JP2000261006A - Semiconductor substrate for forming schottky junction, semiconductor element using the same, and schottky battier diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor element having Schottky junction structure superior in backward direction characteristic, without sacrificing the forward direction characteristic, a Schottky barrier diode, and a semiconductor substrate for forming a Schottky junction for forming the diode. SOLUTION: In a Schottky barrier diode, in which a metal layer is formed on an epitaxial layer surface of a semiconductor substrate for forming a Schottky junction in which an epitaxial layer containing impurities, is formed on a silicon substrate, the concentration of an impurity in the epitaxial layer is constant from the surface to a prescribed depth, gradually increasing from the prescribed depth toward the silicon substrate, and sharply increasing toward the substrate in the vicinity of the silicon substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor substrate for forming a Schottky junction in which an epitaxial layer containing a specific impurity is formed on a semiconductor layer of one conductivity type, and a metal layer provided on the surface of the semiconductor substrate. The present invention relates to a semiconductor element having a Schottky junction structure and a Schottky barrier diode.

[0002]

2. Description of the Related Art A Schottky junction structure formed by forming an epitaxial layer containing a specific impurity on a semiconductor layer of one conductivity type and further providing a metal layer on the epitaxial layer has a structure as shown in FIG. It is applied to Schottky barrier diodes (SBDs) because of its excellent properties such as low forward voltage (V _F ), low power consumption, and little current flow even when reverse voltage (V _R ) is applied. Have been.

In recent years, a Schottky junction structure having more excellent characteristics has been demanded. In order to improve the reverse characteristics in the Schottky junction structure, that is, to reduce the current generated by the reverse voltage, a reverse voltage is generated at the boundary between the epitaxial layer and the metal layer when a reverse voltage is applied. It is important to lower the electric field strength as much as possible, and it is known that it is effective to lower the electric field strength to lower the impurity concentration of the epitaxial layer.

[0004]

However, when the concentration of impurities in the epitaxial layer is reduced, the resistance increases and the forward characteristics decrease, that is, the forward voltage increases. In order to improve the forward characteristics, that is, to lower the forward voltage (V _F ) shown in FIG. 3 in order to counter this phenomenon, conventionally, a method of selecting a metal having a low barrier height as a barrier metal has been adopted. ing. However, if a metal having a low barrier height is selected, the current (I _R ) flows more easily at a lower reverse voltage (V _R ), and the reverse characteristics deteriorate.

As described above, in the prior art, in order to improve the reverse characteristics, the forward characteristics have to be sacrificed.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a semiconductor device and a Schottky barrier diode having a Schottky junction structure having excellent reverse characteristics without sacrificing forward characteristics, and a method for forming the same. It is an object of the present invention to provide a Schottky junction forming semiconductor substrate.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is directed to a method for forming a Schottky junction in which an epitaxial layer containing a specific impurity is formed on a semiconductor layer of one conductivity type. Wherein the concentration of the impurity in the epitaxial layer is constant from the surface of the epitaxial layer to a predetermined depth, and gradually increases from the predetermined depth toward the one conductivity type semiconductor layer. And

According to a third aspect of the present invention, there is provided a Schottky junction structure in which an epitaxial layer containing a specific impurity is formed on a semiconductor layer of one conductivity type and a metal layer is provided on the surface of the epitaxial layer. In the semiconductor device, the concentration of the impurity in the epitaxial layer is constant from the surface to a predetermined depth, and gradually increases from the predetermined depth toward the one conductivity type semiconductor layer.

According to the Schottky junction structure formed by using the semiconductor substrate of the first aspect and the semiconductor element of the third aspect, the impurity concentration of the epitaxial layer is constant from the surface to a predetermined depth. Since the depth gradually increases from the depth toward the one-conductivity-type semiconductor layer, the boundary portion with the metal layer is the lowest, and the distance increases from the metal layer. The electric field intensity at the boundary portion when a reverse voltage is applied is affected by the impurity concentration of the entire epitaxial layer. However, by decreasing the concentration at the boundary portion as in claim 1 or 3, The electric field intensity in the region can be suppressed, and the reverse characteristics can be improved.

In addition, even if the impurity concentration is low near the surface of the epitaxial layer, the concentration gradually increases, so that it is not necessary to lower the average value of the impurity concentration of the entire epitaxial layer as compared with the prior art, so that the resistance value does not increase. In addition, the forward characteristics do not deteriorate. Furthermore, by making the concentration of the boundary portion with the metal layer constant, in the manufacturing process,
Even if the thickness of the epitaxial layer varies,
The impurity concentration at the boundary between the epitaxial layer and the metal layer can be kept constant, and a product with stable forward and reverse characteristics can be supplied.

According to a second aspect of the present invention, in the semiconductor substrate for forming a Schottky junction according to the first aspect, the impurity concentration of the epitaxial layer is such that the impurity concentration of the one conductivity type semiconductor is near the one conductivity type semiconductor layer. It is characterized by rapidly increasing toward the layer.

The invention described in claim 4 is the same as the invention described in claim 3.
In the semiconductor device described in (1), the impurity concentration of the epitaxial layer rapidly increases in the vicinity of the one conductivity type semiconductor layer toward the one conductivity type semiconductor layer.

According to the Schottky junction structure formed by using the semiconductor substrate according to the second aspect and the semiconductor element according to the fourth aspect, the impurity concentration of the epitaxial layer is
In the vicinity of the one-conductivity-type semiconductor layer, the resistance value rapidly increases toward the one-conductivity-type semiconductor layer, so that the resistance value in this region can be reduced. As a result, good forward characteristics are obtained.

According to a fifth aspect of the present invention, in the semiconductor device according to the third or fourth aspect, a crystal axis of a main surface of the one conductivity type semiconductor layer is a <100> plane.
Thus, a semiconductor device having a small reverse current (IR) can be obtained.

According to a sixth aspect of the present invention, in the semiconductor device according to any one of the third to fifth aspects, the constant concentration is smaller than an _ND value obtained by the following equation (1). And

(Equation 2) (In the above formula (1), V _bi is the built-in potential, epsilon _s is the dielectric constant of the epitaxial layer, q is the elementary charge, V _R is the reverse voltage, k is the Boltzmann constant, T is the absolute temperature, Em Schottky The electric field strength at the junction interface is shown.)

Here, in equation (1), V _bi , ε _s ,
q, k, and T are constants, and V _R uses a reverse voltage value that needs to withstand practical use. E _m is the critical electric field strength is a limit as practically acceptable may be referenced to, for example, may be used if the 28V / [mu] m if the silicon substrate. More precisely, it depends on the impurity concentration of the epitaxial layer. By substituting each number in this manner, the maximum allowable impurity concentration is obtained. Conventionally, in the Schottky barrier diode is used as a product, it is slightly different depending upon the particular application etc., based essentially impurity concentration N _D values obtained in this way, the constant for the entire epitaxial layer Many have impurity concentrations. That N _D obtained from equation (1) it is the regarded as conventional amounts of concentrations. According to the sixth aspect of the present invention, since the constant impurity concentration near the surface of the epitaxial layer is lower than that of the related art, the electric field intensity in the region at the boundary with the metal layer can be sufficiently suppressed. , Good reverse characteristics. In this case, from the N _D obtained from equation (1), for example, if less than half, it is possible to further small electric field intensity.

According to a seventh aspect of the present invention, in the semiconductor device according to the sixth aspect, a graph showing the depth of the epitaxial layer by one of the horizontal axis and the vertical axis and the electric field intensity by the other is prepared. In this case, a graph of the electric field strength of the epitaxial layer obtained by applying a predetermined reverse voltage and an area surrounded by the horizontal axis and the vertical axis are obtained by calculating the impurity concentration of the epitaxial layer from the above equation (1). when constant at N _D value which is substantially equal to the area obtained in the same manner, and the electric field strength in the epitaxial layer surface portion, the concentration of impurities in the epitaxial layer by the formula N _D value obtained from (1) When it is constant, the electric field strength is similarly lower than that obtained.

Here, the electric field strength at the time of creating a graph can be obtained, for example, by obtaining a space charge from the impurity distribution of the epitaxial layer, and from this value by the Poisson equation. Alternatively, it may be obtained by actually measuring. The term “similarly obtainable area” or “similarly obtainable electric field strength” means that the impurity concentration is constant at the _ND value, but the structure such as the film thickness and shape of the epitaxial layer and the metal layer is completely different. Similarly, when a reverse voltage of the same magnitude is applied, it refers to the graph of the electric field strength, the area enclosed by the horizontal and vertical axes, and the electric field strength at the epitaxial layer surface.

According to the seventh aspect of the present invention, a graph of the electric field strength when a predetermined voltage is applied and the areas enclosed by the horizontal axis and the vertical axis are substantially equal to each other. average resistance value of the epitaxial layer 6 semiconductor device would impurity concentration that equivalent to that of the semiconductor device of approximately constant across conventionally N _D value. If the average resistance values are equal, the forward characteristics are also equal. Also, the electric field intensity of the epitaxial layer surface portion, the impurity concentration is lower than that of the conventional degree of semiconductor elements constant across at N _D value, reverse characteristic becomes excellent than conventional. Therefore, the semiconductor device according to claim 7 has excellent reverse characteristics without sacrificing the forward characteristics as compared with the conventional device.

According to an eighth aspect of the present invention, there is provided a Schottky barrier diode comprising the semiconductor device according to any one of the third to seventh aspects.

According to the invention, a Schottky barrier diode having excellent reverse characteristics without sacrificing forward characteristics is provided.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows an example of the Schottky barrier diode of the present invention. The Schottky barrier diode 1 shown in FIG. 1 includes a silicon substrate 2, an epitaxial layer (hereinafter, epilayer) 3, a metal layer 4, guard rings 5, 5, oxide films 6, 6, and electrodes 7, 8.

The silicon substrate 2 is manufactured by adding arsenic or antimony or the like as impurities to a material cut out from a silicon bulk crystal at a specific concentration in a process of pulling an ingot by using a known technique. Semiconductor substrate (one conductivity type semiconductor layer). The epi layer 3 is an n-type semiconductor substrate formed by epitaxial growth and made of a silicon thin film having a thickness of, for example, 3 to 10 μm, and contains the same impurities as the silicon substrate 2. The concentration of this impurity is controlled during the step of epitaxial growth, and has a distribution in the depth direction characteristic of the present invention. This will be described later. The semiconductor substrate for forming a Schottky junction according to the present invention includes a silicon substrate 2 and an epi layer 3. Metal layer 4
Is a thin film of a metal such as chromium, molybdenum, tungsten, or the like formed by a method such as vacuum evaporation or sputtering, and functions as a barrier metal.

The guard rings 5, 5 are p-type semiconductor regions, and are formed in order to prevent a phenomenon that the withstand voltage characteristic against the reverse voltage at the edge of the metal layer 4 is lower than that at the center. Oxide films 6 and 6 are made of a silicon oxide film and serve as an insulating film and a protective film. The electrode 7 is on the anode side, and the electrode 8 is on the cathode side. Therefore,
In the Schottky barrier diode 1, when a voltage is applied so that a current flows from the top to the bottom in the figure, the forward voltage (V
_F), a reverse voltage (V _R) in case of applying a voltage in the opposite direction.

As described above, the epi layer 3 has an impurity concentration distribution unique to the present invention. This density distribution will be described with reference to FIG. The solid line in FIG. 2 indicates the impurity concentration distribution (a), the space charge distribution (b), and the electric field when a predetermined reverse voltage is applied to the epi layer 3 of the Schottky barrier diode 1 in FIG. The intensity (c) is shown corresponding to the position in the depth direction, and (b) and (c) are values obtained by calculation using a known method. In each of FIGS. 2A, 2B, and 2C, the portion between the alternate long and short dash lines corresponds to the epi layer 3, the left side of the section corresponds to the metal layer 4, and the right side corresponds to the silicon substrate 2. FIG. 2A,
The vertical axes of (b) and (c) are relative values.

[0026] The dotted line in Figure 2, except that a constant impurity concentration in the epitaxial layer with N _D value calculated from the following equation (1), the virtual Schottky having exactly the same structure as FIG. 1 This is data obtained for the barrier diode in the same manner as the solid line.

(Equation 3) In the above formula (1), V _bi is the built-in potential, epsilon _s is the dielectric constant of the semiconductor substrate, q is the carrier particles one charge, V _R is the reverse voltage, k is the Boltzmann constant, T is the absolute temperature, E _m is 4 shows the electric field strength at the Schottky junction interface.

Equation (1) is expressed by Physics of Semiconductor D
evices, 1981, John Wiley & Sons (Reference 1), which has been established as a theoretical formula of a Schottky barrier diode. That is, in equation _{(1), V bi, ε} s, q, k, T is a constant, V _R may be used voltage value of the reverse direction used in the calculation in FIG. 2 (c). E _m may be the basis of the critical electric field intensity is the limit that is practically acceptable, for example, if the silicon substrate 2
About 8 V / μm may be used temporarily. By substituting each number in this manner, the maximum allowable impurity concentration is obtained. The Schottky barrier diode used as a conventional product has a slight difference depending on the specific application, but basically, the impurity concentration of the epi layer is kept constant based on the impurity concentration obtained in this way. Therefore, the virtual Schottky barrier diode can be said to be equivalent to a conventional product.

The value of the electric field strength in FIG. 2 (c) is calculated from the value of the space charge based on the impurity concentration based on the above-mentioned document 1 and the "Semiconductor circuit design technology" (Nikkei BP) which described the schematic solution.
In accordance with Poisson's equation disclosed in US Pat. That, E _m of formula (1) than those used to determine the concentration N _D which is a measure of the impurity concentration of the epitaxial layer, FIG. 2
It has nothing to do with the value of (c).

As shown in FIG. 2A, the impurity concentration of the epitaxial layer 3 of the present invention is constant in the S region from the surface in contact with the metal layer 4 to a predetermined depth. If the S region is not provided and the impurity concentration distribution of the epi layer 3 is suddenly graded, if the film thickness of the product varies somewhat,
The impurity concentration at the boundary with the metal layer 4 varies, and the characteristics of the Schottky junction differ from product to product. By providing the constant region, even if the thickness of the epi layer 3 varies in the manufacturing process, the impurity concentration at the boundary between the epi layer 3 and the metal layer 4 can be kept constant, and the forward direction can be maintained. Products with stable characteristics and reverse characteristics can be supplied. In order to obtain stable forward and backward characteristics between products, the film thickness of the S region should be, for example, about 0.5 to 1 micron when the epi layer 3 has the above-mentioned film thickness.

The impurity concentration of the epi layer 3 gradually increases in the Q region continuous with the S region, and sharply increases in the R region near the silicon substrate 2. Thereafter, in the silicon substrate 2, the impurity concentration is kept at a constant high concentration. On the other hand, in the virtual Schottky barrier diode, as shown by the dotted line in FIG.
And the impurities are contained in a constant concentration in in N _D value obtained. As shown in FIG. 2 (a), the concentration of the S region is set to a value lower than N _D value.

The impurity concentration shown in FIG.
In the graph (c), the solid line (Schottky barrier diode 1) has a vertical axis (Y axis) and a horizontal axis (X axis).
And the area A (the area indicated by oblique lines) surrounded by the graph of the electric field strength is the Y-axis and the X-axis with respect to the dotted line (virtual Schottky barrier diode), and the area B (shown by dots) enclosed by the graph of the electric field strength Area) is controlled to be substantially equal to This area is a value determined by the resistance value of the epi layer and the applied voltage. That is, the average resistance value of the epi layer of the Schottky barrier diode 1 is equal to that of the virtual, ie, conventional, Schottky barrier diode. As described above, the average concentrations of the impurities are also substantially equal.

A semiconductor substrate on which the crystal axis of the main surface of the silicon substrate before forming the epitaxial layer is the <100> plane and on which the epitaxial layer is formed just like the Schottky barrier diode 1 is used. When the characteristics of the semiconductor device obtained were measured, the reverse current (I _R ) was larger than that of the conventional crystal axis <111> plane.
Experimental results were obtained which were reduced to 1/2 to 1/5. In general, it is known that when an insulating film is formed on the crystal axis <100> plane, the phenomenon of attracting electrons to the semiconductor surface by cations present in the insulating film becomes smaller than that of the crystal axis <111> plane. . In the present invention, it is considered that the reverse current is greatly reduced due to a synergistic effect with the surface impurity concentration distribution of the epitaxial layer.

According to the above-described Schottky barrier diode 1 of the present invention, the impurity concentration of the epitaxial layer 3 is constant in the S region from the surface to a predetermined depth, and gradually in the Q region continuous with the S region. And rapidly increases in the R region closest to the silicon substrate 2. Therefore, the concentration is lowest at the boundary with the metal layer 4, and increases as the distance from the metal layer 4 increases. In addition, the graph of the electric field strength when a predetermined reverse voltage is applied and the area formed on the X axis and the Y axis are equivalent to those of the conventional Schottky barrier diode. In other words, the amount of impurities contained in such an amount that the average resistance value of the epi layer becomes about the same as the conventional one is
The density distribution shown in FIG. 2A and the density of the S region are made smaller than the _ND value to obtain the density distribution shown in FIG.
As shown in (1), the maximum electric field strength at the boundary between the epi layer and the metal layer can be made smaller than in the conventional case, and as a result, the reverse characteristics are improved. Further, since the resistance value of the entire epi layer is about the same as the conventional one, the forward characteristics are not sacrificed.

The present invention is not limited to the above embodiment. For example, the concentration change in the Q region of the epi layer may not be linear, but may be a curve or a step. Good.

[0035]

According to the Schottky junction structure formed by using the semiconductor substrate of claim 1 and the semiconductor device of claim 3, the impurity concentration of the epitaxial layer is constant from the surface to a predetermined depth. Since the width gradually increases from a predetermined depth toward the one-conductivity-type semiconductor layer, the boundary portion with the metal layer is the lowest, and the distance increases from the metal layer. The electric field strength at the boundary portion when a reverse voltage is applied is influenced by the impurity concentration of the entire epitaxial layer. However, as described in claim 1 or 3, the concentration at the boundary portion is particularly reduced. The electric field intensity in the region can be suppressed, and the reverse characteristics can be improved.

In addition, even if the impurity concentration is low near the surface of the epitaxial layer, the concentration gradually increases, so that it is not necessary to lower the average value of the impurity concentration of the entire epitaxial layer as compared with the conventional case, and the resistance value does not increase. In addition, the forward characteristics do not deteriorate.

According to the Schottky junction structure formed by using the semiconductor substrate according to the second aspect and the semiconductor element according to the fourth aspect, the impurity concentration of the epitaxial layer is
In the vicinity of the one-conductivity-type semiconductor layer, the resistance value rapidly increases toward the one-conductivity-type semiconductor layer, so that the resistance value in this region can be reduced. As a result, good forward characteristics are obtained.

According to the fifth aspect of the present invention, since the crystal axis of the main surface of the one conductivity type semiconductor layer is set to <100>, the crystal axis of the epitaxial layer surface also becomes the <100> plane, and the reverse current is reduced. Dramatically reduced.

According to the sixth aspect of the present invention, the constant impurity concentration in the vicinity of the surface of the epitaxial layer is lower than the conventional level, so that the electric field intensity in the region at the boundary with the metal layer is sufficiently suppressed. And the reverse characteristics are good.

According to the seventh aspect of the present invention, the fact that the graph of the electric field strength when a predetermined voltage is applied and the areas enclosed by the horizontal axis and the vertical axis are substantially equal to each other means that: average resistance of the epitaxial layer of the seventh semiconductor devices would impurity concentration that equivalent to that of the semiconductor device of approximately constant across conventionally N _D value. If the average resistance values are equal, the forward characteristics are also equal. Also, the electric field intensity of the epitaxial layer surface portion, the impurity concentration is lower than that of the conventional degree of semiconductor elements constant across at N _D value, reverse characteristic becomes excellent than conventional. Therefore, the semiconductor device according to claim 7 has excellent reverse characteristics without sacrificing the forward characteristics as compared with the conventional device.

According to the eighth aspect of the present invention, a Schottky barrier diode having excellent reverse characteristics without sacrificing forward characteristics is obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a Schottky barrier diode of the present invention.

FIGS. 2A and 2B show the characteristics of the Schottky barrier diode of FIG. 1 in the depth direction, where FIG.
(B) shows the space charge, and (c) shows the electric field strength when a reverse voltage is applied.

FIG. 3 is a diagram showing characteristics of a general Schottky junction structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Schottky barrier diode 2 Silicon substrate (one conductivity type semiconductor layer) 3 Epitaxial layer 4 Metal layer 5 Guard ring 6 Oxide film 7, 8 Electrode

Claims (8)

[Claims] 1. A semiconductor substrate for forming a Schottky junction in which an epitaxial layer containing a specific impurity is formed on a semiconductor layer of one conductivity type, wherein the concentration of the impurity in the epitaxial layer is predetermined from the surface of the epitaxial layer. A Schottky junction forming semiconductor substrate, wherein the semiconductor substrate is constant up to a predetermined depth, and gradually increases from the predetermined depth toward the one conductivity type semiconductor layer. 2. The method according to claim 1, wherein the impurity concentration of the epitaxial layer is:
2. The semiconductor device according to claim 1, wherein in the vicinity of the one-conductivity-type semiconductor layer, the size increases rapidly toward the one-conductivity-type semiconductor layer.
4. The semiconductor substrate for forming a Schottky junction according to claim 1. 3. A semiconductor device having a Schottky junction structure in which an epitaxial layer containing a specific impurity is formed on a semiconductor layer of one conductivity type and a metal layer is provided on the surface of the epitaxial layer, A semiconductor element, wherein the impurity concentration is constant from the surface to a predetermined depth, and gradually increases from the predetermined depth toward the one-conductivity-type semiconductor layer. 4. The epitaxial layer according to claim 1, wherein
4. The semiconductor device according to claim 3, wherein in the vicinity of the one-conductivity-type semiconductor layer, the size increases rapidly toward the one-conductivity-type semiconductor layer.
A semiconductor device according to item 1. 5. A crystal axis of a main surface of the one conductivity type semiconductor layer is a <100> plane.
A semiconductor device according to item 1. 6. The method according to claim 3, wherein the predetermined density is smaller than an _ND value obtained from the following equation (1).
6. The semiconductor device according to any one of items 5 to 5. (Equation 1) (In the above formula (1), V _bi is the built-in potential, epsilon _s is the dielectric constant of the epitaxial layer, q is the elementary charge, V _R is the reverse voltage, k is the Boltzmann constant, T is the absolute temperature, Em Schottky The electric field strength at the junction interface is shown.) 7. An epitaxial layer obtained by applying a predetermined reverse voltage when a graph showing the depth of the epitaxial layer on one of the horizontal axis and the vertical axis and showing the electric field strength on the other is prepared. the area of surrounded by the graph with the horizontal axis and the vertical axis of the electric field strength, if the concentration of the impurity of the epitaxial layer was fixed by the formula N _D value obtained from (1),
Substantially equal to the obtained analogously area, and electric field intensity in the epitaxial layer surface portion, when the concentration of the impurity of the epitaxial layer was fixed by the formula N _D value obtained from (1), similarly obtained field 7. The semiconductor device according to claim 6, wherein the strength is lower than the strength. 8. A Schottky barrier diode, which is the semiconductor device according to claim 3.