JPH02191377A - Variable capacitance diode element - Google Patents

Abstract

PURPOSE:To improve a saturation tendency of capacitance-voltage characteristics by causing a depletion layer extending at the side of PN junction to from buried layers having high resistivity on a semiconductor substrate between a semiconductor substrate and an epitaxial layer with the exception of a part which is directly below PN junction so that its depletion layer may extend downward that is deeper than the main surface of the semiconductor substrate. CONSTITUTION:Buried layers 3 of a first conductivity type having resistivity that is higher than that of a semiconductor substrate 1 are formed at peripheral parts with the exception of a part which is directly below a diffusion layer 5 of the first conductivity type at a boundary part between the semiconductor substrate and an epitaxial layer 4. Consequently, a depletion layer 10 extends to the semiconductor substrate 1 as impressed voltage VR increases and reaches each buried layer 3. Since it extends deeply into the semiconductor substrate 1, the main surface of the depletion layer is not saturated. The saturation tendency of capacitance-voltage characteristics is thus improved without deteriorating a performance index.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a variable capacitance diode element, and improves the saturation tendency of the capacitance-voltage characteristic of the variable capacitance diode element, and reduces high frequency series resistance R5. Moreover, the figure of merit Q is improved.

[Conventional technology]

Generally, many variable capacitance diode elements are manufactured with a planar structure. Hereinafter, a conventional variable capacitance diode element will be explained based on FIG.

In FIG. 3, an epitaxial layer 4 having a resistivity of about 1 Ωcm, which is N-type and has a higher resistivity than the semiconductor substrate 1, is formed by vapor phase growth on a low-resistance N-type semiconductor substrate 1 to a thickness of 4.
The main surface of this epitaxial N4 is thermally oxidized to form a semiconductor substrate with a thickness of about 1 to 2 μm.
A thermal oxide film 2 (Si0g film) of μm thickness is formed. after that,
An opening is formed by an etching process, and an N-type impurity element is implanted into the epitaxial layer 4 through the opening at an acceleration voltage of 130 KeV and a dose of (2 to 3) x 10 "am-" by ion implantation. In the ion implantation process, impurity elements may be implanted through the oxide film of 100 to 3,000 layers.Next, heat treatment is performed that also serves as annealing to recover lattice defects and carrier recovery caused by ion implantation. Then, an N9 type diffusion layer 5 having a higher impurity concentration than the epitaxial layer is formed. Next, the diffusion layer 5 includes the surface exposed portion of the diffusion layer 5.
A P″9 type diffused layer 8 is formed which is shallower than the diffusion depth of , and a PN junction is formed between the P″ type diffused layer 8 and the N9 type diffused layer 5 and the epitaxial layer 4. Electrodes are formed on the front and back sides to form a variable capacitance diode element.

[Problem to be solved by the invention] In the above-mentioned conventional variable capacitance diode element, P
If the impurity concentration of the ++ type diffusion layer 8 is sufficiently higher than the impurity concentration of each of the N゛ type diffusion layer 5 and the epitaxial layer 4, then when a reverse bias voltage vl is applied, the depletion in the P41 type diffusion layer 8 will be reduced. The width of the layer is very narrow and negligible compared to the spread of the depletion layer in the region of the N-type diffusion layer 5 and the epitaxial layer 4. The variable capacitance Cj of the variable capacitance diode element is the junction capacitance ICJ due to the depletion layer 9 generated in the N0 type diffusion layer 5 and the P++ type diffusion layer 8PN junction J1.
1 and a junction capacitance Cj2 due to the depletion N10 generated at the PN junction Jt formed by the N-type epitaxial layer 4 and the P++ type scattering layer 8. Since the impurity concentration of the epitaxial layer 4 is lower than that of the N0 type diffusion layer 5, the expansion width of the depletion layer 10 of the epitaxial layer is larger than the expansion width of the depletion rtJ9 of the diffusion layer 5. By increasing or decreasing the reverse bias voltage (applied voltage) ■ true, the depletion layer 9,
The spread width of 10 increases or decreases. Therefore, the capacitance Cjl,
When CJz is varied, the variable capacitance @Cj, which is the combined capacitance thereof, is varied.

On the other hand, the variable capacitance Cj of the variable capacitance diode element is expressed by the following relational expression.

However, W is the width of the depletion layer, N (x) is the impurity concentration, and K
, is the dielectric constant of the semiconductor substrate, ε. is the dielectric constant in vacuum (8,
85x 10-”F/m”), q is the electron charge (
1,60X l O-'' C), Φ represents the diffusion potential of the PN junction, and n represents an index determined by the slope of the impurity concentration.

From these equations (1) and (2), the expansion of the depletion layer is P″3
It is clear that it depends on the impurity concentration of the semiconductor layer 4.5 forming the PN junction with the type diffusion 11B.

Therefore, when the applied voltage VII is applied to the variable capacitance diode element, the width W of the depletion layer 10 of the epitaxial N4, which has a lower impurity concentration than the diffusion layer 5, expands, ! becomes larger than the width WjI of the depletion layer 9 extending to the diffusion layer 5, and when the applied voltage V is further increased, the depletion layer 10 around the N1 type diffusion layer 5 extends and collides with the semiconductor substrate l, and then the applied voltage Even if the voltage (2) is increased, the expansion of the depletion layer 10 does not proceed any further, showing a tendency to saturation of the capacitance-voltage characteristic (
1) It is clear from equation (2).

To explain this state using the diagram showing the capacitance-voltage characteristics in Figure 4, in the case of a conventional variable capacitance diode element, for example, when the applied voltage (1)8 exceeds about 15V, the slope of the curve becomes gentler. As shown by (A) in Figure 4, there is a tendency to saturate at the applied voltage V, and the capacitance is saturated at C8. In this way, in the conventional variable capacitance diode element, the depletion layer 10 extending from the periphery reaches the semiconductor substrate 1 and the capacitance is saturated.
There is a drawback that the voltage change ratio of the capacitor Cj becomes small.

In order to improve these problems, it is possible to prevent the depletion layer 10 from colliding with the semiconductor substrate 1 by increasing the thickness of the epitaxial layer 4, but there is a drawback that the region immediately below the diffusion layer 5 becomes thicker. This has the drawback that the high-frequency series resistance R8 increases and the figure of merit Q decreases, and the epitaxial layer 4 cannot be made thicker than the conventional one. Therefore, the problem is to improve the saturation tendency of the capacitance-voltage characteristics without increasing the high-frequency series resistance R3 and impairing the figure of merit Q.

The present invention has been made to solve the above-mentioned problems, and its main purpose is to improve the saturation tendency of capacitance-voltage characteristics that is common in conventional variable capacitance diode elements, and to improve the saturation tendency of high-frequency series resistance. The present invention provides a variable capacitance diode element with a small R8 and which does not reduce the figure of merit Q.

[Means to solve problems]

The variable capacitance diode element of the present invention includes an epitaxial layer of a first conductivity type formed on a semiconductor substrate of a first conductivity type and having a higher resistivity than the semiconductor substrate, and an epitaxial layer of a first conductivity type having a resistivity lower than that of the epitaxial layer diffused in the epitaxial layer. A diffusion layer of the first conductivity type of the resistor and a resistor having a higher specific resistance than the semiconductor substrate formed at the boundary between the semiconductor substrate and the epitaxial layer except for directly under the diffusion layer of the first conductivity type. A buried layer of a first conductivity type, and a second conductivity type having a lower resistivity than the first diffusion layer forming a PN junction with the first diffusion layer covering the exposed main surface of the first diffusion layer. and electrodes formed on the front and back surfaces of the semiconductor substrate.

[Effect]

The variable capacitance diode element of the present invention has a PN of a semiconductor substrate.
A buried layer with an impurity concentration similar to that of the epitaxial layer is formed in the semiconductor substrate between the epitaxial layer and the portion directly below the junction, so that the depletion layer extending to the side of the PN junction is deeper than the main surface of the semiconductor substrate. A buried layer with high specific resistance is formed so as to extend downward to improve the saturation tendency of the capacitance-voltage characteristics.

〔Example〕

FIGS. 1 and 2 regarding the variable capacitance diode element of the present invention.
This will be explained based on the manufacturing process using figures.

An N-type diffusion layer 3 serving as a buried layer is formed by ion implantation in an N-type low resistivity semiconductor substrate 1 using a thermal oxide film 2 as a mask. The diffusion layer 3 is located at the periphery of the diffusion layer to be formed in the next step, except for directly below it (FIG. 1a).
. An N-type epitaxial layer 4 having a resistivity higher than that of the semiconductor substrate 1, for example, around 1Ω, is formed on the semiconductor substrate 1 by a vapor phase growth method. The thickness t is the same as the conventional one if it is used in a high frequency band.
The thickness should be approximately 5 μm. Note that this value varies depending on the electrical function of the element. A buried layer 3 is formed in this manufacturing process. An N-conductivity type semiconductor layer and an N-conductivity type semiconductor layer are formed in the buried N3 (FIG. 1b). After a thermal oxide film (330 g film) which is an insulating film for surface protection and has a thickness of about 1 to 2 μm is formed on the main surface of the epitaxial layer 4 by thermal oxidation, an opening 6 is formed by etching.

N-type impurity elements (phosphorus, arsenic, etc.) are implanted into the opening 6 by ion implantation at an accelerating voltage of 130 KeV and a dose of (2 to 3) x 10"ell-" (7). Inject epitaxial N4 through. Thereafter, a heat treatment is performed that combines lattice defect recovery by ion implantation and annealing for carrier recovery, and thermal diffusion is performed to form a diffusion layer 5 (see FIG. 1C). Using the thermal oxide film formed in the opening 7 in this diffusion process, a P-type impurity element (boron, etc.) is applied at an accelerating voltage of 20 KeV and a dose of (
5-B) Ion implantation and heat treatment are performed under the conditions of " A variable capacitance diode element is formed through such a manufacturing process.

In the variable capacitance diode element of the present invention, as shown in FIG. A depletion jW10 due to the PN junction J2 is generated as shown in FIG. As the applied voltage 18 increases, the depletion layer 10 extends toward the semiconductor substrate 1, reaches the buried layer 3, and extends deep into the semiconductor substrate 1, so that it is not saturated at its main surface. Moreover, if the thickness t of the epitaxial layer 4 is the same as that of the conventional one, the thickness of the epitaxial layer 4 below the diffusion layer 5 is the same as that of the conventional one, and the high frequency series resistance R5 is It is possible to improve the saturation tendency of the capacitance-voltage characteristics without increasing the capacitance-voltage characteristics, that is, without decreasing the figure of merit Q.

In the voltage-junction capacitance characteristic of the conventional variable capacitance diode element, as shown in FIG. 4(a), the capacitance C8 becomes saturated at the applied voltage 3. In contrast, in the variable capacitance diode element of the present invention, as the applied voltage increases, the depletion layer IO extends and reaches the buried N3, as shown in FIG. 4(b).
In order to extend deeply into the inside of the semiconductor substrate 1, the applied voltage V
, the depletion layer extends without reaching saturation on its main surface, and the applied voltage v reaches the N'' type semiconductor layer of the semiconductor substrate l.
It does not become saturated until it reaches I.

According to the thermal theory, according to the variable capacitance diode element of the present invention, it is clear that in order to improve the high frequency series resistance R1 and the figure of merit Q, the epitaxial layer 4 should be made thinner than the conventional one, and the buried N3 should be formed. There are advantages that can be easily achieved by doing so.

〔effect〕

The variable capacitance diode element according to the present invention has good characteristics that do not saturate unless the applied voltage exceeds 25 V, whereas the capacitance-voltage characteristics of conventional variable capacitance diode elements tend to be saturated at an applied voltage of about 20 V. This shows that. Thermal theory also has the effect of suppressing the saturation tendency of the capacitance-voltage characteristics even in variable capacitance diode elements used at low voltages of about several volts.

The variable capacitance diode element of the present invention has a depletion layer formed by forming a buried layer having an N- or N-type conductor around the periphery except for the lower part of the PN junction without changing the thickness t of the epitaxial layer 4. does not saturate on the main surface of the semiconductor substrate 1, so the advantage is that the saturation tendency of the capacitance-voltage characteristics can be significantly improved without increasing the high frequency series resistance R8 and without impairing the figure of merit Q. has.

Thermally, since the thickness t of the epitaxial layer 4 can be made thinner, the thickness of the epitaxial layer 4 between the diffusion IW 5 and the semiconductor substrate 1 can be made thinner than in the conventional case. Therefore, since the high frequency series resistance R8 can be set small, it is also possible to improve the figure of merit Q.

[Brief explanation of the drawing]

1A to 1D are cross-sectional views showing the manufacturing process of the variable capacitance diode element of the present invention, FIG. 2 is a cross-sectional view for explaining the expansion of the depletion layer of the variable capacitance diode element of the present invention,
FIG. 3 is a sectional view of a conventional variable capacitance diode element, and FIG. 4 is a diagram showing the capacitance-voltage characteristics of the variable capacitance diode element. 1: Semiconductor substrate, 2: Thermal oxide film, 3: Buried layer, 4: Epitaxial layer, 5: N-type diffusion JW, 8: P-type diffusion layer, 9
, 10: Depletion layer, J, , J,: PN junction Fig. 1

Claims (1)

[Claims] an epitaxial layer of a first conductivity type formed on a semiconductor substrate of a first conductivity type and having a resistivity higher than that of the semiconductor substrate; a diffusion layer, a first conductivity type buried layer having a higher specific resistance than the semiconductor substrate, which is located at the boundary between the semiconductor substrate and the epitaxial layer, and is formed in the peripheral area except directly under the first conductivity type diffusion layer; a second conductivity type having a lower specific resistance than the first diffusion layer for forming a PN junction by the first diffusion layer covering the exposed main surface of the first diffusion layer; A variable capacitance diode element comprising a diffusion layer and electrodes formed on the front and back surfaces of the semiconductor substrate.