1. Field of Invention
The present invention relates to a fabrication method for an integrated circuit. More particularly, the present invention relates to a method for fabricating a high voltage (HV) MOS device.
2. Description of Related Art
A technique for fabricating HV MOS devices is described in the article entitled "High Voltage Thin layer Devices (RESURF Devices)," IEDM Proceedings, 1979, pp 238-241. This technique uses a shallow lightly doped region between the drain and channel regions of the device. This shallow lightly doped region is referred to as a drift region because of the low amount of current carriers that are available due to the low level of impurity doping, while the device is known as a Reduced Surface Field, RESURF, device.
RESURF techniques are utilized in manufacturing high voltage N-channel Lateral Double Diffused MOS, LDMOS, devices and high voltage, P-channel LDMOS, devices. However, problems exist in the manufacture of such RESURF LDMOS devices.
FIG. 1 is a schematic, cross-sectional diagram illustrating a conventional LDMOS structure.
Referring to FIG. 1, a p-type substrate 100 is provided with a field oxide layer 102 formed on the surface thereof, wherein the field oxide layer 102 serves to increase the channel length between a N-type source region 104 and a N-type drain region 106. A N-type lightly doped region 108 which serves as a drift region for carriers after voltage application to the device is formed below the field oxide layer 102 in the substrate 100. A P-type lightly doped region 112 is formed below a gate electrode 110 and surrounding the N-type source region 104 to increase the internal electric field of the device. As a result, the trans conductance of the device is improved under this circumstance. However, a denser distribution of electric line with larger curvature is produced below a channel region 114 and around the edge of the drift region 108 to result in a potential crowding when the device is operated at a high voltage. The depletion region formed by the drift region 108 is not sufficient to lessen the distribution of the electric line produced by high voltage; thus the increased electric field causes an electrical breakdown adjacent to the channel region of the device instead of in a bulk region. The bulk region is indicated as a part of the substrate 100 at the drain end of the device.
Conventionally, the problem of electrical breakdown is solved by reducing the doping concentration of the drift region 108 as a way to increase the size of the depletion region, while such size increase of the depletion region results in an increase in breakdown voltage. However, the current driving performance of the device is decreased when the concentration in the drift region 108 is decreased, while the electrical breakdown rarely occurs in the bulk region. Furthermore, since the channel length 114 cannot be precisely controlled using the gate electrode 110 that serves as a mask, it is difficult to increase the electric field strength, which in turn affects the conductance of the device.