1. Field of the Invention
The present invention relates to semiconductor devices and more particularly, to a trench Schottky diode with differential oxide thickness and a process for fabricating such a device.
2. Discussion of the Related Art
Schottky diodes are well known and are made with different topologies, including a trench topology as typically shown in U.S. Pat. No. 5,612,567 by Bantval Baliga. The process for manufacturing a trench Schottky diode requires a large number of mask layers and manufacturing steps. However, U.S. patent application Ser. No. 10/193,783, by the present inventor, presented an inventive process for fabricating trench Schottky diodes in which a reduced number of steps and fewer mask layers are required.
Referring to the Figures, in which like reference numerals refer to like elements, there is shown in FIG. 1 an example trench Schottky diode fabricated according to the process of the aforementioned application, FIG. 1 being a reproduction of FIG. 11 of that application. As seen, the device of FIG. 1 includes a silicon wafer 10 with a plurality of mesas 54 separating a plurality of trenches 30, including a termination trench region 60. A thin uniform oxide layer (SiO2) 44 lines the sidewall and bottom surfaces of each trench 30 and termination region 60, forming gate oxides and a termination oxide, respectively. A P type polysilicon 48 fills each trench 30 thereby forming a plurality of electrodes that under reverse bias, reduce reverse biased leakage currents and increase the reversed bias blocking voltage. A Schottky barrier metal 50 covers the active region, forming Schottky contacts 52 that extend over the tops of the mesa regions 54. Finally, anode electrode 56 and a cathode electrode (not shown) extend over the top and bottom surfaces, respectively, of the device.
In a trench Schottky device, such as that shown in the example device of FIG. 1, the oxide layer is formed simultaneously along the sidewall and bottom surfaces of each trench 30 and termination region 60 and thereby has the same uniform thickness throughout. However, the oxide layer along the sidewall and bottom surfaces of the trenches and termination region accomplish different purposes. More specifically, depending on the thickness of the oxide layer in each of these regions, the electrical and structural characteristics of the resulting device are affected in different ways.
For example, the gate oxide on the bottom surface of each trench 30 serves to screen the high electric field regions located at the trench bottom corners. In general, the higher the oxide thickness in this critical region, the higher the reverse breakdown voltage the device can sustain. Accordingly, a thicker gate oxide is desired along the bottom surface of the trenches.
On the contrary, the gate oxide grown on the sidewall of each trench 30 mainly affects the pinch-off characteristics of the reversed bias leakage current. Specifically, during reverse bias, the oxide on the sidewall of a trench serves to transfer the anode voltage from the trench electrode to the mesa region 54. However, part of the negative anode voltage is dropped in the oxide, with the balance of the voltage serving as the pinch-off voltage for the mesa conductive region.
Accordingly, a thin gate oxide thickness is desired along the trench sidewall in order to minimize the voltage dropped in the oxide and allow a reduced reverse biased leakage pinch-off voltage. In addition, a thin oxide thickness along the trench sidewall means less silicon is consumed during the oxidation process, thereby increasing the conduction area in the mesa regions 54. As is known, an increased mesa region means a higher active area, which is beneficial towards reducing the forward voltage drop of the device during forward conduction.
As for the termination oxide in termination region 60, it serves several purposes. From an electrical perspective, the oxide along the bottom surface of the region serves as a field oxide for the field plate termination. In general, it is desirable to have this field oxide thick enough to minimize the electric field crowding that occurs in the oxide underneath the edge 58 of the metal field plate and that is responsible for breakdown voltage walk-out. From a mechanical perspective, this portion of the field oxide underneath the edge of the metal field plate experiences high stress during temperature cycle reliability tests of the packaged device. Again, a thicker field oxide is desirable to avoid oxide rupture under the edge of the field plate during these tests.
As seen, it is disadvantageous to use the same oxide thickness across all regions of the trench Schottky device. In particular, it is desirable to have the oxide layer that lines the sidewalls of the trenches and termination region thinner than the oxide layer that lines the bottom surfaces of these trenches.
Nonetheless, it should also be noted that others have disclosed trench type devices that use a gate oxide of variable thickness; in particular, U.S. Pat. No. 6,236,099 by Milton Boden and U.S. Pat. No. 6,580,123 by Naresh Thapar disclose such devices. However, both patents are directed at MOSgated devices, including MOSFETs and IGBTS, and not at trench Schottky devices. In addition, these patents only teach the use of a variable gate oxide and not the use of a variable termination oxide.
The process by which Thapar forms the variable gate oxide should also be noted. In particular, this process includes first depositing a silicon nitride layer on the trench sidewall and bottom surfaces. The nitride layer at the bottom surface is then etched away and an oxide layer grown thereupon to a desired thickness. The nitride layer on the trench sidewall is then removed and a subsequent oxide layer is grown on the sidewall to a desired thickness.