Abstract:
A high voltage device. A first-type semiconductor substrate having at least a gate formed thereon is provided. The high voltage comprises a second-type first diffusion region in the semiconductor region, a second-type second diffusion region within the first diffusion region, a second-type third diffusion region under the second diffusion region, a field oxide layer on a part of the second diffusion region, and a first-type source/drain region under a surface between the field oxide layer and the gate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 87105065, filed Apr. 3, 1998, the full disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a semiconductor device, and more particularly, to a high voltage device formed by a method which employs doping with different kind of dopant of different concentration to change electric field of a semiconductor device. 
     2. Description of the Related Art 
     In FIG. 1, a cross sectional view of a complementary metal oxide semiconductor (CMOS) is shown. In an N-type semiconductor substrate 100, using dopant diffusion or ion implantation, a P-type region 102 is formed. A PMOS transistor is formed on the N-type semiconductor substrate 100, and an NMOS transistor is formed on the P-type region. The NMOS transistor comprises a gate 104 and a P-type source/drain region 106, and the PMOS transistor comprises a gate 104 and an N-type source/drain region 108. The NMOS transistor is isolated from the PMOS transistor by a field oxide layer 110. 
     As the devices are designed and developed towards a smaller and smaller dimension, the channel length becomes shorter and shorter. The reduced channel length causes a faster operation speed of a device and other short channel effects. According to the formula &#34;E=V/D&#34;, in which E is the electric field, V is the voltage, and d is the channel length, providing a constant voltage, the electric filed is increased abruptly with reducing the channel length. The abrupt increase of electric field accelerates the electron in the channel. As a consequence, the electric breakdown occurs. 
     In the application of radio frequency (RF), a high power gain is required to improve the frequency response. A direct method to increase the power gain is to increase the transconductance g m . While the transconductance g m  is increased, the electric field across the channel is increase. In Reverse, the transconductance g m  is increased by increasing the electric field across the channel. However, considering the limitation due to short channel effect and electric breakdown, it is difficult to obtain a high transconductance g m . 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a high voltage device. In the device, the carrier transportation along longitudinal direction is avoided. Therefore, the formation of a parasitic bipolar junction transistor is avoided. The effect and performance of the device is thus maintained without being degraded. 
     To achieve these objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed towards a high voltage device. A first-type semiconductor substrate having at least a gate formed thereon is provided. The high voltage comprises a second-type first diffusion region in the semiconductor region, a second-type second diffusion region within the first diffusion region, a second-type third diffusion region under the second diffusion region, a field oxide layer on a part of the second diffusion region, and a first-type source/drain region under a surface between the field oxide layer and the gate. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross sectional view of a conventional complementary MOS; and 
     FIG. 2A to FIG. 2F shows a cross sectional view, in which a method of forming a high voltage device in a preferred embodiment according to the invention is shown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2A, on a semiconductor substrate 200, a gate oxide layer 202, a polysilicon layer 204 and a first oxide layer 206 is formed. In the embodiment, an N-type substrate is adapted. However, it is to be noted that the type of the substrate is not restricted as described in this embodiment. The gate oxide layer 202 is, for example, formed by thermal oxidation with a thickness of about 500 Åto 1500 Å. The poly-silicon layer 204 is, for example, formed by chemical vapor deposition (CVD) with a thickness of about 3000 Åto 7000 Å. The first oxide layer 206 is formed, for example, by thermal oxidation with a thickness of about 300 Åto 700 Å. Using a mask, the first oxide layer 206, poly-silicon layer 204 and gate oxide layer 202, so that an opening 208 is formed to exposed the substrate 200 therewithin. The opening 208 has a sidewall denoted as 208&#39;. A gate is formed by stacking the first oxide layer 206, poly-silicon layer 204 and gate oxide layer 202. 
     In FIG. 2B, via the opening 208, a first self-aligned ion implantation and drive-in process is performed, so that a first diffusion region in the substrate 200. In this embodiment, a P -   diffusion region 210 is formed with a concentration less than and equal to about 10 17  /cm 3 . Again, it is to be noted that the diffusion region is not restricted as the embodiment. The substrate 200 is pumped with nitrogen. A second oxide layer 212 is formed on the structure shown in FIG. 2A. The second oxide layer 212 is, for example, formed by thermal oxidation. The surface being directly pumped with nitrogen has a slower rate for oxide growth. As a consequence, the second oxide layer 212 on the first oxide layer 206 and the substrate 200 within the opening 208 is thinner than the second oxide layer 212 on the sidewall 208&#39;. 
     In FIG. 2C, a second ion implantation and drive-in process is performed, so that a second diffusion region is formed with a concentration less than or equal to about 10 18  /cm 3 . Due to the second oxide layer 212 on the sidewall 208&#39;, the second diffusion region is formed within the P -   diffusion region 210. The second diffusion region is, for example, a P diffusion region 214 doped with P-type dopant. Hence, a channel region comprising two different diffusion regions 210 and 214 is formed. In the channel region, a large transconductance g m  is obtained in P diffusion region 214 , whereas the P 31   diffusion region 210 controls the threshold voltage. 
     In FIG. 2D, a silicon nitride layer 216 is formed on the structure shown in FIG., 2C. The silicon nitride layer 216 is defined as a mask to expose a part of the silicon oxide layer 212. A third ion implantation is performed to form a third diffusion region within the P diffusion region, for example, a P +   diffusion region 218 in this embodiment. 
     In FIG. 2E, a drive-in process is performed, so that P +   ion within the P +   diffusion region 218 diffuses down towards the substrate 200. A deeper P +   diffusion region 218&#39; is formed under the P diffusion region 210 within the range under the bottom surface of the opening 208. A field oxide layer 220 is then formed on the exposed silicon oxide layer, for example, by thermal oxidation. After the formation of the field oxide layer 220, the silicon nitride layer 216 is removed. 
     In FIG. 2F, using a mask, a fourth ion implantation, for example, N +  ion, is performed. An annealing process is then performed, so that a N +   diffusion region is formed as a source/drain region 222 within the P diffusion region 214 under a surface between the field oxide layer 220 and the gate. 
     In this embodiment, an N-type substrate 200 is used. While carriers transport vertically from the N +   source/drain 222 and the P diffusion region to the N type substrate 200 without the formation of the P +   diffusion region 218&#39;, a parasitic bipolar junction transistor which degrades the quality of the device is formed. In the invention, a deeper P +   diffusion region 218&#39; is formed to avoid the vertical transportation of carriers. Therefore, the majority of carriers transports horizontally through the channel regions. The formation of a parasitic bipolar junction transistor is prevented. 
     In the invention, the diffusion regions are formed using self-aligned ion implantation without additional mask, so that the process is simplified with a lower cost. The misalignment caused during exposure is avoided. 
     Another feature of the invention is that a higher transconductance g m  is obtained by forming a P +   diffusion region. The threshold voltage is controlled by the formation of the P -   diffusion region. 
     Yet, another feature of the invention is that a deeper diffusion region is formed. The vertical transportation of carriers towards the substrate is prevented, so that the formation of a parasitic bipolar junction transistor is avoided. Therefore, the majority of carriers transports horizontally through the channel region. The performance of the device is enhanced. 
     Other embodiment of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.