MOS technology power device with low output resistance and low capacity and related manufacturing process

Abstract: A MOS technology power device comprises a plurality of elementary functional units, each elementary functional unit comprising a body region (3) of a first conductivity type formed in a semiconductor material layer (2) of a second conductivity type having a first resistivity value. Under each body region (3) a respective lightly doped region (20) of the second conductivity type is provided having a second resistivity value higher than said first resistivity value.

Description: The present invention relates to a MOS technology power device with low output resistance and low capacity, and to a related manufacturing process. A primary goal of the designers of MOS technology power devices is to reduce as far as possible the output resistance (or "on" resistance) and the various capacitances associated with the device structure. These parameters can be reduced increasing the integration density of the elementary functional units (polygonal cells or stripes) which constitutes a MOS technology power device, by exploiting photolitographic techniques and manufacturing processes more and more similar to those of the Very Large Scale of Integration (VLSI) technologies. However, the physical structure of the MOS power devices limits the increase of the integration density. These limits can be better understood considering the distinct components of the on resistance of a MOS power device, which are: the channel resistance Rc, which is the component associated with the channel region of the MOS power device; the accumulation region resistance Racc, which is the component associated with the surface region of those portions of the common drain layer (i.e. the lightly doped epitaxial layer wherein the elementary functional units are formed) comprised between the body regions of the elementary functional units; the JFET resistance Rjfet, which is the component associated with those portions of the common drain layer comprised between the depletion regions of the body regions of the elementary functional units; and the epitaxial layer resistance Repy, which is the component associated with those portions of the common drain layer beneath the body regions of the elementary functional units. The channel resistance Rc and the accumulation region resistance Racc (both associated to regions near the surface of the common drain layer) can be reduced by scaling down the dimensions of the elementary functional units and by employing photolithographic machines with better optical resolution. Differently, the JFET resistance Rjfet and the epitaxial layer resistance Repy can be reduced only modifying the physical structure of the MOS power devices. In fact, reducing the distance between the elementary functional units (cells or stripes), the Rjfet component increases strongly, the increase being more pronounced the higher the resistivity of the common drain layer. This means that in order to prevent the on resistance from strongly increasing, the minimum distance to which the elementary functional units of the MOS power device must be kept increases with the increase of the resistivity of the common drain layer. By way of example, in devices designed to operate in a voltage range of approximately 60 V, the distance between the elementary functional units can be comprised between 4 µm and 10 µm, while in the case of devices for higher voltages of about 500 V, being the common drain layer more resistive, such a distance is comprised between 15 µm and 20 µm. Therefore, if in the attempt to increase the integration density it is desired to reduce the distance between the elementary functional units (cells or stripes), so that the gate-drain (or feedback) capacitance can be reduced, without however increasing the output resistance of the MOS power device, it is necessary to increase the doping concentration of the common drain layer. This however reduces the breakdown voltage of the MOS power device. One known technique to overcome this drawback is described in the United States Patent No. 4376286: the doping concentration in the portions of the common drain layer between the elementary functional units is increased by means of an implant of N type dopants, without affecting the doping concentration of the common drain layer beneath the body regions of the elementary functional units. In this way, it is possible to reduce the distance between the elementary functional units (and consequently reducing the feedback capacitance of the MOS power device), without increasing the Rjfet component of the on resistance. This technique has the limitation that only the JFET component of the on resistance can be reduced, but not the epitaxial layer resistance Repy. Furthermore, an additional mask is required in the manufacturing process, to prevent the N type dopants from being implanted at the edge of the power MOS device chip. FR-A-2.666.932 (corresponding to the preamble of claim 1) discloses a MOS power device with a lightly doped region below the body region. In view of the state of the art described, it is an object of the present invention to provide a MOS technology power device with a low output resistance and low capacity, without negatively affecting the breakdown voltage. According to the present invention, such object is achieved by means of a MOS technology power device as defined in claim 1. Thanks to the present invention as claimed in claim 1, and specifically to the presence of the lightly doped regions under the body regions of the elementary functional units, a MOS technology power device is provided which, for a given breakdown voltage, has a common drain layer with a lower resistivity than that which would be necessary in a conventional MOS power device with the same breakdown voltage. The reduced resistivity of the common drain layer not only determines a decrease of the JFET component Rjfet, but also of the epitaxial layer component Repy of the output resistance of the MOS power device; furthermore, it is possible to reduce the distance between the elementary functional units without increasing said JFET component, thus reducing the gate-drain capacitance of the MOS power device. The structure according to the present invention is particularly suitable for MOS technology power devices for low voltages (30-200V), in which the dimension of the elementary functional units is comparable with the residual thickness of the epitaxial layer under the body regions. These and other features of the present invention will be made more evident by the following detailed description of one particular embodiment, illustrated as a non limiting example in the annexed drawings, wherein: Fig. 1 is a cross-sectional view of a MOS technology power device according to the present invention; Figs. 2 to 5 are cross-sectional views similar to Fig. 1 showing intermediate steps of a manufacturing process according to the present invention; Fig. 5A is a cross-sectional view similar to fig. 5, illustrating another embodiment of the manufacturing process; Fig. 6 is a comparative diagram showing doping profiles in the case of a conventional MOS technology power device and in the case of the present invention; Fig. 7 is another comparative diagram showing doping profiles in the regions between elementary functional units of the MOS technology power device; Fig.8 is a comparative diagram showing the electric field distribution in the case of a conventional MOS technology power device and in the case of the present invention. With reference to the drawings, and specifically to Fig. 1, a MOS technology power device chip according to the present invention comprises a heavily doped semiconductor substrate 1, over which a lightly doped semiconductor layer 2 is formed, for example by means of an epitaxial growth. In the shown example, referring to the case of an N channel power MOSFET, both the substrate 1 and the epitaxial layer 2 are of the N conductivity type; differently, in a P channel power MOSFET both the substrate 1 and the epitaxial layer 2 would be of the P conductivity type. Also, the substrate 1 and the epitaxial layer 2 could be of opposite conductivity types, as in the case of an Insulated Gate Bipolar Transistor (IGBT). The epitaxial layer 2 forms a common drain layer for elementary functional units of the MOS power device. Each elementary functional units comprises a body region 3 of the P conductivity type (or, more generally, of the opposite conductivity type of the epitaxial layer 2). The body regions 3 can have a polygonal layout (e.g. square or hexagonal), as in the case of "cellular" MOS power devices, or alternatively they can be represented by elongated stripes (in which case Fig. 1 shows a cross-section in a direction transversal to the elongated stripes). Inside each body region 3, heavily doped source regions 4 of the N conductivity type (i.e. of the same conductivity type as the epitaxial layer 2) are provided. The top surface of the epitaxial layer 2 is covered by an insulated gate layer comprising a thin gate oxide layer 5 and a polysilicon layer 6. Openings are provided in the insulated gate layer over each body region 3. The insulated gate layer is covered by an insulating material layer 7 in which contact windows are provided over each body region 3 to allow a source metal layer 8 to contact the source regions 4 and the body regions 3. A drain metal layer 9 is also provided on the bottom surface of the substrate 1. In the epitaxial layer 2, beneath each body region 3, a region 20 of the same conductivity type as but having a higher resistivity than the epitaxial layer 2 is provided which extends downwardly substantially for the whole thickness of the epitaxial layer 2, to the substrate 1. Thanks to the presence of the regions 20 beneath the body regions 3, it is possible to reduce the resistivity of the epitaxial layer 2 without decreasing the breakdown voltage of the MOS power device, because the breakdown voltage of the MOS power device depends on the resistivity and on the thickness of the portions of the common drain layer beneath the body regions, not on the portions of the common drain layer between the body regions. In other words, the presence of the lightly doped regions 20 under the body regions 3 allows to achieve the desired breakdown voltage even with an epitaxial layer having a lower resistivity than that necessary with the prior art structure. As a consequence of the decreased resistivity of the epitaxial layer 2, both the JFET component Rjfet and the epitaxial layer component Repy of the output resistance Ron of the MOS power device are reduced, because the current flux I coming from the source regions and flowing towards the substrate 1 encounter a lower resistance. Also, it is possible to reduce the distance d (Fig. 1) between adjacent elementary functional units without the drawback of an increase of the Rjfet component of the output resistance of the MOS power device. Figs. 6 and 7 show the doping profiles of the different semiconductor regions along the directions x only, respectively, in Fig. 1 in the case of a conventional MOS technology power device structure (dash-and-dot line) and in the case of the present invention (continuos line). Fig. 8 is a diagram showing the profile of the electric field E in the two cases of Figs. 6 and 7: it is possible to see that in the structure of the present invention the breakdown voltage is higher (the area subtended by the curve of the electric field E is higher in the case of the structure of the present invention (continuous line) than in the case of a conventional structure (dash-and-dot line)). A manufacturing process according to the invention provides for the epitaxial growth of the lightly doped layer 2 over the heavily doped substrate 1. The thickness of the epitaxial layer 2 depends on the voltage class of the MOS power device to be fabricated; for example, for low voltage devices the epitaxial layer 2 can have a thickness of about 4 or 5 µm. However, while according to the prior art the resistivity of the epitaxial layer is determined on the basis of the desired breakdown voltage of the MOS power device (for example 1 ohm x cm for a breakdown voltage of 60 V), in the present invention the epitaxial layer 2 has a resistivity which is lower than that necessary to achieve the same desired breakdown voltage (for example 0.6 ohm x cm). Over the surface of the epitaxial layer 2 a thin oxide layer 5 is formed, for example by means of a thermal growth. A polysilicon layer 6 is then deposited over the oxide layer 5 (Fig. 2). The polysilicon layer 6 and the oxide layer 5 are then selectively removed from the surface of the epitaxial layer 2 to form openings 10. This step involves the deposition of a photoresist layer 11, the selective exposition of the photoresist layer 11 to a light source by means of a mask carrying the pattern of the openings 10, the selective removal of the photoresist layer 11, and the etching of the polysilicon and oxide layers 5, 6 where they are not covered by the photoresist layer 11 (Fig. 3). The openings 10 can have a polygonal layout (for example square or hexagonal), or they can be elongated stripes. The body regions 3 of the elementary functional units of the MOS power device are then formed. To this purpose, a P type dopant such as boron is implanted, using the polysilicon and oxide layers 5, 6 (and if necessary also the photoresist layer 11) as a mask, in a dose ranging from 5 x 10 13 to 5 x 10 14 atoms/cm 2 , with an implantation energy in the range 80-200 keV (Fig. 3). A subsequent thermal diffusion of the dopants leads to the formation of body regions 3 (Fig. 4) with a surface concentration in the channel region of approximately 10 17 atoms/cm 3 , necessary to achieve the desired threshold voltage of the MOS power device. In alternative, the body regions 3 can be formed by means of two distinct implants of boron in different doses and at different energies, still using the polysilicon and oxide layers 5, 6 as a mask. For example, the first implant can involve a dose of a P type dopant in the range 10 13 -10 14 atoms/cm 2 with an energy of approximately 80 keV and is used to control the dopant concentration at the surface of the body stripes, exspecially in the channel regions, to set the desired threshold voltage of the MOS power device. The second implant involves for example a dose of P type dopant in the range 10 14 -10 15 atoms/cm 2 with an energy comprised between 100 keV and 300 keV, such that the peak concentration of the dopants can be located at a prescribed depth, namely under the source regions which will be formed in a later step. A subsequent thermal diffusion process at a temperature in the range 1050-1100 °C for 0.5 to 2 hours determines the lateral diffusion of the dopant introduced with the first implant, to form the channel regions of the body regions extending under the gate oxide layer; the vertical diffusion of the dopant introduced with the second implant does not alter the threshold voltage of the MOS power device, because the dopant ions reach the surface with a concentration lower than the concentration of the dopant introduced with the first implant (in fact, the peak dopant concentration of the dopant introduced with the first implant is located substantially at the surface of the drain layer 2). The vertical and lateral diffusion of the dopants introduced with the second implant forms the heavily doped deep body portions of the body regions, reducing the resistivity of the body regions under the source regions. A dopant of the P conductivity type, preferably one having a high diffusivity such as aluminum, is now implanted into the epitaxial layer 2 using the polysilicon and oxide layers 5, 6 (and if necessary the photoresist layer 11) as a mask. The implant dose is suitable to partially compensate, but not to invert, the N type doping level of the epitaxial layer, so to substantially increasing the resistivity of those portions of the epitaxial layer 2 wherein such a dopant is implanted. The implantation energy (ranging from 700 keV to 1 MeV) is such as to locate the peak concentration of the dopant as close as possible to the body-drain junction (1,5 - 2 µm from the surface of the epitaxial layer 2) (Fig. 5). In alternative, the implant mask for the high-diffusivity dopant could be formed by another photoresist layer 111 with smaller openings 100 than the openings 10 in the polysilicon and oxide layers 5, 6 (Fig. 5A). A high dose of an N type dopant (such as arsenic or phosphor) is then selectively implanted into the body regions 3 to form the source regions 4. The N type dopant is then made to diffuse by means of a thermal process. During such thermal process, the source dopant diffuses for a depth of about 0,4-0,5 µm in the case of arsenic, or about 0,6-0,7 µm in the case of phosphor; during the same thermal process, the high-diffusivity dopant diffuses for a depth of about 1,5-2 µm, distributing in a controlled manner under all the body regions 3 substantially to the substrate 1, modifying the doping profile of the epitaxial layer 2 under the body regions 3. The following process steps involves the formation of a layer of insulating material 7 over the whole surface of the chip, the openings of contact windows in the insulating layer 7 over the body regions 3, and the formation of a source metal layer 8 and a drain metal layer 9. If the thermal budget of the thermal diffusion process used to diffuse the source dopant is not sufficient to completely diffuse the high-diffusivity dopant, for example in the case of high-voltage devices with a thick epitaxial layer, it is possible to modify the thermal diffusion process of the source dopant, or to invert the described sequence of steps, implanting the high-diffusivity dopant before the step of formation of the body regions 3, to exploit the thermal diffusion process of the body regions.

Claims: MOS technology power device comprising a plurality of elementary functional units, each elementary functional unit comprising a body region (3) of a first conductivity type formed in the surface of a semiconductor layer (2) of a second conductivity type having a first resistivity value and functioning as a common drain layer, and under each body region (3) a respective lightly doped region (20) of the second conductivity type having a second resistivity value higher than said first resistivity value, characterized in that said lightly doped region (20) is a diffused region with a variable dopant concentration which increases shifting from said body region (3) down to the end of said lightly doped region (20). MOS technology power device according to claim 1, characterized in that said second resistivity value of the lightly doped regions (20) under the body regions (3) determines a breakdown voltage of the MOS technology power device. MOS technology power device according to claim 2, characterized in that said lightly doped regions (20) of the second conductivity type contains dopants of a first conductivity type in a concentration suitable to partially compensate, but not to invert the conductivity type, the concentration of dopant of the semiconductor layer (2) of the second conductivity type. MOS technology power device according to claim 3, characterized in that said dopants of the first conductivity type contained in said lightly doped regions (20) of the second conductivity type have a high diffusivity in the semiconductor layer (2). MOS technology power device according to claim 3, characterized in that said semiconductor layer (2) is superimposed over a heavily doped semiconductor substrate (1), said lightly doped regions (20) under the body regions (3) extending substantially to the semiconductor substrate (1). MOS technology power device according to claim 5, characterized in that said semiconductor substrate (1) is of the second conductivity type. MOS technology power device according to claim 5, characterized in that said semiconductor substrate (1) is of the first conductivity type. MOS technology power device according to claim 6 or 7, characterized in that said first conductivity type is the P type, and said second conductivity type is the N type. MOS technology power device according to claim 8, characterized in that said dopants of the first conductivity type contained in said lightly doped regions (20) of the second conductivity type are aluminum atoms. MOS technology power device according to claim 6 or 7, characterized in that said first conductivity type is the N type, and said second conductivity type is the P type. Process for the manufacturing of the MOS technology power device according to claim 1, comprising the steps of a) forming a lightly doped semiconductor layer (2) of a first conductivity type and with a first resistivity value over a heavily doped semiconductor substrate (1); b) forming over the lightly doped semiconductor Layer (2) a conductive insulated gate layer (5,6); c) selectively removing the insulated gate layer (5,6) to open windows (10) over selected portions of a surface of the lightly doped semiconductor layer (2); d) forming body regions (3) of a second conductivity type in said lightly doped semiconductor layer (2) using the remaining portions of said insulated gate layer (5, 6) as a mask; e) forming source regions (4) of the first conductivity type in said body regions (3), said process providing for forming under each body region (3) a respective lightly doped region (20) of the first conductivity type substantially extending in the semiconductor layer (2) to the semiconductor substrate and having a second resistivity value higher than the first resistivity value of the lightly doped semiconductor layer (2), characterized in that said lightly doped regions (20) of the first conductivity type under the body regions (3) are formed by implanting into the lightly doped semiconductor layer (2) of the first conductivity type a dopant of the second conductivity type in a dose suitable to partially compensate, but not to invert, the concentration of dopant of the first conductivity type of the lightly doped semiconductor layer (2). Process according to claim 11, characterized in that said dopant of the second conductivity type implanted to form said lightly doped regions (20) under the body regions (3) is a dopant having high diffusivity in the lightly doped semiconductor layer (2). Process according to claim 12, the step of formation of the source regions (4) comprising a selective implant of a dopant of a first conductivity type and a subsequent thermal diffusion of the implanted dopant, characterized in that said high diffusivity dopant implanted to form the lightly doped regions (20) under the body regions (3) is implanted before the formation of the source regions (4), and is thermally diffused in the lightly doped semiconductor layer (2) substantially down to the semiconductor substrate (1) during the thermal diffusion of the dopant forming the source regions (4). Process according to claim 12, the step of formation of the body regions (3) comprising an implant of a dopant of the second conductivity type through said windows (10) in the insulated gate layer (5,6) and a subsequent thermal diffusion of the implanted dopant, characterized in that said high diffusivity dopant implanted to form the lightly doped regions (20) under the body regions (3) is implanted before the formation of the body regions (3), and is thermally diffused in the lightly doped semiconductor layer (2) substantially down to the semiconductor substrate (1) during the thermal diffusion of the dopant forming the body regions (3). Process according to claim 13 or 14, characterized in that said high diffusivity dopant is implanted through said windows (10) in the insulated gate layer (5,6). Process according to claim 13 or 14, characterized in that said high diffusivity dopant is implanted into a surface area of the lightly doped semiconductor layer (2) through an opening (100) being smaller than said windows (10) in the insulated gate layer (5,6). Process according to anyone of claims 12 to 16, chracterized in that said semiconductor substrate (1) is of the first conductivity type. Process according to anyone of claims 12 to 16, characterized in that said semiconductor substrate (1) is of the second conductivity type. Process according to claim 17 or 18, characterized in that said first conductivity type is the N type, and said second conductivity type is the P type. Process according to claim 19, characterized in that said high diffusivity dopant is aluminum. Process according to claim 20, characterized in that said aluminum dopant is implanted with an energy ranging from 500 keV to 1 MeV. Process according to claim 17 or 18, characterized in that said first conductivity type is the P type, and said second conductivity type is the N type.

Inventors: FERLA GIUSEPPE, FRISINA FERRUCCIO, RINAUDO SALVATORE, FERLA, GIUSEPPE, FRISINA, FERRUCCIO, RINAUDO, SALVATORE
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CONSORZIO PER LA RICERCA SULLA, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H01L29
IPC codes: H01L21, H01L29