JPS6327868B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a semiconductor element for radiation or photodetection, particularly sensitive to low-energy radiation or light.

Surface barrier type semiconductor devices have long been known as semiconductor devices for radiation detection, and these are made by coating a semiconductor substrate with a metal that forms a shot barrier.
For example, gold is used for a p-type silicon substrate, and aluminum, for example, is used for a p-type silicon substrate. Of these, gold generally has the disadvantage of low mechanical strength and is easily peeled off from the substrate, and aluminum also has the disadvantage of poor environmental resistance, particularly chemical resistance. Especially for detecting low-energy radiation and light,
Unlike the case for high-energy radiation, if a semiconductor device is housed in a sealed metal can, the radiation and light will be absorbed or reflected by the can walls, so it is best to use it in an exposed state or in a simple package. must be sensitive to atmospheric oxygen. The shot barrier metal in the surface barrier type is formed with an extremely thin layer of 1000 Å or less in order to reduce the absorption of radiation and light, so it is easily attacked by the above-mentioned oxygen, and therefore it is inevitable that the device characteristics will change over time. Ta.

Furthermore, pn junction type radiation detection elements have been known for a long time. This kind of thing is shown in Figures 1 and 2.
As shown in the figure, a diffusion layer 2 having the opposite conductivity is formed in a silicon substrate 1, a pn junction is formed between the diffusion layer 2 and the substrate 1, and an electrode 3, A depletion layer 5 is formed in the diffusion layer 2 and the substrate 1, especially in the latter by applying a reverse bias voltage through the electrode 4, and when radiation R is incident on this depletion layer, this generates charged pairs as shown in the figure. , which is based on the principle of detecting current pulses generated by the movement of this charged pair. Note that Fig. 1 shows a mesa-type pn junction radiation detection element, and Fig. 2 shows a planar-type pn junction radiation detection element.
6 in FIG. 2 indicates a metal oxide layer, particularly a silicon oxide film. In this p-n junction type, there is less of the problem of aging that occurs in the surface barrier type described above, but the metal electrode 3 acts as an obstacle to incident low-energy radiation and light, and the amount of incident light to the depletion layer is small. Therefore, there is a drawback that the sensitivity decreases. In addition, the diffusion layer 2 needs to be diffused with a high impurity concentration to improve conductive connection with the electrode 3, and since the depletion layer does not spread much into the diffusion layer 2, radiation and light that are annihilated in this layer are does not contribute to the generated current pulse. In this way, the electrodes and the diffusion layer for forming the pn junction form an insensitive layer to radiation, and the pn junction type has a large insensitive layer width and therefore has the disadvantage of low sensitivity. Furthermore, in p-n junction types, 800
It is necessary to go through a heat treatment process at a high temperature of ~1200°C, and this treatment process tends to cause crystal lattice defects and irregularities. Such crystal defects reduce the lifetime of charged bodies generated within the depletion layer, contributing to a decrease in the sensitivity of the detection element. In addition, this type of device originally requires the depletion layer to be sufficiently expanded, but even if the above-mentioned crystal defects occur accidentally at the pn junction interface, even if they are trivial, the result is to suppress the depletion layer from expanding. In addition, in severe cases, the reverse leakage current may increase significantly, causing the device to become defective.

Next, comparing the advantages and disadvantages of the mesa type shown in FIG. 1 and the planar type shown in FIG. 2, in the latter case, radiation can directly enter the depletion layer 5 through the silicon oxide film 6, so
The former is clearly superior to the case where the diffusion layer 2 and electrode 3 cover the entire wafer in that the area of the dead layer is smaller. However, even with the planar type shown in FIG. 2, there are still many problems in terms of energy spectrum characteristics. This point will be explained below.

A radiation detection element originally measures the energy of a single radiation beam by the peak value of a current pulse, and counts the number of incident radiation beams by the number of current pulses. In the case of the planar type shown in Figure 2,
Radiation that directly enters the depletion layer 5 through the silicon oxide film 6 enters the depletion layer with almost the same original energy, whereas radiation that enters the depletion layer 5 through the electrode 3 and the diffusion layer 2 loses its original energy. The energy is considerably reduced and enters the depletion layer, so that an energy value lower than the energy value to be measured will be counted. This is shown in FIG. The horizontal axis in Figure 3 is scaled by an amount proportional to the measured pulse height, that is, the energy value of the radiation, and the vertical axis is the number of pulses, that is, the number of radiation, corresponding to each proportional energy value on the horizontal axis. . Curve A in Figure 3 shows the results of measuring alpha rays emitted by isotope ²⁴¹ Am using the planar element in Figure 2.
The peak value of the curve A on the right side originally corresponds to the energy possessed by this alpha ray, whereas the peak value of the curve A on the left side corresponds to an energy smaller than this original value, and the difference between the two energies is is determined to correspond to the energy consumed by the electrode 3 and the diffusion layer 2. In this way, in the conventional planar radiation detection element, an irregular spectrum appears in addition to the normal spectrum in the measured energy spectrum characteristics, and as shown in the example in Figure 3, the count value of the irregular spectrum is more normal. It may even be larger than the count value of the spectrum.

Based on the recognition of the drawbacks of the prior art as mentioned above,
An object of the present invention is to manufacture a semiconductor element for detecting radiation or light that is highly sensitive, has little aging, and is suitable for detecting or measuring radiation or light in the low energy range.

In order to achieve the above-mentioned object, the manufacturing method according to the present invention includes a step of forming a strong p-type diffusion layer at the periphery of one main surface of a silicon substrate exhibiting weak p-type, which is close to an essential semiconductor; forming an n-type diffusion layer in a part of the region excluding the peripheral edge;
forming a metal oxide film on the one main surface; implanting positive charge ions into the metal oxide film after forming the metal oxide film; and implanting ohmic ions into the n-type diffusion layer through the metal oxide film. The step of forming one electrode in contact with the substrate on the one main surface side and the step of forming the other electrode in ohmic contact with the substrate on the other main surface are combined. Hereinafter, in order to facilitate understanding of the above-described steps, an outline of a semiconductor device manufactured according to the present invention will first be explained.

In the present invention, a silicon single crystal substrate with high resistivity is used as the substrate of the semiconductor element in order to sufficiently spread the depletion layer within the substrate of the semiconductor element. The specific resistance value is preferably 10,000 ohm centimeters or more, preferably about 20,000 ohm centimeters.

This type of silicon single crystal can be any conventional single crystal with a high specific resistance.
In particular, it is preferable to use a material in which silane is purified using a molecular sieve in the material gas stage. Such crystals have recently become available, and in addition to having the advantage of high specific resistance, they also have the advantage of having fewer distortions and defects in the crystal lattice since there is no need to add impurities during the crystallization stage. In other words, in the radiation detection element of the present invention, when impurities are introduced during the manufacturing process, lattice defects cause defects at the pn junction surface and the depletion layer in the vicinity must not be disturbed. The above-mentioned materials with few lattice defects are preferred.

Next, as a means for forming a depletion layer as a radiation detection site, a metal oxide film, such as a silicon oxide film, grown or deposited on the above-mentioned high resistivity wafer is used. As is well known, this type of metal oxide film produces a so-called inversion layer on the wafer surface directly below it. Although the occurrence of such an inversion layer is considered undesirable in MOS type devices, the inventor of the present invention discovered that when such a metal oxide film is combined with the above-mentioned high resistivity wafer, the wafer surface directly under the film is We discovered for the first time that the inversion layer that occurs in the wafer greatly expands the depletion layer within the bulk of the wafer and has a significant effect on improving radiation detection performance, and focused on the active use of the above-mentioned inversion layer. It is something.

The structure of a detection element based on this idea is shown in FIGS. 4 and 5. In FIG. 4, reference numeral 11 denotes a p-type silicon substrate with a high resistivity. A metal oxide film 12 is deposited or grown on one main surface 1a of the substrate, and an inversion layer 13 is formed on the substrate surface directly below this film. induced. In order to connect this inversion layer 13 and one electrode 15, a ring-shaped n-type diffusion layer 14 is diffused around the inversion layer 13 in the example shown in FIG. Furthermore, a p ⁺ layer 16 of the same conductivity type as the substrate 11 is diffused around this n-type diffusion layer, that is, around the periphery of the main surface 11a, to prevent an inversion layer from being formed in this portion. Similarly, a p ⁺ layer is diffused over the entire surface from the other main surface 11b of the substrate 11, and the other electrode 18 is formed thereon.
is deposited. In order to detect or measure radiation or light using a detection element having such a configuration, a reverse bias voltage, for example, a DC voltage of about 20 V, is applied in advance between one electrode 15 and the other electrode 18. As a result, the depletion layer 19 spreads over almost the entire area of the bulk of the substrate 11, as shown in the figure. When radiation or light enters the depletion layer 19 as shown, positive and negative charged bodies PN are generated within the depletion layer 19,
This charged body moves towards one or the other electrode under the influence of the electric field in the depletion layer, and this charge movement is detected in the form of a current pulse in the reverse biasing circuit.

FIG. 5 shows a detection element having a structure different from that in FIG.
An inversion layer 13 is disposed approximately in the center of the area a, and an inversion layer 13 extends directly under the metal oxide layer 12 around the metal oxide layer 12 . This inversion layer ends up extending to the periphery of the substrate 11. Further, the electrode 18 on the opposite side passes through the side surface 11c of the substrate.
This may lead to a so-called short-circuit phenomenon and increase leakage current when a reverse bias voltage is applied to the element. Therefore, a p ⁺ diffusion layer having the same conductivity type as the substrate is provided at the periphery of one main surface 11a of the substrate 11. The configuration of the remaining portions is almost the same as in the case of FIG.

As described above, the device manufactured according to the present invention utilizes a depletion layer that spreads in a high resistivity silicon substrate under a metal oxide film. During the formation of a silicon oxide film in a device, the amount of positive charge incorporated into the oxide film continues to decrease, and if an attempt is made to actively use an inversion layer as in this case, the oxide film formed using recent technology will However, the disadvantage is that it is difficult to form an inversion layer under the film, and the degree of formation tends to vary. FIG. 6 shows an example of this, and shows the energy spectrum characteristics when alpha rays from the isotope ²⁴¹ Am are incident on the element configured as described above. Curve C in the diagram
and D are for the case where the reverse bias voltage of the element is OV and 20V, respectively, and as can be easily seen from the figure, the pulse height distribution clearly differs depending on the bias voltage. This indicates that the ability of the metal oxide film to induce an inversion layer is insufficient, and it also means that the detection characteristics will be distorted if the reverse bias voltage fluctuates during measurement, which is clearly a problem. .
Let's examine this phenomenon somewhat theoretically. According ^to the well _- known formula for the spreading depth of the depletion layer _, Reverse bias voltage applied to the radiation detection element q: Electron charge NA: Acceptor concentration in the wafer Furthermore, φT=φFP+φFN φFP: Fermi potential of the wafer φFN: Due to the fermi potential of the inversion layer induced on the wafer surface Therefore, the depth Xd of the depletion layer can be calculated. In this example, the specific resistance of the wafer is 20,000 ohm centimeters, and the amount of positive charge contained in the silicon oxide film is 1 x 10 ¹⁰ /cm ² , so VR = 0, that is, a reverse bias voltage is applied to the radiation detection element. When there is no voltage, Xd = 28 micrometers, and when a reverse bias voltage of Vd = 20V is applied, Xd = 204 micrometers. Now, since the depth of the depletion layer when there is no bias is shorter than the range of the alpha rays mentioned above (approximately 30 micrometers), all the energy of the alpha rays does not disappear within the depletion layer, so all of the alpha rays are current pulses. 20V cannot contribute to the generation of
When applying a bias of This explains the phenomenon described above, that is, the pulse height distribution when a bias is applied is observed at a position where the energy is higher than when no bias is applied.

In this way, when the amount of positive charge in the metal oxide film is small, the range in which the depletion layer expands becomes narrower, but for example, if the amount of positive charge in the oxide film is one order of magnitude higher than in the previous case, 1×
10 ¹¹ /cm ² , the depth of the depletion layer is 31 micrometers even when there is no bias.From this meaning, the amount of positive charge in the metal oxide film is above a certain concentration, for example, when detecting the alpha rays mentioned above, the depth of the depletion layer is 31 micrometers. It is necessary that the area is at least ×10 ¹¹ /cm ² . For this reason, the present invention employs a step of implanting positively charged ions into the metal oxide film.

Embodiments of the present invention will be described below based on the drawings.
FIG. 7 shows a manufacturing process diagram according to the present invention, and the structure of the element corresponds to that shown in FIG.
In the figure, the same parts as in FIG. 4 are given the same reference numerals. In Figure A, 11 is a p-type silicon substrate with a high resistivity, for example, a silicon substrate with a resistivity of 20,000 ohm centimeter, which is a weak p-type silicon substrate that is close to an essential semiconductor, and is made from a material obtained by refining the silane used as the raw material gas using a momon sieve. It is a board. In the present invention, the specific resistance value is preferably 10,000 ohm centimeters or more in order to effectively expand the depletion layer. Note that the shape of the substrate may be circular or rectangular regardless of the shape, and the thickness of the substrate is desirably greater than the range of the radiation to be detected or the depth through which light can penetrate. In step b, a known thermal oxide film 12 is formed on the entire surface of the silicon substrate. As this metal oxide film, a CVD oxide film and various metal oxide films formed by a sputtering method can be used, but for the sake of simplicity, the case where the above-mentioned thermal oxide film is applied will be explained here. That is, after cleaning the silicon substrate 11 shown in a of the figure, a silicon film 12 is formed in an atmosphere containing a high temperature oxide film of oxygen or water vapor. Although there is no particular problem with the thickness of the oxide film, it is generally desirable to have a thickness of 0.5 micrometer or more.

In step c, the above-mentioned oxide film 12 is applied to the peripheral edge of one main surface 11a of the substrate, the other main surface 11b, and the side surface 1.
1c is removed by a known photoetching method, and then a p ⁺ diffusion layer 16 is diffused over the entire surface from which the oxide film has been removed by, for example, boron diffusion.
This p ⁺ diffusion layer 16 is used as a stopper layer on one main surface side to prevent the above-mentioned inversion layer 13 from spreading to the periphery of the substrate, and on the other main surface side to obtain good ohmic contact with the electrode metal. , it is not necessary to diffuse it particularly thickly, it may be on the order of several micrometers, and the impurity concentration may be at the same level as that used in a normal p ⁺ layer. Also, p ⁺ at the side surface 11c
There is no particular need to provide a diffusion layer.

In step d, an oxide film 20 is again formed for masking on the entire surface of the substrate by, for example, a thermal oxidation method, and then a ring-shaped window 20 is formed by photoetching.
a, and the n ⁺ layer 14 is diffused by diffusing phosphorus or the like. Since this diffusion layer 14 is in conductive contact with the inversion layer induced under the oxide film 12 and in order to maintain close ohmic contact with one of the electrodes 15 to be deposited later, the diffusion depth is The thickness is usually several micrometers, and the impurity concentration is about the same as that of a normal n ⁺ layer. Now, the oxide film formed up to this point usually contains positive charges of about 1 to 10 × 10 ¹⁰ /cm ² , but as mentioned above, this positive charge concentration is insufficient to induce a strong inversion layer. In addition to being insufficient, there are many variations in the concentration value itself, and for quality control purposes, it is necessary to increase this value above a certain value to reduce the variation. Therefore, in the present invention, in the next step e, positive charges 21 are injected into the oxide films 12 and 20 on the one main surface 11a side of the substrate by ion implantation. The conditions for ion implantation are the thickness of the oxide film.
At 0.5 micrometers, boron or phosphorus positive ions are implanted at an acceleration voltage of 30 KeV and a dose of 1×10 ¹² /cm ² or more. As a result, the oxide films 12 and 20 contain a sufficient concentration of positive charge to induce the inversion layer 13 under the film, and the variation thereof can also be managed. In the final step e, an electrode material such as aluminum is deposited on one main surface 11a using a vacuum evaporation method, and one electrode 15 having a predetermined ring shape is formed using an ordinary photoetching method to form an n ⁺ diffusion layer 14. Make ohmic contact. The other electrode 18 is also deposited on the other main surface 11b, but for this purpose, aluminum or the like may be deposited during the vacuum evaporation on the one main surface, and when mounting the element on the case. Separately for convenience in soldering
A composite electrode layer of Cr-Ni-Au may also be deposited.

In the semiconductor element for radiation or photodetection manufactured by the manufacturing method of the present invention as described above, the radiation or the like enters the depletion layer widely spread within the substrate through the metal oxide film that is almost transparent to the radiation or light. ,
It can accept radiation, etc. with virtually no attenuation, and therefore has higher sensitivity than conventional pn junction type detection elements. Particularly when detecting or measuring low-energy radiation such as low-energy X-rays, gamma rays, alpha rays, beta rays, or light, the fact that this insensitive layer is small is a great advantage.
The metal oxide film used in the present invention is also useful as a stabilizing protective film on the surface of a semiconductor, and does not have the drawbacks of metal thin films that are prone to oxidation deterioration in surface barrier type sensing elements, and does not deteriorate over time. has few advantages. If the metal oxide film is carefully selected or the composite film as described above is used to provide stabilization protection, the sensing element according to the present invention can be used over time even when exposed to the atmosphere unless the environment is particularly bad. no change. Further, in this case, since a case that absorbs radiation or the like is not used, there is an advantage that the sensitivity is further increased. Curve B shown as a solid line in Figure 3 shows the results of measuring alpha rays emitted by the isotope ²⁴¹ Am using the detection element with the structure shown in Figure 4. Curve B on the right side of the figure shows the normal energy. The curve B on the left side is the peak of the count value correctly measured in the energy range due to the alpha rays received through the dead layer. Contrary to the conventional pn junction type detection element shown in the figure, the normal count value is much larger than the irregular count value, indicating that the energy spectrum characteristics have been greatly improved by the present invention. In the case of the detection element having the structure shown in FIG. 5, the area of the dead layer can be made even smaller than in this example, so that the above-mentioned characteristics can be further improved. Of course, even in the case of a detection element with the structure shown in Figure 4, even if a mask that can completely absorb low-energy radiation is placed over the insensitive layer at the periphery of the detection element shown in Figure 3, the detection sensitivity will not decrease significantly. Therefore, the problem of energy spectrum characteristics can be substantially solved by such means.

Furthermore, by ion-implanting positive charges into the oxide film in the present invention, the detection characteristics of the device are improved, and variations in characteristics are reduced, resulting in a significant improvement in manufacturing yield. Figure 8 shows isotope detection by the radiation detection element manufactured by the method of the present invention.
This figure shows the characteristics when measuring the alpha rays emitted by ²⁴¹ Am, and corresponds to Figure 6 in the previous scene. As shown in the figure, the peak positions of the curve E when the bias voltage is 0V and the lower part of the curve when the bias voltage is 20V are exactly the same, and even when there is no bias, an inversion layer with a sufficient concentration is induced directly under the oxide film. This proves that the depletion layer extends into the bulk of the substrate to more than about 30 micrometers, which is the range of the alpha line within the substrate.

As explained above, the detection element manufactured by the method of the present invention has excellent characteristics not found in conventional surface barrier type or pn junction type elements, and the method of the present invention allows the detection characteristics of such an element to be constant. This has many effects, such as greatly improving manufacturing yield.

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views of mesa-type and planar-type pn junction radiation detection elements, respectively.
FIG. 3 is a diagram showing a comparison of energy spectrum characteristics between a pn junction type sensing element and a sensing element having the structure shown in FIG. 4 manufactured by the method of the present invention. FIG. 6 is a diagram showing the difference in characteristics when no bias is applied and when a bias is applied without using the ion implantation method, and FIG. 7 is a manufacturing process diagram using the method of the present invention. , FIG. 8 is a diagram showing a comparison of the characteristics of the detection element manufactured by the method of the present invention when no bias is applied and when a bias is applied. In the figure, 11: silicon substrate, 12, 20: metal oxide film, 13: inversion layer, 14: n-type diffusion layer, 15: one electrode, 16: p-type diffusion layer, 18: other electrode, 19: depletion Layer 21: Positive ions to be implanted; R: Radiation or light.

Claims (1)

[Claims] 1. Utilizing an inversion layer induced on the surface of a metal oxide film formed on a high resistivity silicon substrate and a depletion layer extending from the inversion layer, radiation or light incident into the depletion layer is A method for manufacturing a semiconductor device for detection, the method comprising: forming a strong p-type diffusion layer on the periphery of one main surface of a silicon substrate exhibiting weak p-type, which is close to an essential semiconductor; a step of forming an n-type diffusion layer in a part of the region to be removed, a step of forming a metal oxide film on the one principal surface, and ion-implanting positive charges into the film after forming the metal oxide film. a step of forming one electrode on the one main surface side that penetrates the metal oxide film and makes ohmic contact with the n-type diffusion layer; and a step of forming one electrode on the one main surface side that makes ohmic contact with the substrate on the other main surface. 1. A method of manufacturing a semiconductor element for detecting radiation or light, the method comprising the step of forming an electrode.