JPH11135808A - Manufacture of surge protective element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a surge protective element whose breakover voltage is made low and whose capacitance is not increased as compared with conventional cases. SOLUTION: A first-conductivity region 11, whose impurity concentration is higher than that of a first-conductivity substrate 10 is formed through diffusion in a part on the first main face of the substrate. A first-conductivity layer 10a whose impurity concentration is lower than that of the region 11 is formed over the whole first main face of the semiconductor substrate 10, including the region 11. A second-conductivity region 12 is formed at a depth which joins the region 11 from the surface of the layer 10a. A second-conductivity region 13 is formed on the second main face of the semiconductor substrate 10 so as to face the region 12. A first-conductivity region 14 is formed inside the region 12. An electrode 16 which comes into ohmic contact with the region 12 and with the region 14 is formed, and an electrode 17 which comes into ohmic contact with the region 13 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a surge protection element for protecting an electronic device against a surge. More specifically, the present invention relates to a method for manufacturing a surge protection element having a conductivity type of npnp or pnpn structure.

[0002]

2. Description of the Related Art In recent years, as electronic devices have been digitized and their power consumption has been suppressed, the withstand voltage itself of electronic devices tends to decrease. Therefore, a surge protection element of an electronic device is required to start operating at a lower surge voltage to protect the electronic device. Conventionally, in order to start the operation of this type of surge protection element at a low voltage, a silicon wafer that is a starting material of the surge protection element has been selected to have a high impurity concentration, that is, a wafer having a low resistivity. As another means, as shown in FIG. 1B, after forming the same n-type region 2 having a high impurity concentration on a part of the first main surface of the n-type silicon substrate 1 by a normal diffusion process, P-type regions 3 and 4 are respectively formed on the first main surface and the second main surface which are both surfaces of the substrate 1, an n-type region 5 is further formed in the region 3, and ohmic regions are formed on the regions 5 and 3. A surge protection element has been proposed in which the contacting electrode 6 is formed while the ohmic contacting electrode 7 is formed in the region 4 of the second main surface. In this surge protection element, the region 2 is formed to locally increase the impurity concentration of the substrate,
A low breakover voltage was realized.

[0003]

However, in the former surge protection device, although the breakover voltage can be lowered by using a wafer having a high impurity concentration, the junction capacitance is increased, and the signal applied to the circuit is reduced. There was a problem that transmission was delayed. Further, even if an extremely low breakover voltage, for example, about 10 V, is to be realized, it is extremely difficult to manufacture a silicon wafer having an impurity concentration commensurate with it, and even if a wafer having an extremely high impurity concentration can be manufactured, the base diffusion for this wafer is difficult. In addition, there has been a problem that the emitter cannot be substantially diffused. n
n sandwiched between p layers in surge protection element of pnp structure
The layer generally needs to be thin to some extent from the viewpoint of securing appropriate current capacity and response speed. In the latter surge protection element, it is necessary to form the regions 3 and 4 deep, that is, thick, in order to reduce the thickness of the layer 1a sandwiched between the region 3 and the region 4. Prior to this, the formation of the region 2 is performed. Diffusion must be performed further deeply, which makes the heat treatment difficult. In addition, since the junction between the region 2 and the region 3 for determining the breakover voltage is at a deep position from the substrate surface, the interface impurity concentration of this junction is set to be too high as long as the n-diffusion region 2 is formed by the ordinary thermal diffusion method. Can not. In other words, the controllable range of the breakover voltage is narrow, so that a surge protection element with a very low breakover voltage cannot be obtained. An object of the present invention is to provide a method of manufacturing a surge protection element that can lower the breakover voltage and does not increase the capacitance as compared with the related art.

[0004]

The invention according to claim 1 is
As shown in FIG. 1A, a semiconductor substrate 10 of a first conductivity type
Of the first principal surface having a higher impurity concentration than that of the substrate.
A first region 11 of a conductivity type is formed, and a first conductivity type layer 10 a having a lower impurity concentration than the first region 11 is formed over the entire first main surface of the semiconductor substrate 10 including the first region 11. A second region 12 of the second conductivity type is formed at a depth from the surface of the semiconductor substrate 10 to join the first region 11, and a second region 12 of the second conductivity type is formed on the second main surface of the semiconductor substrate 10 so as to face the second region 12. 3 areas 13
Is formed, a fourth region 14 of the first conductivity type is formed in the second region 12, a first electrode 16 in ohmic contact with the second region 12 and the fourth region 14 is formed, and a third region 13 is formed. This is a method for manufacturing a surge protection element that forms a second electrode 17 that makes ohmic contact. In FIG. 1, the first conductivity type is n,
Although the surge protection element of the npnp structure is illustrated assuming that the second conductivity type is p, the surge protection element of the pnpn structure can be similarly formed by changing n of the conductivity type to p and p to n.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention basically has a conductivity type of npn.
This is a method for manufacturing a two-terminal surge protection element made of a semiconductor having a p or pnpn structure. The present invention relates to npnpn or pn
The present invention can also be applied to a bidirectional element having a pnp structure. The layer 10a is preferably formed by epitaxial growth with the same impurity concentration as the substrate 10. The thickness of the layer 10a and the impurity concentration profile of the region 11 are determined by a desired breakover voltage and the thickness of the region 12.

[0006]

Next, an embodiment of the present invention will be described with reference to the drawings. <Embodiment> As shown in FIG. 1A, an n-type silicon substrate 10 is prepared, and a region 11 having a concentration distribution (depth profile) corresponding to a target breakover voltage is provided.
From the first main surface of the substrate 10 to an n-type impurity (for example, phosphorus).
Is formed by diffusing under predetermined conditions. Next, a silicon layer 10a having the same conductivity type and impurity concentration as the substrate 10 is formed on the entire first main surface by, for example, epitaxial growth.

Next, a p-type impurity (for example, boron) is diffused from the surface of the layer 10a to form a region 12 having _a junction Ja with the region 11, that is, exceeding the thickness (depth) of the silicon layer 10a. . Further, a p-type impurity is diffused from the second main surface to form a region 13 facing region 12. Area 12
And 13 may be formed simultaneously, that is, in one heat treatment step. Impurity concentration profile of J _a vicinity of an interface determines the breakover voltage of the device. Then, area 1
After forming the n-type region 14 in the region 2, the regions 12 and 14
And an electrode 16 that makes ohmic contact with the region 13 is formed. Thereby, the surge protection element of the embodiment is obtained.

<Comparative Example> As shown in FIG. 1B, a silicon substrate 1 having the same impurity concentration as the substrate used in the embodiment and having a thickness corresponding to the thickness of the silicon layer 10a is prepared. The region 2 is formed by diffusing an n-type impurity from the first main surface of the substrate under the same heat treatment conditions as when forming the region 11 in the embodiment. Next, regions 3 and 4 are formed from both surfaces of the substrate 1 by diffusing p-type impurities under the same heat treatment conditions as when forming the regions 12 and 13 in the embodiment. The junction J _b of region 2 and region 3,
Similarly J _a in example, the impurity concentration profile of the vicinity of the interface determines the breakover voltage of the device.

Subsequently, an n-type region 5 is formed in the region 3 under the same heat treatment conditions as when the region 14 in the embodiment is formed, and an electrode 6 in ohmic contact with the regions 3 and 5, and an ohmic contact in the region 4 are formed. An electrode 7 to be in contact is formed. Thereby, the surge protection element of the comparative example is obtained.

<Comparative observation> Regions 14 and 1 in the embodiment
2, 10b and 13 constitute the four main layers of the npnp structure, and in the comparative example, the regions 3, 5, 1b and 4 correspond thereto. The regions corresponding to each other in the example and the comparative example have the same impurity diffusion conditions at the time of formation, and have the same thickness and impurity concentration of the substrate (including the epitaxial layer) to be subjected to the diffusion process. 2
The thickness and the impurity concentration profile are almost the same in most parts except for the local part related to the above. On the other hand, the region 11 in the example has the same impurity concentration profile as the region 2 in the comparative example due to the same diffusion conditions, but is located deeper in the substrate by the epitaxial layer. Based on the above, the junction for determining the breakover voltage of the surge protection element, that is, the regions 11 and 12 in the embodiment,
Comparing the junction (J _a ) with the junction (J _b ) between the regions 2 and 3 in the comparative example, the example has a higher impurity concentration at the interface, and therefore has a lower breakover voltage.

[0011]

As described above, according to the manufacturing method of the present invention, a range in which a normal impurity diffusion technique can be used for a local high concentration region in a substrate provided for controlling a breakover voltage. In addition to being able to adjust the impurity concentration profile of the region within the substrate, it is also possible to arbitrarily adjust the position (depth from the substrate surface) of the region in the substrate by applying an epitaxial growth technique or the like. The degree of freedom in designing the surge protection element having a voltage is improved, and specifically, a low breakover voltage can be realized irrespective of the thickness and the impurity concentration profile of the four main layers constituting the npnp structure. As a result, more specifically, in obtaining a lower breakover voltage as compared with the former former surge protection device,
Since it is not necessary to use a high-concentration substrate as a starting material, it is possible to avoid difficulties such as a high temperature and a long time in the diffusion of impurities during manufacturing, and also, functionally, the capacitance is suppressed low, and signal delay in an applied circuit. Do not cause problems such as Also, for example, in order to obtain appropriate current capacity and response, np
Even if the n-layer sandwiched by the p-layers in the np structure is made thin, that is, the upper and lower p-layers are made thicker, the local high-concentration region in the substrate is located at a correspondingly deeper position, so that the break-over voltage of the surge protection element becomes higher. The interface impurity concentration of the junction which determines the above can easily be increased, and thus a lower breakover voltage can be easily realized.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view showing a manufacturing step of a surge protection element according to an embodiment of the present invention in the order of steps. (B) Sectional drawing which shows the manufacturing process of the surge protective element of a comparative example in order of a process.

[Explanation of symbols]

 Reference Signs List 10 silicon substrate (semiconductor substrate) 10a epitaxial layer 11 first region 12 second region 13 third region 14 fourth region 16 first electrode 17 second electrode

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H02H 9/04

Claims (1)

[Claims] A first conductive type first region having an impurity concentration higher than that of the first conductive type semiconductor substrate formed on a part of a first main surface of the first conductive type semiconductor substrate; Forming a first conductivity type layer (10a) having a lower impurity concentration than the first region (11) on the entire first main surface of the semiconductor substrate (10) including (11); and a surface of the layer (10a). A second region (12) of the second conductivity type is formed at a depth having a junction with the first region (11), and the second region (12) is formed on a second main surface of the semiconductor substrate (10). A third region (13) of the second conductivity type is formed facing the first region, a fourth region (14) of the first conductivity type is formed in the second region (12), and the second region (12) and A method for manufacturing a surge protection device, comprising: forming a first electrode (16) in ohmic contact with a fourth region (14); and forming a second electrode (17) in ohmic contact with the third region (13).