparatus, as shown inFIG. 5(g) and FIG.4.

While an N-channel MOS type semiconductor apparatus had been shown as an example above, a P-channel MOS type semiconductor apparatus, can be manufactured similarly. Also, an N-channel MOS type semiconductor apparatus and a P-channel MOS type semiconductor apparatus may be used to manufacture a CMOS type semiconductor apparatus. Further, low-concentration dopant may be doped and a high-concentration dopant may be doped using a spacer provided in the gate electrode to manufacture an LDD structure semiconductor.

The thus manufactured TFT10is provided with a polysilicon layer11from which the residual nickel22has been removed from the surface and near the surface thereof, and a nitrate film12, dense and having low leakage current and low interface level, as a gate insulating film, meaning that further improved transistor characteristics such as improvement of mobility, reduction of current density, and reduction of TFT driving voltage can be expected over a TFT50to be described later. The formation of an oxide film by nitric acid oxidation method and the surface treatment method for removing the same is effective for the removal of metal catalysts or contaminants other than nickel, for example, copper (Cu) and iron (Fe).

Third Embodiment

A thin film transistor (referred to as the TFT50), which is a third embodiment, will be described in detail hereinafter, with reference toFIGS. 6 through 8.

As shown inFIG. 6, besides the fact that the nitrate film12xincluding the residual nickel22was formed on the surface of the polysilicon layer11, and this nitrate film12xwas used as the gate insulating film without being removed, other components are similar to those of the thin film transistor10. That is, the TFT50is obtained by a manufacturing method similar to that of the TFT10except for the fact that the surface treatment process ofFIGS. 1(b) and (c) is excluded. In this case, preferably the residual nickel22is reduced by techniques such as gettering.

Now, the characteristics of the TFT50are described with reference toFIGS. 7 and 8. In the manufacturing process of the TFT50, donor atoms and acceptor atoms were doped respectively to form two types of TFT, a P-channel TFT50(hereinafter referred to as the P-chTFT) and an N-channel TFT (hereinafter referred to as the N-chTFT) on the same glass substrate20to evaluate their characteristics. That is, the TFT50is a CMOS type semiconductor apparatus.

FIG. 7is a characteristics chart illustrating the relationship between the drain voltage (Vds) [V] and the drain current (Ids) [mA].FIG. 7shows the characteristic curve obtained by applying gate-source voltage (Vgs) in steps of 1 volt [V] between 1 and 8 [V], and the characteristic curve when Vgs=−2.5 V for P-chTFT, and when Vgs=3.5 V for N-chTFT are shown by bold lines, respectively. The size of the TFT50subjected to evaluation had a gate length (channel length) (L) of 2 [μm] and a gate width (W) of 20 [μm] in the case of both P-chTFT and N-chTFT. According to the characteristic curves inFIG. 7, drain current (Ids) is not generated when the gate voltage is not applied, and therefore it can be understood that the TFT50is an enhancement type.

FIG. 8is a characteristics chart illustrating the relationship between gate-source voltage and the drain current (solid line), as well as the gate current (dashed line). Specifically, the solid line shows the relationship between gate-source voltage (Vgs) [V] and the drain current (Ids) [A] when the voltage between source electrode/drain electrode is set to 0.1V. The TFT50has almost no gate leakage current as shown by the dashed line, and as shown by the characteristic curve in solid line, it has been confirmed that it operates sufficiently at a voltage as low as 3 V.

While the CVD insulating film13had been described as having the thickness of 40 nm in the foregoing, thickness of the insulating film formed on the nitrate film12may be made, for example, 20 nm, or 10 nm, or less than 10 nm to realize a high-performance TFT capable of operating at a low voltage of 3V or less. More specifically, high-performance TFT can be realized such that the threshold voltage is approximately 0.2 to 1.0 in the case of the N-chTFT, and the threshold voltage is approximately −0.2 to −1.0 in the case of the P-chTFT. Further, the insulating film formed on the nitrate film12may be of a composition other than SiO2, for example, high permittivity films such as SiON, SiN, and hafnium oxide (HfO).

Although the TFT formed on a glass substrate had been illustrated as an example in the above description, a flexible substrate may also be used instead of a glass substrate. Also, the TFT may be such that is used for a semiconductor apparatus of a three-dimensional structure formed by laminating on top of a substrate made of single-crystal silicon and the like. Further, the semiconductor substrate may be minute crystallized silicon and amorphous silicon.

Technical ideas of the present invention may also be applied to solar cells of single-crystal silicon, polysilicon, and the like.

INDUSTRIAL APPLICABILITY

The present invention may be applied to thin film semiconductor apparatus such as the TFT having silicon dioxide layer on the surface of the polysilicon layer and to the manufacturing method of the same. Further, such thin-film semiconductor apparatuses may be adopted in highly integrated functional devices such as a highly functional integrated circuit, a semiconductor memory device, and various liquid crystal displays.

DESCRIPTION OF THE REFERENCE NUMERALS