JPH03108378A - Variable capacity diode device - Google Patents

Abstract

PURPOSE:To improve high frequency properties, and also to enlarge capacity varying ratio by forming a diffusion layer, which nearly determines capacity ratio, in hollow columnar shape. CONSTITUTION:An N-conductivity type high resistivity epitaxial layer 2 is formed on a high impurity concentration N-conductivity semiconductor substrate 1. An N- conductivity type high impurity concentration circular hollow columnar diffusion layer 3 is formed to reach the substrate 1 from the main surface of the semiconductor base substance of this layer 2. A P-conductivity type high impurity concentration diffusion layer 4 is formed circularly by diffusion so that it may cover this layer 3, and by the layers 3 and 12, PN junction J2 is formed. An SiO2 film 51, which is the SiO2 film covering the center of an element formed in this diffusion process and covers the element main surface exposed part of PN junction and the element center, and an SiO2 film 5, which is formed on the periphery of the hollow column and covers the part where PN junction is exposed to the element main surface, are left as protective films. Next, the exposed part of the layer 4 and the film 51 are coated with a conductor film 6 so as to form an electrode. To this film 6, a gold wire 7 is force- bonded by heat. According to this constitution, it is possible to form the layer 4 extremely thin, and the capacity varying ratio can be enlarged.

Description

[Detailed Description of the Invention] [Industrial Application Field of the Invention] The present invention relates to a variable capacitance diode device used as a tuning element suitable for high frequencies, and has a high figure of merit Q,
This relates to a variable capacitance diode device with a large variable capacitance ratio.

[Conventional technology]

FIGS. 5a and 5b show a plan view and a sectional view of a conventional variable capacitance diode device.

In FIGS. 5a and 5b, a semiconductor substrate (N") 10 whose semiconductor body is of N conductivity type and has a high impurity concentration, and an epitaxial layer (N") whose semiconductor substrate is of N conductivity type and has a low impurity concentration are formed on the semiconductor substrate 10. -) 20 semiconductor layers.

A diffusion layer (N'') 30 of Ni conductivity type and high impurity concentration is diffused into the epitaxial layer 20 from the main surface of the semiconductor substrate so as to reach the semiconductor substrate IO. A junction J1 is formed by diffusing the diffusion layer (P'') 40 which is a type and has a high impurity concentration. The silicon dioxide film formed on the main surface of the semiconductor substrate in the diffusion process is removed in the center of the device, and the P exposed on the main surface is removed.
The silicon dioxide film 50 covering the N junction J is left as a protective film for the junction. A conductor film 60 made of aluminum or the like is deposited on the exposed portion of the main surface to form an electrode. A gold wire 70 is wire-bonded to this conductor film 60 to form a variable capacitance diode device.

In variable capacitance diode devices for high frequencies, the maximum capacitance value C
Wax must be set to several pF or less. That is,
It is necessary to set the bonding area extremely small. For example, F
When attempting to obtain a maximum capacitance value Cmax of several pF in a manufacturing process adjusted for a variable capacitance diode device for M radio receivers, according to the conventional example shown in Fig. 5, the diameter of the junction area S0 needs to be reduced to about 80 μm.

However, it becomes necessary to form the conductor film 60 for wire bonding in an extremely large area relative to the bonding area. Usually, the size of the gold ball 71 of the gold wire 70 is about 3 to 5 times the surface shape of the gold wire, and the diameter of the gold wire is 25 μm.
In the case of m, the diameter of the gold ball is 75 to 125 μm, and the area of the conductor film 60 is usually set correspondingly.

In the conventional variable capacitance diode device, the area of the diffusion layer 40 is smaller than the area of the tip of the capillary from which the gold wire is drawn out during wire bonding, and stress is applied to the diffusion layer 40 during thermocompression bonding of the gold wire. . In addition, in order to make the capacitance variable ratio as large as possible without deteriorating the figure of merit Q, it is necessary to make the junction depth of the diffusion layer 40 shallow. It is layered.

[Problem to be solved by the invention]

In recent years, in variable capacitance diode devices for high frequencies,
A high figure of merit Q is required, and the capacitance value is also required to be smaller and smaller. For example, in a variable capacitance diode device used in AM radio, etc., the maximum capacitance value C□8 is about 350 to 650 pF.
As the frequency band used becomes higher, such as the UHFTV band, a variable capacitance diode device is required to have a maximum capacitance value CmaN of about 0.1 to 15 pF, and the junction area becomes extremely small. According to the embodiments shown in FIGS. 5a and 5b, the area of the outer peripheral wall of the cylindrical diffused diffusion layer 30 to obtain the required maximum capacitance value C1ax becomes extremely small. As is clear from the skin effect of high frequencies, since the outer circumferential area of the diffusion layer 30 is small, the high frequency resistance increases, resulting in a decrease in the figure of merit Q.

Also, during wire bonding, the gold ball is attached to the conductor film 6.
It is necessary to prevent the conductor film from protruding from 0, and the area of the conductor film becomes unnecessarily large compared to the bonding area. Therefore, unnecessary stray capacitance may occur between the epitaxial layer 20 and the conductive film 60 extending in the shape of a brim from the joint, which has the drawback of adversely affecting high frequency characteristics.

Furthermore, the junction area is set to be small to accommodate high frequencies, and the diffusion depth of the diffusion layer 40 is set to 0.0 to obtain a large variable capacitance ratio.
In order to make it shallower than 1 μm, during wire bonding, when lowering the capillary that pulls out the gold wire 70 onto the chip and thermocompression bonding the gold vA70 to the conductor film 60, the tip of the capillary is placed on the bonding surface. Because the gold wire 70 is larger in size, the stress during thermocompression bonding of the gold wire 70 causes the diffusion layer 40 to
There is a risk of damage to the bonding process, and this is the cause of a considerable number of defects during bonding.

The present invention has been made to improve the above-mentioned drawbacks, and its main purpose is to provide a variable capacitance diode device with good high frequency characteristics and a large variable capacitance ratio.

Another object of the present invention is to provide a variable capacitance diode device that can improve the figure of merit Q.

Another object of the present invention is to provide a variable capacitance diode device having a structure in which the element is not destroyed by stress during wire bonding.

[Means to solve problems]

The variable capacitance diode device of the present invention is intended to solve the above-mentioned problems, and has a hollow cylindrical shape formed in a high resistivity epitaxial layer of a first conductivity type formed on a semiconductor substrate of a first conductivity type. A first diffusion layer of a first conductivity type is formed, and a second diffusion layer of a second conductivity type is formed in a shallow annular shape so as to cover the first diffusion layer of a first conductivity type to form a junction. death,
a first insulating film layer covering a junction located inside the second diffusion layer; a second insulating film layer covering a junction at an outer peripheral portion of the second diffusion layer; It includes a main surface exposed portion of the layer and a conductive film covering the first insulating film layer.

[Effect]

The variable capacitance diode device of the present invention has a first diffusion layer formed by diffusing an impurity element of the same conductivity type in a high concentration into an epitaxial layer having a high specific resistance. By making it columnar, it prevents the drop in high frequency resistance based on the skin effect and improves the figure of merit Q.
In addition, the first and second diffusion layers are not formed in the portion where the capillary presses the main surface of the semiconductor substrate during wire bonding, so that the second diffusion layer is formed thin in order to increase the variable capacitance ratio. Even if this happens, the shallowly formed diffusion layer will not be destroyed by the stress caused by the pressure of the capillary.

〔Example〕

FIGS. 1a and 1b are views showing one embodiment of a variable capacitance diode device of the present invention, FIG. 1a is a plan view of the variable capacitance diode device, and FIG. 1 is a sectional view thereof. be.

In Figure 1, an epitaxial layer (N-) 2 of N conductivity type and high resistivity is grown in vapor phase on a semiconductor substrate (N'') 1 of which the semiconductor substrate is of N conductivity type and has a high impurity concentration. has been formed.

From the main surface of the semiconductor substrate of the epitaxial layer 2, an annular or hollow cylindrical diffusion layer (N'') 3 of N4 type and high impurity concentration is diffused so as to reach the semiconductor substrate 1. A diffusion layer (P'') 4 of P conductivity type and high impurity concentration is formed in an annular shape so as to cover it, and a PN junction J2 is formed between the diffusion layer 3 and the epitaxial layer 2. This is a silicon dioxide film that covers the center of the device formed in a diffusion process, and includes an epitaxial layer 2 and an annular diffusion layer 4.
PN formed on the inner periphery of the hollow cylinder formed by
A silicon dioxide film 5I that covers the exposed part of the element main surface of the junction and the central part of the element, and a silicon dioxide film 5I that covers the part where the PN junction formed on the outer periphery of the hollow cylinder is exposed to the element main surface are left as protective films. The exposed portion of the annular diffusion layer 4 from which the silicon dioxide film has been removed and the silicon dioxide film 5 at the center of the element. A conductive film 6 made of aluminum or the like is deposited thereon to form an electrode. A gold wire 7 drawn out from the capillary is connected to this conductor film 6 by thermocompression bonding, and the gold wire 7 is connected to an external lead or the like to form a variable capacitance diode device.

Generally, the maximum capacitance value Cma* of a variable capacitance diode device
is determined by the product of the junction area S and the impurity concentration coefficient α of the junction surface. Therefore, in order to reduce the maximum capacitance value Cmax, the junction area S or the impurity concentration coefficient α can be decreased. The maximum capacitance value Cmax is the product of the junction area S and the impurity concentration coefficient α of the junction surface, and has the following relationship: Cmax ocS cx −−−−−−−−−−=−
(1) From the relationship in equation (1), if you want to reduce the maximum capacitance value Cmax and increase the capacitance change ratio, make the junction depth of the P conductivity type diffusion layer 4 shallow, and reduce the junction area S or impurity It is necessary to reduce the concentration coefficient α. but,
There is a limit to reducing the impurity concentration coefficient α due to the impurity concentration of the semiconductor substrate. That is, if an attempt is made to increase the sheet resistance of the semiconductor substrate by adjusting the impurity concentration of the semiconductor substrate, which determines the minimum capacitance value Cm1n, to a low value, the figure of merit Q will decrease, so it is decided to set the impurity concentration of the semiconductor substrate low. has its limits. From this point of view, reducing the junction area S is generally an effective means for reducing the maximum capacitance value Cmax. In addition, in order to increase the variable capacitance ratio, there is a limit to controlling the impurity concentration, so by forming a shallow junction, the change in the impurity concentration between the surface and the semiconductor substrate surface can be increased, and the variable capacitance ratio can be increased. It has a large value.

In the embodiment shown in FIG. 1, the epitaxial layer 2 has a relatively high resistivity, and a hollow cylindrical diffusion layer 3 is formed in the epitaxial layer 2 in consideration of the skin effect. Maximum capacitance value Ctma
If x is adjusted to be equal to that of the conventional example, it is only necessary to form a joint surface S, which has the same area in the embodiment shown in FIG. 1 as that of the conventional example shown in FIG. The junction area S of the diffusion layer 3 of the variable capacitance diode device shown in FIG.
It is expressed as follows.

St = π(r, tr −) , −−−−−・
-・-・(2) (However, r is the radius of the outer circumferential circle, and r2 is the radius of the inner circumferential circle, which are set taking into consideration the dimensions of the capillary.) Incidentally, the conventional variable capacitance shown in Fig. 5 In the diode device, if the radius of the surface of the diffusion layer 30 is r, the junction area S0 is
It is expressed as the following formula.

S, = πr, 2 ・・−・−・・−・−1−−−
−−・−−−−−−−・(3) Therefore, equation (2) and (3
), the following relational expression is obtained.

πr32=π (r,!-r22)r3=r I
−r z” −−−−−−−−−−(4) By setting the relationship as shown in equation (4), it is possible to easily form a device with the same maximum capacitance value Cmax as that shown in FIG. 5. Also, the surface area of the outer periphery of the hollow cylindrical diffusion layer 3 is greatly expanded,
High frequency resistance becomes small due to the skin effect, and the figure of merit Q does not deteriorate. Rather, it is possible to further improve the figure of merit Q by making the high frequency resistance smaller.

Furthermore, since the part pressed by the capillary during wire bonding is within the circumference of the diameter rt, the diffusion layers 3 and 4 are formed on the outer periphery excluding this part. No breakdown of the bond occurs.

FIG. 2 is a plan view showing another embodiment of the variable capacitance diode device of the present invention, in which the diffusion layer 3 is double in the embodiment of FIG. diffusion region 3I,
3□ is formed, and a P conductivity type diffusion N4 is formed to cover the diffusion regions 3I and 3□. Although not shown, the diffusion layer 3 may be formed in double or more layers.

By forming the diffusion regions 38, 3□ in this way, it is possible to further reduce the high frequency resistance based on the skin effect, improve the figure of merit Q, and prevent the element from being destroyed by thermocompression of the gold wire during wire bonding. do not have.

Thermal theory, as shown in Figure 3, the diffusion N3 shown in Figure 1 is
It is clear that the diffusion layer 33 may be rectangular in plan view. It is also obvious that the square diffusion regions 33 may be formed in duplicate as in the embodiment shown in FIG.

Furthermore, FIG. 4 shows that the diffusion layer 34 is formed so that substantially circular diffusion regions are scattered on the circumference when the diffusion N3 shown in FIG. 1 is viewed from above, and that the diffusion layer 3 is covered. This is an example in which a P conductivity type diffusion layer 4 is formed.

〔effect〕

In the variable capacitance diode device of the present invention, the diffusion layer that approximately determines the variable capacitance ratio is not cylindrical as in the conventional case, but is annular or hollow cylindrical in order to prevent the element from breaking due to stress during wire bonding. Since it is possible to expand the area of the outer peripheral wall, it is possible to reduce high frequency resistance and improve the figure of merit Q. However, by forming the diffusion layer in an annular shape, since no diffusion layer is formed in the center of the element, it is possible to form an extremely shallow (diffusion layer), and the variable capacitance ratio can be made thick ( Is possible.

[Brief explanation of drawings]

FIG. 1a is a plan view of an embodiment of the variable capacitance diode device of the present invention, FIG. 1 is a sectional view of the embodiment of the variable capacitance diode device of the present invention, and FIGS. 1 is a plan view of a diffusion layer showing another embodiment of the diffusion layer shown in the embodiment;
FIG. 5a is a plan view showing an example of a conventional variable capacitance diode device, and FIG. 5 is a sectional view showing an example of a conventional variable capacitance diode device. 2: an epitaxial layer; 3.4: Diffusion layer, 5,5. : Silicon dioxide film. 6: Conductive film, 7: Gold wire

Claims (4)

[Claims] (1) In a variable capacitance diode device, a high resistivity semiconductor layer of a first conductivity type formed on a semiconductor substrate;
Impurity elements are diffused at a high concentration from the conductive type semiconductor layer,
A first semiconductor layer of a first conductivity type having a hollow cylindrical shape is provided in the semiconductor layer.
a shallow annular second conductivity type second diffusion layer formed to cover the hollow columnar first diffusion layer, and a semiconductor main surface surrounded by the second diffusion layer. a relatively thick first insulating film covering an inner junction exposed at the semiconductor main surface formed by the semiconductor layer and the second diffusion layer; and the second semiconductor layer exposed at the semiconductor main surface. a bond formed by the second diffusion layer and a second diffusion layer covering the outer circumferential surface of the bond;
A variable capacitance diode device comprising an insulating film. (2) A variable capacitance diode device, wherein the first diffusion layer is formed in the shape of a plurality of hollow cylinders, and the second diffusion layer is an annular diffusion layer covering the first diffusion layer. (3) the first diffusion layer has a hollow rectangular shape;
A variable capacitance diode device characterized in that the diffusion layer is a hollow rectangular diffusion layer that covers the first diffusion layer. (4) The first diffusion layer is a diffusion layer in which columnar diffusion layers are distributed dotted around the circumference, and the second diffusion layer has an annular shape so as to cover the first diffusion layer. A variable capacitance diode device characterized by: