1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing it, further detailedly relates to lateral double diffused (LD) MOS transistor technique as a high-voltage device utilized for IC for driving a liquid crystal for example and others.
2. Description of the Related Art
LD MOS transistor structure means structure that a new diffused region is formed by diffusing impurities different in a conductive type in a diffused region formed on the side of the surface of a semiconductor substrate and difference in diffusion in a lateral direction between these diffused regions is utilized for effective channel length and a device having the structure is a device suitable for reducing ON-state resistance because a short channel is formed.
FIG. 15 is a sectional view for explaining a conventional type LD MOS transistor and for an example, N-channel LD MOS transistor structure is shown. The description of P-channel LD MOS transistor structure is omitted, however, both structures are different only in a conductive type and it is well-known that they are the similar structure.
As shown in FIG. 15, a reference number 51 denotes one conductive type of, for example, P-type semiconductor substrate, 52 denotes an N-type well region, an LP layer 53 (composing a P-type body region) is formed in the N-type well region 52, an N-type diffused region 54 is formed in the LP layer 53 and an N-type diffused region 56 is formed in an LN layer 55 (composing a drift region) in the N-type well region 52. A local oxide film 57 and a gate electrode 59 via a gate insulating film 58 are formed on the surface of the substrate and a channel region 60 is formed in the superficial region of the LP layer 53 immediately under the gate electrode 59.
The N-type diffused region 54 functions as a source region and the N-type diffused region 56 functions as a drain region. Also, a reference number 61 denotes a P-type layer for applying electric potential to the LP layer 53 and 62 denotes a layer insulation film.
In the LD MOS transistor, the dopant density of the surface of the semiconductor substrate (the LN layer 55) is increased by diffusing the dopant into the surface to form the LN layer 55 as a drift region , current on the surface of the LN layer 55 readily flows and the withstand voltage can be increased.
However, as shown in FIG. 15, the end of the LP layer 53 to be a P-type body region of the LD MOS transistor is under the gate electrode 59 and a range in which the threshold voltage can be regulated exists under an active region.
Therefore, electrostatic focusing at the end of the LP layer 53 and the effect of an electric field from the gate electrode 59 are joined, local current focusing is caused and the driving ability is deteriorated.
Also, as high voltage is applied between the N-type diffused region 56 to be a drain region and the gate electrode 59, the gate insulating film 58 is required to be thickened for high withstand voltage and it prevents further miniaturization.
FIG. 16 is a sectional view for explaining the basic configuration of a conventional semiconductor device.
As seen from FIG. 16, an isolation film (not shown) and a first and a second insulating gate 152 and 153 are formed on a semiconductor substrate 151 of a first conduction type, e.g. P-type. A gate electrode 154 is patterned to extend from the first gate insulating film 152 onto a part of the second gate insulating film 153. Lightly-doped source/drain regions 155 and highly-doped source/drain regions 156 constitute a LDD (Lightly Doped Drain) structure. For brevity of illustration, only the side of the drain region is shown. A source/drain electrode is kept in contact with the source/drain region 156.
Meanwhile, the locations of the density of an electric field at each voltage Vgs of the above semiconductor device through device simulation have been found by the inventors of this invention. It have been found out that the semiconductor device exhibits different breakdown voltage characteristics according to the distribution of the impurity density of the lightly-doped source/drain region 155. Specifically, as seen from FIGS. 17(a) and 17(b), where the surface density of the source/drain region is relatively low (e.g. about 5xc3x971016/cm3), the substrate current Isub have two peaks (double hump structure) as the voltage Vgs increases (FIG. 17(a)). Incidentally, FIG. 17(a) is a characteristic graph (Vds=60 V) showing the substrate current Isub versus the voltage Vgs at the above density. FIG. 17(b) is a characteristic graph of the current Ids versus the voltage Vds.
First, the first peak (1) of the substrate current Isub shown in FIG. 17(a) occurs owing to generation of an electric field from the drain region 155 toward the gate electrode 154 when the voltage Vgs less than the voltage Vds. The electric field is concentrated at the first region (1) in FIG. 16.
When the voltage Vgs=the voltage Vds, the potential difference between the drain region 155 and the gate electrode 154 disappears so that substrate Isub becomes minimum.
When the voltage Vgs greater than the voltage Vds, the resistance of the first region (1) shown in FIG. 16 due to induction of carriers by the voltage Vgs is small whereas application of a voltage to the depletion layer in the second region (2) in FIG. 16 is large due to resistance division. As a result, the electric field in the second region (2) in FIG. 16 is dominant. Thus, the substrate current Isub increases again to provide its second peak (2) shown in FIG. 17(a).
In this way, where the surface density of the lightly-doped source/drain region 155 is lower, the first peak (1) is low. Therefore, this is efficient for the drain breakdown voltage in the range where the voltage Vgs is low. However, the second peak (2) of the substrate Isub is relatively high so that the drain breakdown voltage cannot be maintained in the range where the voltage Vgs is high.
On the other hand, where the surface density of the source/drain region 155 is relatively high (e.g. about 1xc3x971017/cm3), as seen from FIG. 18(a), the substrate current Isub has a single peak at a certain Vgs. However, the drain breakdown voltage cannot be maintained in the range where the voltage Vgs is low. Incidentally, FIG. 18(a) is a characteristic graph (Vds=60 V) showing the substrate current Isub versus the voltage Vgs at the above density. FIG. 18(b) is a characteristic graph of the current Ids versus the voltage Vds.
Accordingly, where the lightly-doped source/drain region 155 is relatively low, the breakdown voltage in the range where the voltage Vgs is high cannot be maintained (region (I) in FIG. 17(b)). On the other hand, where the lightly-doped source/drain region 155 is relatively high, the breakdown voltage in the range where the voltage Vgs cannot be maintained (range (II) in FIG. 18(b)).
In order to relax the density of the electric field in the high voltage MOS transistor structure (N-channel), the end of the lightly-doped source/drain region 155 was retracted from the end of the second gate insulating film 153 by about 2 xcexcm (removal width H), thereby realizing a high breakdown voltage of about 80 V. This is because the drain voltage is trapped at the portion of the removal width H because of an increased parastic resistance at this portion so that the electric filed applied to the transistor body can be increased, thereby realizing a high breakdown voltage of the transistor.
However, the process for realizing the breakdown voltage of 95 V which is being developed involves a further increased electric field as compared with the above process for realizing the breakdown voltage of 80 V so that the end of the lightly-doped source/drain region 155 must be further retracted from the end of the second gate insulating film 153.
In this case, the increase of the removal width H leads to a very large triode threshold voltage (Vt) of the transistor of about 10 V. This makes it very difficult to deal with the transistor.
This invention has been accomplished in order to overcome the inconveniences of the prior art described above and aims at providing a reliable semiconductor device with reduced local density of a current.
Another object of this invention is to provide a semiconductor device which can optimize the distribution of an impurity density in a lightly-doped source/drain region according to a location of the density of an electric field at each of the above voltages Vgs, and a method of manufacturing such a semiconductor device.
Another object of this invention is to provide a semiconductor device which can reduce a threshold voltage without increasing an electric field applied to a transistor body, and a method of manufacturing such a semiconductor device.
The invention is made to solve the problems and a semiconductor device according to the invention is provided with a gate electrode formed on a semiconductor substrate via a gate insulating film, a first conductive type of body region formed so that the body region is adjacent to the gate electrode, a second conductive type of source region and a channel region respectively formed in the first conductive type of body region, a second conductive type of drain region formed in a position apart from the first conductive type of body region and a second conductive type of drift region formed so that the drift region surrounds the drain region and is characterized in that the first conductive type of body region is formed up to the side of the drain region through an active region under the gate electrode and a second conductive type of impurities layer is formed from the drain region to a part before the active region.
Also, a method of manufacturing the semiconductor device is characterized in that it includes the following processes, first, a second conductive type of well region is formed by introducing (for example, implanting and diffusing) impurities of a second conductive type into/in the first conductive type of semiconductor substrate, and a low density first conductive type of impurities layer and a low density second conductive type of impurities layer are formed at an interval by respectively implanting and diffusing impurities of a first conductive type and impurities of a second conductive type into/in the second conductive type of well region. Next, after impurities of a second conductive type are implanted from the low density second conductive type of impurities layer to a part of the low density first conductive type of impurities layer to form an ion implanted layer, a region on the substrate is selectively oxidized to form a local oxide film and a middle density second conductive type of impurities layer is formed by diffusing the ion implanted layer. A gate insulating film is formed in a region except the local oxide film, and after a gate electrode is formed from the gate insulating film to the local oxide film, impurities of a second conductive type are implanted using resist films respectively having an opening on a source formation region formed in the low density first conductive type of impurities layer and on a drain formation region formed in the low density second conductive type of impurities layer as a mask to form high density source/drain regions.