text: The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device. More specifically, the invention relates to an insulated-gate field-effect transistor and a bipolar transistor, both formed on an SOI substrate.
  In a high breakdown voltage semiconductor device formed on the SOI (Silicon On Insulator) substrate, the magnitude of the breakdown voltage of the device determines the thickness of a silicon layer and the thickness of a buried insulating film. The silicon layer is the active region where the device is formed, and is formed over a support silicon substrate via the insulating film. The breakdown voltage of an N-channel insulated-gate field-effect transistor is herein defined as a voltage applied to a drain electrode therein when an electric current flowing through the drain electrode has become 10 μA through the application of the voltage to the drain electrode. In this case, the gate width of the field-effect transistor measured with an IWATSU semiconductor curve tracer TT-508 is 50 μm, and source and gate electrodes and the back of the SOI substrate where the device is formed are set at the ground level or 0 V. Alternatively, the breakdown voltage of a P-channel insulated-gate field-effect transistor is herein defined as a voltage applied to a source electrode or a gate electrode therein when a current flowing through the source electrode has become 10 μA through the application of voltages applied to both the source and gate electrodes. In this case, the gate width of the field-effect transistor measured with the above-mentioned curve tracer is 50 μA, and a drain electrode and the back of an SOI substrate is set at the ground level (0 V). Alternatively, the breakdown voltage of an NPN bipolar transistor is herein defined as a voltage applied to a collector electrode therein, or BVceo when a current flowing through the collector electrode has become 10 μA through the application of the voltage to the collector electrode. The emitter length of the bipolar transistor measured with the curve tracer is 50 μm in this case, and the base electrode is set to open, and an emitter electrode and the back of an SOI substrate are set at the ground level. Still alternatively, the breakdown voltage of a PNP bipolar transistor is herein defined as a voltage applied to an emitter electrode therein when a current flowing through the emitter electrode has become 10 μA through the application of the voltage applied to the emitter electrode. In this case, a collector electrode and the back of an SOI substrate is set at the ground level and a base electrode is set to open by the use of the above-mentioned curve tracer.
  The thicker the silicon layer and the thicker the buried insulating film, a device with an increased breakdown voltage can be fabricated. However, when the buried insulating film becomes thick, a warp in a wafer increases in the manufacturing process of the device. Consequently, the process at the final stage in completion of the device cannot be performed. Further, the larger a wafer diameter, the more manifest this phenomenon becomes. In general, in the wafer with a diameter of 6, 8, or 12 inches, a silicon oxide film is now commonly employed as the buried insulating film. The maximum thickness of the buried insulating film is regarded as approximately 3 μm. For this reason, when a high breakdown voltage device is fabricated, it is necessary to increase the thickness of the silicon layer, which is the active region. However, if the thickness of the silicon layer is increased, it takes much time to form trenches required for isolation of a device-forming region. For this reason, throughput is reduced, so that a cost problem arises. In addition, it becomes difficult to form a completely vertical deep trench and cover the trench with an insulated film tightly.
  FIGS. 2A and 2B show the structure of a high breakdown voltage N-channel MOS field-effect transistor and the structure of a high breakdown voltage NPN bipolar transistor, both using an SOI substrate with an N-type device-forming region and having a breakdown voltage of approximately 200 to 600 V, respectively.
  FIG. 2A illustrates the N-channel MOS field-effect transistor formed on an SOI substrate 101 having an N-type region over a buried insulating film 103. The transistor comprises a high concentration N-type layer 402 and a high concentration p-type layer 401 both contacting a source electrode 201, a gate insulating film 301 and a gate electrode 302, a high concentration N-type layer 403, and a p-type semiconductor layer or a p-body layer 404. The gate insulating film 301 and the gate electrode 302 are in contact with the high concentration N-type layer 402. The high concentration N-type layer 403 is in contact with a drain 202 disposed in a lateral direction via a field oxide film 204 contacting the gate electrode. The p-body layer 404 is in contact with the gate oxide film, and the high concentration N-type and P-type layers contacting the source electrode. As a drain region interposed between the p-body layer and the drain electrode, the N-type substrate is usually employed without alteration. Alternatively the concentration of the drain region may be adjusted by ion implantation and diffusion of phosphorus ions, for example. An n-type layer formed by implantation and diffusion of ions of an element such as phosphorus into the substrate is hereinafter referred to as a WELL and the concentration of the resulting layer is referred to as a WELL concentration.
  FIG. 2B illustrates the NPN bipolar transistor formed on the SOI substrate 101 having the N-type region over the buried insulating film 103. The transistor comprises a collector electrode 205, an emitter electrode 207 and a base electrode 206 disposed via the field oxide film 204, high-concentration N-type layers 411 and 413, a high concentration P-type layer 412, and a P-type base region 414. The high-concentration N-type layer 411 is in contact with the collector electrode. The high concentration N-type layer 413 is in contact with the emitter electrode 207. The high concentration P-type layer 412 is in contact with the base electrode. The p-type base region 414 is in contact with the high concentration N-type layer contacting the emitter electrode and the high concentration P-type layer contacting the base electrode. As a collector region interposed between the p-type base region and the collector electrode, the n-type substrate is usually employed without alteration, or the WELL is formed for use as the collector region. The NPN bipolar transistor is usually what is called a vertical structure with a high concentration N-type layer brought into contact with the buried insulation film. However, the present invention has been made to reduce the thickness of the silicon layer. Accordingly, if the high concentration N-type layer is present in the silicon layer with its thickness reduced, a high breakdown voltage cannot be obtained. Thus, a lateral structure in which the high concentration N-type layer contacting the buried insulating film has been solely removed from the conventional vertical structure is herein defined as a conventional structure.
  Now, a mechanism whereby a breakdown voltage is determined will be described. In the case of the N-channel MOS field-effect transistor illustrated in FIG. 2A, when a voltage is applied to the drain electrode, a depletion layer formed between the p-body layer and the N-type substrate is extending. In addition, a depletion layer is extending from the vicinity of the buried insulating film under the drain electrode as well. If a drain-source distance is short, a rise in the electric field of the depletion layer formed between the p-body layer and the N-type substrate becomes faster than a rise in the electric field of the depletion layer formed in the vicinity of the buried insulating film. If the drain-source distance becomes long, the depletion layer between the p-body layer and the N-type substrate can extend sufficiently large. Accordingly, a rise in the electrical field of the depletion layer formed in the vicinity of the buried insulating film becomes faster than a rise in the electric field of the depletion layer formed between the p-body layer and the N-type substrate. The drain-source distance is affected by the concentration of the substrate as well. When the concentration of the substrate becomes high, even if the drain-source distance is increased, the depletion layer formed between the p-body layer and the N-type substrate cannot extend sufficiently large. Thus, the breakdown voltage is determined from the depletion layer between the p-body layer and the N-type substrate. Suppose selection of the concentration of the substrate is performed so that the breakdown voltage is determined from the extension of the depletion layer formed in the vicinity of the buried insulating film. Then, the maximum breakdown voltage of the device with a given silicon layer thickness and a given buried-insulating-film thickness can be obtained.
  In the case of the NPN bipolar transistor illustrated in FIG. 2B as well, the breakdown voltage is determined either from the electric field of the depletion layer formed between the P-type base layer and the N-type substrate or the electric field of the depletion layer that extends from the buried insulating film immediately under the collector electrode, as in the case of the N-channel MOS field-effect transistor. In the case of the NPN bipolar transistor as well, by selecting the concentration of the substrate such that the breakdown voltage is determined from the extension of the depletion layer from the vicinity of the buried insulating film, the breakdown voltage can be maximized.
  When an N-channel MOS field-effect transistor having the conventional structure is formed on the SOI substrate including a 1.5-μm thick silicon layer and a 3.0-μm thick silicon oxide film employed as the buried insulating film, the breakdown voltage of the device becomes 275 V according to a computer simulation. Thus, it becomes impossible to achieve the breakdown voltage of 300 V. For this reason, if the N-channel MOS field-effect transistor with the breakdown voltage of 300 V is desired, there is no way other than increasing the thickness of the silicon layer or the thickness of the buried oxide film.
  However, in the case of an 8-inch SOI substrate, for example, when the thickness of the buried oxide film exceeds 3.0 μm, a warp in the wafer increases during the manufacturing process of the device, so that manufacture of the device cannot be completed.
  Even when the thickness of the buried oxide film is 3.0 μm as well, in order to reduce a warp on the wafer in the initial state, a thick oxide film with a thickness of 2.0 μm or more should be formed over the region of the SOI substrate surface that is not the device-forming region, at the purchase of the wafer. Then, during the device manufacturing process as well, care should be taken so as not to reduce the thickness of the oxide film as much as possible. Accordingly, a reduction in the thickness of the buried oxide film is desired. On the other hand, when the thickness of the silicon layer becomes thick, it takes additional time to form the trenches for device isolation. Thus, throughput is reduced, and it also becomes difficult to form a vertical deep trench and cover the trench with an insulating film tightly. For this reason, a reduction in the thickness of the silicon layer is also desired.