JPH02150068A - Double diffused mosfet - Google Patents

Abstract

PURPOSE:To form a double diffused MOSFET having characteristics of high breakdown strength and highly saturated current by specifying in view of the edge shape of a gate or effective channel lengths of parts thereof. CONSTITUTION:An n-type epitaxial layer 3 to become a drain-drift region is grown on a substrate 1. A drain wall 4 is so formed as to be brought into contact with an n-type buried layer 2. Polysilicon gate 10 is formed on a gate oxide film 5. The gate 10 has an oblong rectangular window 10a is covered with a P-type channel layer 11 through the window 10a and a resist mask 13 on the corners of the window 10a, and implanted with N-type impurity to be diffused to form a source region 12. The effective channel length of the corner is L'c1, which is longer than the effective channel length Ll at its linear part, thereby forming he enlarging margin of a depleted region (shown by a shaded part) so that a punch-through at a low voltage scarcely occurs.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a double-diffused MO3FET (hereinafter referred to as DMO8), and particularly to a structure that achieves both high breakdown voltage and high saturation current.

C. Prior Art] For example, in a vertical DMO 3, as shown in FIG. After forming a drain wall 4 in contact with this n-type buried layer 2, a gate oxide film 5 is formed.
A polysilicon gate 6 is provided, and using this polysilicon gate 6 as a mask, the rectangular window 6a undergoes p-type diffusion to form the channel layer (body) 7, and then n-type diffusion to form the source region 8, respectively, using a self-alignment line. It was formed.

The basic cell structure in this DMO3 is shown in Figure 1 (
As shown in a), polysilicon gates 6 are aligned vertically and horizontally, and rectangular windows 6a are removed.

[Problem to be solved by the invention]

However, in the double diffusion process, in FIG. 5(a),
When the circled corner is enlarged and shown in FIG. 6(a), the effective value is the difference between the lateral diffusion length of the channel layer 7 in the first step and the lateral diffusion length of the source region 8 in the second step. The channel length is II in the straight part of the edge of the gate 6, while it is L! in the corner part where they intersect perpendicularly. LC, which is shorter. This is because the total amount of impurities at the corners that contribute to lateral diffusion is smaller than that at the straight parts, so the lateral diffusion lengths of the channel layer 7 and source region 8 are both shortened, resulting in an effective channel at the corners. This is because the length Lc+ is shorter than the effective channel length Ll of the straight portion. The peak concentration Npc+ of the channel at the corner is smaller than the peak concentration Npl at the direct portion.

Moreover, as shown in FIG. 6(b), the effective channel length L at the arcuate portion of the edge of the gate 6 adopting a different gate shape
Although C, is longer than the effective channel length II of the corner portion for the same reason as above, it is still shorter than the straight portion L1, and the peak concentration N1)C2 is also small.

In this way, the gate 6 has a concave edge with respect to itself, and the larger the curvature, the shorter the effective channel length is compared to that of the straight part. For example, as shown in FIG.
In the case of a short effective channel length LC at the corner, the withstand voltage characteristics are greatly affected. That is, when the p-type channel layer 7 and the n-type epitaxial layer 3 serving as the drain-drift region are reverse biased and the drain-source voltage increases, the depletion region indicated by diagonal lines in FIG. 7 expands in both directions, and the channel The depletion region in layer 7 contacts source region 8, causing bunch-through. This punch-through letter determines the breakdown voltage, which decreases as the channel becomes shorter and as the impurity concentration in the channel layer 7 decreases. Even if 0MO3 is formed in the same process, the breakdown voltage will differ depending on the shape of the gate edge. For example,
The withstand voltage B vos at the straight part of the gate edge is 160■
The voltage is 130V at the arc portion, and 80V at the corner portion.

On the other hand, the saturation current 10(set) can be increased by lowering the impurity concentration in the effective channel region.
Lowering the impurity concentration also lowers the breakdown voltage.

The interrelationship shown in FIG. 8 is established between the impurity concentration of the effective channel region, the breakdown voltage BVDS, and the saturation current 10 (set).
It was difficult to realize O3.

Therefore, an object of the present invention is to provide a double-diffused MO3FET that has characteristics of high breakdown voltage and high saturation current by taking into consideration the edge shape of the gate or the effective channel length of each part thereof.

[Means to solve the problem]

In order to solve the above problems, the present invention takes measures such that the gate edge has a straight part and a concave curved part with respect to the gate itself, and the effective channel length of the curved part is is set to a longer length than that of the straight part.

Another method is to select a gate shape in which the gate edge is concave or convex with respect to the gate itself and consists only of a curvature portion with uniform curvature, in order to eliminate control of the effective channel length across the gate edge. It is something.

[Effect]

Even if a gate shape is selected in which the gate edge has a straight part and a concave curved part, the effective channel length of the curved part is longer than that of the straight part, so the withstand voltage of the effective channel of the curved part is is never lower than that of the straight portion, and the overall withstand voltage is not limited by the withstand voltage in substantially locally existing curved portions, and as a result, a high withstand voltage can be obtained. Furthermore, on the premise of preventing a drop in breakdown voltage at such a curved portion, the average effective channel length can be shortened to a certain extent compared to the conventional one, thereby making it possible to obtain high saturation current characteristics.

If a gate shape is selected in which the gate edge is concave or convex with respect to the gate itself and consists only of a curvature portion with uniform curvature, a channel with substantially the same effective channel length is automatically obtained through a normal process. Therefore, there is no problem of local breakdown voltage drop, and as a result, the effective channel length can be easily optimized by adjusting the impurity concentration, and 0MO3 with high breakdown voltage and high saturation current characteristics can be obtained.

〔Example〕

Next, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1(a) is a plan view of a first embodiment of the double diffusion MO3FET according to the present invention, and FIG. 1(b) is a cutaway view taken along line Ib-1b in FIG. 1(a). .

l is a p-type substrate, which has an n-type buried layer 2;

An n-type epitaxial layer 3 is grown on the substrate 1 to serve as a drain-drift region.

4 is a drain wall formed so as to be in contact with the n-type buried layer 2. 5 is a gate oxide film, on which a polysilicon gate 10 is formed.

This polysilicon gate 10 has a rectangular window 10a that is enlarged to be relatively vertically long. In this embodiment, the horizontal width of the rectangular window 10a is approximately equal to that of the rectangular window 6a shown in FIG. 5, but the vertical width is set to several times the length. Reference numeral 11 denotes a p-type channel layer formed by diffusion through the rectangular window 10a of the gate 10. Reference numeral 12 denotes a source region formed by covering at least each corner of the rectangular window 10a with a resist mask 13 and doping and diffusing n-type impurities.

The effective channel length at this corner is as shown in Figure 2.
'CI is shorter than the effective channel length Ll in the straight section. That is, in the source diffusion, the resist mask 1 at the corner
Since the lateral diffusion length is suppressed by 3, the effective channel length is increased by that much.

Since the effective channel length at the corner is longer than that at the straight section, even if the reverse bias voltage is increased, the depletion region (indicated by diagonal lines) within the effective channel at the corner, as shown in Figure 2(b), ), and punch-through is less likely to occur at low pressure. Therefore, the limit of the device breakdown voltage is not defined by the breakdown voltage of the channel at the corner, so that a high breakdown voltage can be obtained as a result.

Since the effective channel length in the straight section is constant, even if the concentration in the effective channel region is reduced (shorter channel), for example by reducing the ion implantation dose, the saturation current characteristics will change even if the breakdown voltage remains the same as before. can be made higher. In other words, by preventing a decrease in breakdown voltage in the corners of the channel, it is possible to optimize the shortening of the channel as a whole. It is possible to improve

In addition, in the above embodiment, the number of corners is minimized (
Therefore, a long rectangular window 10a is formed in the gate IO. The number of resist masks 130 is reduced and the effective channel length LC at each corner.

This is to ensure that the production efficiency is high and improve the yield.

FIG. 3(a) is a plan view of a second embodiment of the double-diffusion MO3FET according to the present invention, and FIG. 2(5) is a cutaway view taken along the line IIb-mb in FIG. 2(a). . In this figure, the same parts as those shown in FIG. 1 are designated by the same reference numerals, and the explanation thereof will be omitted.

Gate 20 in this embodiment has a circular window 20a. That is, the gate edge is concave with respect to the gate itself and has a uniform curvature. In the normal double diffusion process, the circular window 20
The lateral diffusion length of the channel layer 21 introduced through a is substantially the same along the gate edge, and likewise that of the source region 22. therefore,
The effective channel length LC1, which is the difference between them, is the same in all parts and is shorter than the effective channel length of the straight part formed by the same process.

According to such a gate shape, the channel layer 21 and the source region 22 are formed by self-alignment using the gate as a mask.
However, since the effective channel length LC3 is necessarily uniform, there is no cause for localized breakdown voltage drop, and a high breakdown voltage can be ensured. Therefore, by increasing the diffusion concentration in the channel portion (increasing the dose of ion implantation), the effective channel length LCs can be independently set to the optimum value, and therefore a high saturation current can be obtained under a high breakdown voltage.

FIG. 4(a) is a plan view showing a third embodiment of the double diffusion MO3FET according to the present invention, and FIG. This is a cutaway diagram.
In this figure, parts that are the same as those shown in FIG. 1 are given the same reference numerals and their explanations will be omitted.

In this embodiment, the gates 30 are formed into independent circular islands. That is, the edge of the gate is convex with respect to the gate itself and has a uniform curvature.

When double diffusion is performed using the self-aligned gate using the circular gate 30 as a mask, a channel layer 31 and a source region 32 are formed. Since the gate edge of the gate 30 is circular, the lateral diffusion length is substantially the same in any part. Therefore, effective channel length L c
a is the same in any arc portion. Also in this embodiment, there is no local non-uniformity in the effective channel length, so a high breakdown voltage can be obtained.

Further, this effective channel length LC4 is longer than the effective channel length of a straight portion formed by the same process using a gate having a straight portion.

Therefore, although it is possible to obtain a sufficiently high breakdown voltage, the saturation current decreases, but by lowering the diffusion concentration in the channel portion (lowering the dose of ion implantation), the value of the effective channel length L C4 can be reduced independently. It can be optimized with
It is possible to achieve both high breakdown voltage and high saturation current while maintaining a certain degree of freedom.

〔Effect of the invention〕

As explained above, the double-diffused MO5FE according to the present invention
T can be used to control the effective channel length of the curved part, which is the cause of low breakdown voltage, to ensure a high breakdown voltage as a whole, or to increase the effective channel length of any part by a normal process in double diffusion using self-line with the gate as a mask. By selecting a gate shape that does not require local control of the effective channel length and eliminating the weak point of low breakdown voltage that defines the overall breakdown voltage, Therefore, it is possible to promote the shortening of the overall channel to some extent while still allowing for a higher withstand voltage, and as a result, it is possible to achieve a harmonious balance between high withstand voltage and high saturation current.

[Brief explanation of the drawing]

FIG. 1(a) is a plan view of the first embodiment of the double diffusion MO3FET according to the present invention, and FIG.
It is a cutaway view along line b-1b. Figure 2 (a) is an enlarged partial plan view showing the corner of the gate in the same embodiment, and Figure 2 (b) is
-■ It is a cutaway view along line b. FIG. 3(a) is a plan view of a second embodiment of the present invention, and FIG. 3(a) is a cutaway view taken along line 1llb--b in FIG. 3(a). FIG. 4(a) is a plan view of a third embodiment of the present invention, and FIG. 4(a) is a cutaway view taken along line 1b to 2b in FIG. 4(a). FIG. 5(a) is a plan view showing an example of a conventional double diffusion MO3FET, and FIG. 5(b) is a plan view showing an example of a conventional double diffusion MO3FET.
It is a cutaway view along the yb line. FIG. 6(a) is an enlarged partial plan view showing the corner of the gate in FIG. 5(a), and FIG. 6(a) is an enlarged partial plan view of the corner of the gate in FIG. FIG. FIG. 7 is a cutaway view taken along the line ■-■ in FIG. Figure 8 shows the diffusion concentration and breakdown voltage in the effective channel region. FIG. 3 is a graph diagram showing the relationship between saturation currents. l p-type substrate, 2 n-type buried layer, 3 n-type epitaxial layer, 4 drain wall, 5 gate oxide film, 10
．． 20 Polysilicon gate, 10a rectangular window, 20
a Circular window, 11.21.31 Channel layer, 12
．． 22.32 Source region, 13 Resist mask, 30 Circular polysilicon gate, LiL'C. LC3, LC4 effective channel length. Figure 1: Effective channel length of straight section Effective channel length of long corner Figure 1: Effective channel length: Effective channel length: 1. Indication of matter f11: 2 Title of the invention: 11j'' Showa 3-.3θ36/3 2 to 0.';, FE engineering 3 correction? r. 11 ('Relationship with 1 (- place name

Claims (2)

[Claims] (1) The edge of the gate under which a channel is to be formed has a straight part and a curved part that is concave with respect to the gate itself, and the effective channel length of the curved part is longer than that of the straight part. A double diffusion MOSFET characterized by: (2) A double diffusion MOSFET characterized in that the edge of the gate under which a channel is to be formed consists only of a curvature portion that is concave or convex with respect to the gate itself and has a uniform curvature.