JPH114012A - Pin photodiode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an adverse effect on a substrate end part, and to suppress a dark current when a depletion layer is formed by applying reverse voltage by a method wherein the interval between the end of the first conductive type semiconductor substrate and the second conductive type impurity region is formed in a specific ratio against the thickness of the first conductive type semiconductor substrate. SOLUTION: A P<+> region 2 is formed on the surface of an N<-> substrate 1, and an N<+> region 3 is formed on the backside of the N<-> substrate 1. Said N<+> region 3 is coupled to an earth 9, a negative potential is applied to the P<+> region 2, and a depletion layer 7 is formed on the N<-> substrate 1. At this point, when the interval between the end 1a of the N<-> substrate 1 and the end 2a of the P<+> region 2 is set at L and the thickness of the N<-> substrate is set at (t), L/t is set at 0.7 or higher and 2 or lower. Accordingly, the affection of the defect generated on the end 1a of the N<-> substrate 1, which is the part cut from a wafer, can be prevented, and the sudden increase of a dark current can also be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silicon PIN photodiode for detecting high-energy radiation such as X-rays, gamma rays, and charged particles.

[0002]

2. Description of the Related Art High-speed response photoelectric conversion elements or X-rays,
Conventionally, PIN photodiodes have been used as semiconductor radiation detecting devices for detecting high-energy radiation such as γ-rays and charged particles. In the PIN photodiode, for example, a P + type region is formed on one surface of a high specific resistance N− substrate, and an N + type region having a higher concentration than the substrate is formed on the opposite surface. Has a structure,
The extension of the depletion layer when a reverse voltage is applied is large, and the energy range of the radiation to be detected is wide, so that high-energy radiation such as X-rays, γ-rays, and charged particles can be detected. Further, it has a feature that it is suitable for high-speed response because the junction capacitance is small.

[0003] Even when the substrate is a P- substrate, the same effect can be obtained by setting the P + type region to the N + region and the N + region to the P + region, and is also called a PIN photodiode. In the following description, an N-substrate will be described as an example. However, in this case, the same applies to a PIN photodiode using a P-substrate.

[0004]

The dark current is a factor that determines the performance of the PIN photodiode, and it is necessary to minimize the dark current. However,
When a large reverse voltage is applied to greatly expand the depletion layer, there is a problem that the dark current increases extremely. Therefore,
An object of the present invention is to realize a PIN diode with a low dark current used for applications such as detection of high-energy radiation such as X-rays, γ-rays, and charged particles by applying a large reverse voltage to greatly expand a depletion layer. And

[0005]

According to the present invention, there is provided a semiconductor substrate of a first conductivity type made of silicon, which has a second conductivity type impurity region partially on a first surface thereof and has a second conductivity type impurity region on a second surface thereof. In a PIN photodiode having a first conductivity type impurity region having a higher impurity concentration than a substrate, a distance (L) between an end of the first conductivity type semiconductor substrate and an end of the second conductivity type impurity region is equal to the first conductivity type. 70 of the thickness (t) of the semiconductor substrate
% Or more (0.7 ≦ L / t)
N photodiode.

Here, the distance (L) between the end of the first conductivity type semiconductor substrate and the end of the second conductivity type impurity region is at least 70% of the thickness (t) of the first conductivity type semiconductor substrate, and 200
% (0.7 ≦ L / t ≦ 2.0). When a reverse voltage is applied to a PIN photodiode to form a depletion layer, the depletion layer extends not only in the direction perpendicular to the substrate but also in a direction parallel to the substrate surface, which reduces the influence of the end surface of the substrate containing a large number of defects due to cutting. The dark current suddenly increases when it is received. However, in the present invention, the edge of the N-substrate and P
By setting the distance from the end of the + region to 70% or more of the depth of the depletion layer, the influence of the end face of the substrate is avoided. Also, N-
The distance between the edge of the substrate and the edge of the P + region is set to 20 times the depletion layer depth.
If it exceeds 0%, the effect of reducing the dark current is completely lost.
It is preferably at most 200%.

Further, even when the PIN photodiode is completely depleted and used, by setting the distance between the edge of the N- substrate and the P + region to be 70% or more of the thickness of the N- substrate, adverse effects on the edge of the substrate can be prevented. Can be avoided. PI of the present invention
The N-photodiode is preferably used by depleting 80% or more of the first conductivity type semiconductor substrate by applying a reverse voltage.

In the PIN photodiode having such a structure, when the substrate is depleted by 80% or more, the S / N ratio is not so different from that obtained when the substrate is completely depleted. The specific resistance of the first conductivity type semiconductor substrate is, for example, 1 kΩ · cm or more. The thickness of the substrate is 300 μm for handling in manufacturing
The specific resistance of the substrate required to form a depletion layer at a thickness of about 100 V is 1 kΩ · cm or more.

Further, in the present invention, on the first surface of the first conductivity type semiconductor substrate, the second first conductivity type impurity region surrounding the second conductivity type impurity region and having a higher impurity concentration than the substrate is provided. May be formed. For example, an N + region having a higher impurity concentration than the substrate is formed on the same substrate surface surrounding the P + region.

By forming this N + region, an increase in dark current at a reverse voltage close to the breakdown voltage can be reduced. Further, in the present invention, a second second conductivity type impurity region may be provided on the first surface of the first conductivity type semiconductor substrate so as to surround the second conductivity type impurity region.

For example, at least one second P + region called a guard ring or the like is formed on the same substrate surface surrounding the P + region. As a result, the breakdown voltage can be increased and a thicker substrate can be completely depleted. Further, in the present invention, on the first surface of the first conductivity type semiconductor substrate,
A second first conductivity type impurity region surrounding the second second conductivity type impurity region and having a higher impurity concentration than the substrate may be provided.

That is, in order to further increase the breakdown voltage, for example, at least one second P + region called a guard ring or the like is formed on the same substrate surface surrounding the P + region and the guard ring is formed. N which has a higher impurity concentration than the substrate on the same substrate surface surrounding the outermost P + region
+ Region is formed. As a result, it is possible to reduce an increase in dark current at a reverse voltage close to a breakdown voltage in a PIN photodiode that can extend a depletion layer extremely deeply with a high withstand voltage.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a sectional view of a PIN photodiode according to one embodiment. On the surface of the N-substrate 1, P
+ Region 2 is formed, and an N + region 3 is formed on the back surface.

In this embodiment, the N-substrate 1 has a thickness t.
Is 300 μm, and the end 1 of the N-substrate 1 is used.
the distance L between a and the end 2a of the P + region 2 is 210 μm or more;
The thickness is set to 600 μm or less, particularly about 500 μm. N-
The end 1a of the substrate 1 is a cut portion from the wafer, and a lot of defects are generated.
By setting a apart by a predetermined distance or more, the influence of defects can be avoided.

Consider a case where a reverse voltage is applied to the PIN photodiode using the P + region 2 as an anode and the N + region 3 as a cathode. That is, for example, when the N + region 3 is coupled to the ground 9 and a negative potential is applied to the P + region 2, a depletion layer 7 is generated on the N− substrate 1, and the depletion layer extends toward the N + region 3 according to the potential. Eventually, the depth becomes W, and eventually reaches N + region 3 to form a complete depletion layer.

Of course, the application of the reverse voltage is not limited to the above, and generally, the potential of the N + region 3 should be higher than that of the P + region 2. Here, the depletion layer 7 extends not only in a direction perpendicular to the surface of the N − substrate 1 but also in a direction parallel to a small percentage. Here, as described above, the thickness of the N− substrate 1 is t, the end 1 a of the N− substrate 1 and the end 2 of the P + region 2.
L / t in a fully depleted state, where L is the distance from
FIG. 2 is a characteristic diagram showing the relationship between the current and the dark current.

From the results shown in FIG. 2, it can be seen that the dark current sharply increases in the range of L / t <0.7. This is because the end 1a of the N-substrate 1 is affected by a large number of defects due to the extension of the depletion layer 7 in a direction parallel to the surface of the N-substrate 1. That is, it is necessary to satisfy L / t ≧ 0.7 in order to avoid the influence of the end 1a of the N-substrate 1 in which a large number of defects are generated.

By the way, increasing the distance L between the end 1a of the N- substrate 1 and the end 2a of the P + region 2 lowers the yield. As is apparent from FIG. 2, when L / t exceeds 2.0, there is no longer any decrease in dark current. Therefore, it is understood that L / t may be 2.0 or less. Of course, even when the depletion layer is not completely formed, if L / t ≧ 0.7, no drastic increase in dark current occurs.

When the depletion layer is formed in an S / N ratio of 80% or more, there is no much difference from the case where the depletion layer is completely formed. Here, whether or not the layer is completely depleted can be determined by measuring the junction capacitance. FIG. 3 is a graph showing a junction capacitance-reverse voltage characteristic of a PIN photodiode. According to this, it is understood that the junction capacitance decreases as the reverse voltage increases, and eventually shows a constant value. When the constant value is reached, the depletion layer is formed.

Depletion of 80% or more means that the junction capacitance is 125% or less when the junction capacitance in the case of complete depletion is 100%. Generally, the thickness of the substrate is 30 due to the handling in manufacturing.
The specific resistance of the substrate needs to be 1 kΩ · cm or more in order to form a depletion layer with this thickness of about 100 V or more. The specific resistance of the substrate is 20 kΩ · cm.
Can be obtained relatively easily, the specific resistance is 1
It is inconvenient to apply a reverse voltage of 100 V or more to those of kΩ · cm or less to make the depletion layer 300 μm or more.

In other words, when a substrate having a specific resistance of 1 kΩ · cm or less is used, the depletion layer becomes 300 μm or less. Therefore, in this case, the distance between the edge of the N− substrate and the edge of the P + region is determined by the thickness of the N− substrate. If it is 70% or more, the useless area is taken and the yield is reduced. (Second Embodiment) FIG. 4 relates to a second embodiment of the present invention.
FIG. 11 is a cross-sectional view of a PIN photodiode in which an N + region having a higher impurity concentration than the substrate is formed on the same substrate surface surrounding the P + region.

As shown in FIG. 4, a P + region 2 is formed on the surface of the N- substrate 1, and an N + region 3 is formed on the back surface. The surface of the N- substrate 1 where the P + region 2 is formed is surrounded by N
An N + region 4 having a higher impurity concentration than the substrate 1 is formed.

Also in this embodiment, the end 1 of the N-substrate 1
The distance L between a and the end 2a of the P + region 2 is set to be 70% or more and 200% or less of the thickness t of the N− substrate 1;
-The influence of the cut end surface of the substrate 1 is prevented. Here, for example, in order to detect X-rays of several tens keV or more, it is necessary to extend the depletion layer by 1 mm or more. For that purpose, even if the specific resistance of the N-substrate is 20 kΩ · cm, a breakdown voltage of several hundred V or more is required. Required. In this case, in general, there occurs a problem that the dark current suddenly increases due to a high breakdown voltage. However, in the present embodiment, by forming the N + region 4, it is possible to prevent such an increase in the dark current. it can.

FIG. 7 shows the dark current-reverse voltage characteristics of the PIN photodiodes of the first embodiment of FIG. 1 and the second embodiment of FIG. 4 by curves a and b, respectively. In addition,
Substrate specific resistance, substrate thickness, area of P + region, substrate edge and P
The distances L from the edges of the + region are all the same. First
The curve a of the PIN photodiode according to the embodiment shows that the dark current sharply increases before the breakdown,
The curve b of the PIN photodiode of the present embodiment shows that the increase in the dark current is small until the breakdown occurs,
Therefore, when a high voltage is applied to extend the depletion layer deeply, as in the present embodiment, the P + having the N + region 4 formed therein is used.
It turned out that the IN photodiode was superior.

(Third Embodiment) FIG. 5 is a sectional view of a high breakdown voltage PIN photodiode according to a third embodiment of the present invention. As shown in FIG. 5, a P + region 2 is formed on the front surface of the N- substrate 1, and an N + region 3 is formed on the back surface.

On the surface on which the P + region 2 is formed, P +
Two ring-shaped P + regions 5 are formed around the region 2 without being in direct contact with the region 2. Also in this embodiment, N
The distance L between the end 1a of the substrate 1 and the end 2a of the P + region 2 is N
-The thickness is set to be 70% or more and 200% or less of the thickness t of the substrate 1 to prevent the influence of the cut end face of the N- substrate 1.

The second P + region 5 provided in the present embodiment is called a guard ring or the like, and has an effect of increasing a breakdown voltage. At least one region may be provided, but as the number of regions increases, the breakdown voltage increases, and the device is usually used in a floating state. The dark current-reverse voltage characteristic of the PIN photodiode of the present embodiment is shown by a curve c in FIG. 7, similarly to the above-described embodiment.
The substrate resistivity, the substrate thickness, the area of the P + region, and the distance L between the edge of the substrate and the edge of the P + region are all the same.

As is clear from FIG. 7, the curve c of the PIN photodiode in which two P + regions 5 are formed as guard rings as in the present embodiment has a high breakdown voltage. (Fourth Embodiment) FIG. 6 is a sectional view of a PIN photodiode having a high withstand voltage and a low dark current according to a fourth embodiment of the present invention.

In the present embodiment, P of the third embodiment shown in FIG.
In the IN photodiode, an N + region 4 having a higher impurity concentration than the N− substrate 1 is formed around the P + region 5 as a guard ring. Also in the present embodiment, the distance L between the end 1a of the N- substrate 1 and the end 2a of the P + region 2 is set to be 70% or more and 200% or less of the thickness t of the N- substrate 1, and the N- substrate 1 prevents the influence of the cut end face.

Further, since the P + region 5 is formed as a guard ring, there is an effect of increasing the breakdown voltage, and further, since the N + region 4 is formed therearound, the breakdown voltage becomes high. This can also prevent an increase in dark current. The dark current-reverse voltage characteristic of the PIN photodiode of the present embodiment is shown by a curve d in FIG. 7, similarly to the above-described embodiment. The substrate resistivity, the substrate thickness, the area of the P + region, and the distance L between the edge of the substrate and the edge of the P + region are all the same.

As is apparent from FIG. 7, when two P + regions 5 are formed as guard rings as in the present embodiment, the breakdown voltage becomes high. It has been found that the increase in dark current near the breakdown is small due to the formation of, and therefore, it is advantageous when a high voltage is applied to extend the depletion layer deeply.

In this embodiment, there are two P + regions as guard rings.
At least one is sufficient, and as the number increases, the effect increases.

[0033]

As described above, according to the present invention, it is possible to realize a PIN diode used in a fully depleted or near depletion layer state and a low dark current.
Used for applications such as high-energy radiation detection such as X-rays and γ-rays of V or more.
It is also possible to realize a low dark current of a PIN photodiode which requires a depletion layer.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a PIN photodiode according to a first embodiment of the present invention.

FIG. 2 is a dark current-L / t characteristic diagram of a PIN photodiode.

FIG. 3 is a graph showing a junction capacitance-reverse voltage characteristic of a PIN photodiode.

FIG. 4 is a cross-sectional view of a PIN photodiode according to a second embodiment of the present invention.

FIG. 5 is a sectional view of a PIN photodiode according to a third embodiment of the present invention.

FIG. 6 is a sectional view of a PIN photodiode according to a fourth embodiment of the present invention.

FIG. 7 is a graph showing dark current-reverse voltage characteristics of the PIN photodiode of each embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... N-substrate 1a ... End of substrate 2,5 ... P + region 2a ... End of P + region 3,4 ... N + region 7 ... Depletion layer 8 ... Applied voltage 9 ... grounding

Claims (7)

[Claims] A first conductive type semiconductor substrate made of silicon;
A PIN photodiode having a second conductivity type impurity region partially on a surface and a first conductivity type impurity region having a higher impurity concentration than the substrate on the second surface.
The distance (L) between the end of the conductive type semiconductor substrate and the end of the second conductive type impurity region is 70% or more (0.7 ≦ L / t) of the thickness (t) of the first conductive type semiconductor substrate. A PIN photodiode. 2. The semiconductor device according to claim 1, wherein a distance (L) between an end of the first conductivity type semiconductor substrate and an end of the second conductivity type impurity region is 70 times the thickness (t) of the first conductivity type semiconductor substrate.
% Or more and 200% or less (0.7 ≦ L / t ≦ 2.0). 3. The PIN photodiode according to claim 1, wherein 80% or more of the first conductivity type semiconductor substrate is depleted by applying a reverse voltage. 4. The PIN photodiode according to claim 1, wherein the first conductivity type semiconductor substrate has a specific resistance of 1 kΩ · cm or more. 5. The second conductive type semiconductor substrate according to claim 1, wherein the second conductive type impurity region surrounds the second conductive type impurity region and has a higher impurity concentration than the substrate on the first surface of the first conductive type semiconductor substrate. P having a first conductivity type impurity region
IN photodiode. 6. The second conductive type impurity region according to claim 1, surrounding the second conductive type impurity region on the first surface of the first conductive type semiconductor substrate. A PIN photodiode, comprising: 7. The second conductive type semiconductor substrate according to claim 6, wherein the second conductive type impurity region is surrounded by the second conductive type impurity region on the first surface of the first conductive type semiconductor substrate. A PIN photodiode having the first conductivity type impurity region described above.