A. Field of the Invention
The present invention relates to a semiconductor device and particularly relates to a structure of a Schottky Barrier Diode (hereinafter abbreviated to SBD) having a trench structure.
B. Description of the Related Art
FIG. 15 shows a sectional structure of a Trench MOS Barrier Schottky diode (hereinafter referred to as TMBS diode) which is an SBD having a trench structure. N-type drift layer 2 and anode electrode 3 are formed in a surface of the TMSB diode. N-type drift layer 2 and anode electrode 3 form a Schottky barrier junction. Active portion trenches 12 having inner walls covered with oxide films 11 respectively are formed at regular intervals in active portion 21 which serves as a current path at the time of current conduction. The inside of each active portion trench 12 is filled with the same metal as that of the anode electrode or with a conductor such as conductive polysilicon. When a reverse bias voltage is applied to the TMBS diode, a depletion layer is spread from the Schottky barrier junction of anode electrode 3 and n-type drift layer 2. When the depletion layer becomes deeper than the bottom of each trench, isoelectric lines are concentrated in oxide film 11 which is formed in the bottom of each trench and which is lower in dielectric constant than silicon, so that electric field intensity in the vicinity of the Schottky barrier junction is relatively lowered. As a result, not only can a barrier height reducing phenomenon be suppressed to reduce a leakage current but also the applied voltage can be shared with the oxide film. Accordingly, a withstand voltage can be improved due to a well-known RESURF (Reduced Surface Electric Field) effect compared with a conventional SBD. Moreover, because the doping concentration of n-type drift layer 2 can be increased due to the RESURF effect, lower on-resistance can be achieved at the same leakage current as that of the conventional SBD regardless of a high withstand voltage.
Voltage withstanding structure portion 22 (an electric field relaxing region formed on the outer circumferential side of active portion 21) is formed in a peripheral region of anode electrode 3. A trench is formed in an end portion (hereinafter referred to as active end portion 19) of anode electrode 3. Anode electrode 3 is terminated in an upper portion of polysilicon 13 embedded in the trench. The trench formed in active end portion 19 is hereinafter referred to as end portion trench 7. P-type guard ring layer 5 is formed between active portion trenches 12 and end portion trench 7 so as to be connected to anode electrode 3. Though not shown, a structure in which p-type guard ring layer 5 is removed may be provided. Particularly when there is no p-type guard ring layer 5, active end portion 19 is terminated in an upper surface of polysilicon 13 embedded in the end portion trench 7 so that active end portion 19 is not in direct contact with n-type drift layer 2. In this manner, electric field intensity is prevented from increasing locally in n-type drift layer 2 near active end portion 19 when a reverse bias is applied to the TMBS diode.
A method of processing a lengthwise end portion of each active portion trench 12 and a positional relation between active portion trench 12 and end portion trench 7 will be described next. FIGS. 16A and 16B are plan views showing the positional relation between active portion trench 12 and end portion trench 7 based on a conventional conception. Although description of a sidewall oxide film formed in each of the trenches and polysilicon 13 embedded in each of the trenches will be omitted, FIGS. 16A and 16B actually include the sidewall oxide film and polysilicon 13. In FIG. 16A, one active portion trench 12 is processed so as to be connected to another adjacent active portion trench 12 at lengthwise end portions of active portion trenches 12. That is, active portion trenches 12 are shaped like a doughnut so that active portion trenches 12 are adjacent to each other. Such end portion processing is a method often seen in the layout of a plurality of long and narrow trench gates in a trench gate MOSFET or IGBT. As shown in FIG. 16B, there is another method in which each active portion trench 12 is terminated at its lengthwise end portion. In this case, the end portion of each active portion trench 12 is terminated with a semi-circular shape having the width of the trench as its diameter.
JP-A-2002-50773
It has been however found that the following problem arises in such a conventional structure. When a high reverse bias is applied between anode electrode 3 and cathode electrode 4, electric field intensity is concentrated in the vicinity of the outer circumferential side bottom of end portion trench 7 of active portion 21. For this reason, an avalanche breakdown occurs not in active portion 21, but in a position Q shown in FIG. 15. Because an avalanche current flows mainly in voltage withstanding structure portion 22 compared with active portion 21, the avalanche current is concentrated in voltage withstanding structure portion 22 so that the withstand voltage is lowered.
As a measure to solve this problem, there is a method of thickening oxide film 11 on the sidewall of each trench. For example, to form oxide film 11 having a thickness of 5000 Å or more, it is necessary to control a gas flow while the temperature is kept at 1000° C. or higher for a long time during an oxidizing process. The processing process per se is difficult. Moreover, when a high reverse bias voltage is applied to the TMBS diode, isoelectric lines are concentrated in oxide film 11 having a low dielectric constant. As a result, the aforementioned good RESURF effect cannot be obtained, so that a withstand voltage increasing and leakage current reducing effect which is a merit of the TMBS diode is lowered.
Moreover, when p-type guard ring layer 5 is in contact with anode electrode 3, the following problem arises. When the value of the forward bias voltage at the time of on operation becomes higher than the content potential of a pn junction formed between p-type guard ring layer 5 and n-type drift layer 2, a forward bias is applied to the pn junction so that minority carriers (positive holes) are injected into n-type drift layer 2. For this reason, when the on operation is switched off, the stored minority carriers are flushed out so that the reverse recovery time becomes very large. Accordingly, the p-type guard ring being in contact with n-type drift layer 2 serves as a factor of disturbing high-speed operation which is one of merits of the TMBS diode.
In addition, there is a problem in the conventional method of processing the lengthwise end portion of each active portion trench 12. When the terminating process shown in FIG. 16A is applied to the TMBS diode, electric field intensity is concentrated in a position M shown in FIG. 16A at the time of application of a reverse bias. That is, because the end portion of each active portion trench 12 is curved with a certain curvature radius so that the shape of the end portion is reflected in isoelectric lines spread toward the outside of the doughnut shape, the isoelectric lines are curved. As a result, because electric field intensity is proportional to a spatial gradient of electrostatic potential, electric field intensity in the outside of active portion trench 12 increases compared with that in the inside of active portion trench 12 shaped like a linear stripe. Electric field intensity is maximized in the position M farthest from adjacent trenches inclusively of end portion trench 7. Accordingly, not only does avalanche occur easily in the position M but also the leakage current increases in the position M because of a well-known Schottky barrier reducing phenomenon. In addition, in the method shown in FIG. 16B, the end portion of each active portion trench 12 is terminated with a semi-circular shape having the width of the trench as its diameter but the curvature radius of the semi-circular shape is very small. As a result, electric field intensity increases remarkably largely in the end portion of the trench as described above. In addition, stress is generated among the semiconductor layer (n-type drift layer 2) around active portion trenches 12, oxide film 11 formed on the sidewall of each active portion trench 12 by thermal oxidation or the like, and polysilicon 13 embedded in the inside of oxide film 11 (see FIG. 15). Because this stress increases according to decrease in curvature radius of each trench end portion, cracks 14 are frequently generated in mesa region 18 as shown in FIG. 16B.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.