METHOD OF MANUFACTURING A RESURF SEMICONDUCTOR DEVICE, AND A SEMICONDUCTOR DEVICE MANUFACTURED BY SUCH A METHOD

Abstract: An epitaxial layer with a doping of approximately 10<12> atoms per cm<2> is used in accordance with the resurf condition for the high-voltage circuit element in high-voltage integrated circuits of the resurf type. If the circuit comprises a zone which is provided in the epitaxial layer, which is of the same conductivity type as the substrate, and to which a high voltage is applied, the doping between this zone and the substrate must in addition be sufficiently high for preventing punch-through between the zone and the substrate. A known method of complying with these two requirements is to make the epitaxial layer very thick. It is found in practice, however, that this method is often not very well reproducible. According to the invention, the epitaxial layer is provided in the form of a high-ohmic layer which is doped from the upper side (3a) and from a buried layer (3b). The buried layer is blanket-deposited, which dispenses with a masking step, and is locally redoped by the island insulation zone (4).

Description: METHOD OF MANUFACTURING A RESURF SEMICONDUCTOR DEVICE, AND A SEMICON¬ DUCTOR DEVICE MANUFACTURED BY SUCH A METHOD The invention relates to a method of manufacturing a high-voltage semiconductor device of the resurf type whereby a silicon body with a substrate of a first conductivity type is provided at a surface with a surface layer of the opposite, second conductivity type whose thickness and doping concentration are chosen such that the surface layer can be depleted at least locally throughout its thickness without breakdown, whereby d e layer is subdivided into a number of mutually separated islands by means of insulation zones of the first conductivity type which are diffused into die surface layer, and whereby a circuit element widi a surface zone of the first conductivity type which extends from the surface into the surface layer over part of me diickness of the surface layer is provided in at least one of said islands. The invention also relates to a semiconductor device manufactured by d is method. Such a method and a device manufactured thereby are known inter alia from the article "A versatile 700-1200-V IC process for analog and switching applications" by A.W. Ludikhuize, published in IEEE Transactions on Electron Devices, Vol. 38. No. 7, July 1991, pp. 1582-1589. The resurf principle is described inter alia in the article "High voltage thin layer devices (resurf devices)" by J.A. Appels and H.M.J. Vaes, (Techn. Digests IEDM 1979, pp. 238/241) and is based on the phenomenon that electrical breakdown in a comparatively thin layer is avoided by means of depletion of the layer tiiroughout its thickness, so that an electric field distribution is obtained at me surface such diat die breakdown voltage approaches its theoretical maximum value. It follows from calculations that the product N*d of the thickness d and doping concentration N must be of the order of IO 12 atoms per cm 2 for resurf. Owing to die high breakdown voltage, the resurf principle is highly suitable for use in high-voltage semiconductor devices such as indicated in die above publication by Ludikhuize. This describes integrated circuits in which the surface layer is provided in the form of an n-type epitaxial layer of silicon on a p-type silicon substrate. Various circuit elements for high-voltage applications are provided in die epitaxial layer, such as transistors of the LDMOST (lateral DMOST) type. Such a transistor has a p-type channel region or back-gate region in which an n-type source is formed. The drain lies at some distance from the back-gate region, separated therefrom by an interposed drift region. As is noted in the cited publication, some transistors are used under operating conditions different from those for other transistors. Thus the same voltage (ground) is applied to the back-gate region and to the substrate in a number of transistors, which may give rise to a high voltage between back gate and drain. In odier transistors, for example transistors operated in the source-follower mode or transistors at die high-voltage side in bridge circuits, a very high voltage may also be applied to me back-gate region. A high voltage may be present between the substrate and die back-gate region in mis situation, which renders it imperative diat no electrical breakdown (punch-through) occur between the substrate and the back-gate region. A similar situation can arise in the source of HV p-channel MOS transistors. To avoid punch-mrough, the product N*D, in which D is the ickness of the portion of die epitaxial layer between substrate and back-gate region, must be sufficiently great. This is achieved in me known device in mat the epitaxial layer is made comparatively thick while N has a low value, so that sufficient charge remains below the comparatively thin back-gate region; in die embodiment described, however, the epitaxial layer has a thickness of no more man 23 μm for N « 7*10 14 per cm 3 . It is found in practice, however, that it is comparatively difficult to manufacture such a device widi a sufficient reproducibility owing to fluctuations in the doping, i.e. to keep fluctuations of me total doping N*d within 20% . It is an object of the invention inter alia to provide a method of manufacturing a semiconductor device of me resurf type which has a higher reproducibility and a higher accuracy than known methods. According to die invention, a method of the kind described in the opening paragraph is for this purpose characterized in mat die surface layer of the second conductivity type is formed partly mrough diffusion of impurities which are introduced into me semiconductor body from me surface and partly through diffusion of impurities from a buried layer which is provided over die entire surface of the surface layer with a doping concentration which is lower man the doping concentration of the insulation zones of the first conductivity type and which is redoped by the buried zones of die first conductivity type at die areas of me insulation regions. The impurities may be diffused into the surface layer by means of previously implanted regions, so that the total doping of the layer can be well controlled. Doping of the layer partly from a buried layer achieves diat a sufficient impurity of the second type is present also under the surface zone of the first type, for example the back-gate region of a LDMOST, for preventing punch-through between the surface zone and me substrate. Moreover, the process does not or substantially not become more complicated than the known process because the implanted regions can be provided without masks and it is usually sufficient to provide minner epitaxial layers. An embodiment of a method according to the invention, in which the influence of the epitaxy process on the total doping in the layer is small, is characterized in mat the surface layer is formed in a comparatively weakly doped epitaxial layer of me first or second conductivity type which is deposited on me substrate after me substrate has been provided at its surface with me buried zones of the first and me buried layer of the second conductivity type. These and other aspects of the invention will be explained in more detail with reference to an embodiment and a drawing, in which: Fig. 1 is a cross-section of a high-voltage semiconductor device manufactured by a method according to the invention; and Figs. 2 to 7 show this device in a number of stages of its manufacture. Fig. 1 shows in cross-section an integrated high-voltage circuit of the resurf type provided in a silicon body 1 wi i a p-type substrate 2 which is provided with an n-type surface layer 3 at its surface. In accordance widi the resurf condition, the composition of the layer 3 is such that it can be depleted at least locally throughout its thickness before punch-mrough occurs. A favorable field distribution is obtained dirough mis depletion, as is known, in particular at the surface, so that the punch-through voltage approaches the intrinsic breakdown voltage of silicon and is influenced to a lesser degree by local field concentrations. To obtain me resurf effect, me mickness d and the doping concentration N of d e surface layer 3 are so chosen mat the product N*d is approximately equal to 2*10 12 atoms per cm 2 . The surface layer 3 is subdivided into a number of mutually electrically insulated islands by means of p-type zones 4 which extend from the surface down to die substrate. The insulation zones 4 each have a region 4a which is formed from the surface and a region 4b which is formed from the substrate, d e regions 4a and 4b forming a coherent region owing to diffusion into the surface layer. A high-voltage circuit element is provided in each of the islands depicted, in mis example in the form of a lateral doubly diffused MOS transistor, called LDMOST for short hereinafter. The transistors are of a conventional construction and will accordingly be only briefly described. Transistor T t in me island on me right comprises a back-gate region 5 formed by a p-type surface zone 5. The source zone is formed by an n-type zone 6 provided in me p-type zone 5. The drain zone of the transistor is formed by me n-type surface zone 7 provided at a distance from me back-gate region 5 in the n-type surface layer 3, while me interposed portion of die surface layer 3 forms me drift region of the transistor. The portion of the back-gate region 5 situated between the source 6 and die drift region forms the channel region which is electrically insulated from die overlying gate 9 by the gate oxide 8. The source is contacted by a source electrode 10 which in mis example is also connected to me back-gate region 5 and to a diagrammatically indicated connection s. As me drawing shows, me electrode 10 extends to above the drift region and forms a field plate there which effects a favorable field distribution in die drift region adjacent the back-gate region, mus increasing the punch-dirough voltage. The gate is connected to a gate connection g. The drain 7 is connected in a usual manner to the drain electrode 11 and, via mis electrode, to me drain connection d. Transistor T 2 provided in the island on the left has a consunction which is substantially identical, with a p-type back-gate region 12, an n-type source 13, an insulated gate 15, and an n-type drain 14, separated from me back-gate region 12 by an interposed portion of the surface layer 3 which forms the drift region. The source and the back-gate region are provided widi a contact 16 which is connected to a connection s. The drain 14 is provided with a contact 17 and a drain connection d. The transistors may be connected, for example, in series so as to form a half-bridge circuit, where die source 13 of T 2 is grounded and the drain 7 of T x is connected to me high supply voltage, while drain 14 of T 2 is connected to source 6 of Tj. The transistors may then be switched on and off alternately by means of suitable control signals at the gate electrodes, so mat a current in a first direction alternating widi a current in me other direction can be passed mrough a load element connected to die junction between source 6 and drain 14. In such a situation, a very high voltage may be applied to the drain 14 of T 2 , and thus a high reverse voltage across die pn junctions between the n-type island and the p- type insulation regions 4 and the p-type back-gate region 12. This voltage can be accommodated thanks to the resurf principle, possibly in combination wid breakdown voltage raising, weakly doped p-type extensions (not shown) along the edges of the p-type zones. There is no voltage between die substrate 2 and die back-gate region 12, so that accordingly no breakdown will occur between die substrate and the back-gate region 12. As Fig. 1 shows, the back-gate region 12 may adjoin the p-type insulation zone 4 and may be connected to die substrate via this zone. In transistor T,, breakdown between die n-type island and the p-type insulation zone 4 can also be avoided dirough the resurf principle in a manner analogous to that in T 2 . In addition, there may be a high voltage between the back- gate region 5 and the substrate 2 in Tj. To prevent punch-through occurring between the regions 2 and 5, the semiconductor layer 3 must have so much doping below the back-gate region 5 that the punch-mrough voltage is higher than the supply voltage. This is achieved in a simple and reproducible manner in that the semiconductor layer 3 according to the invention is doped from two sides, indicated diagrammatically in Fig. 1 widi die reference numerals 3a for the portion doped from d e upper side and 3b for die portion doped from the substrate side. The process of manufacturing the device of Fig. 1 will now be described with reference to Figs. 2 to 7. It starts, as shown in Fig. 2, with a high-ohmic p-type silicon substrate 2 with a resistivity of, for example, 30 β.cm for 300 V applications. The resistivity is chosen to be higher for higher-voltage applications, for example 100 O.cm for 700 V. The upper surface of the substrate is doped with an n-type impurity 20 through implantations of P ions, indicated diagrammatically with arrows 19. The doping is carried out over the entire surface, i.e. also where in a later stage the p-type insulation regions 4 will be formed, so that a mask can be dispensed with. The doping level is approximately 0.7* IO 12 atoms per cm 2 . In a next step (Fig. 3), an implantation mask 21 is provided on the substrate with openings at the areas of the zones 4b to be formed, after which die p-type doped zones 22 are formed through implantation with boron ions, diagrammatically indicated with arrows 22. The doping level is, for example, approximately 3*10 12 atoms per cm 2 , or more, so mat the previously provided n-type zone 20 is redoped. Obviously, the order of the two doping steps may alternatively be reversed, the p-type zones 22 being first provided through masked implantation and d e n-type zones 20 subsequently in a blanket implantation. The mask 21 is then removed in a usual manner, whereupon possibly a heating step is carried out in order to restore lattice damage caused by the ion implantation and, if necessary, to form an alignment marker for subsequent process steps. Then the surface layer 3 is deposited in die form of an epitaxial layer in a usual manner on the surface of the substrate. The thickness of die layer 3 is, for example, approximately 7 μm. The doping of the layer 3 in this example is weakly n- type, but it may alternatively be weakly p-type, if so desired. During die deposition, the impurities provided in me buried zones 20 and 22 may already diffuse a little into d e epitaxial layer 3, as indicated in Fig. 4. The surface of the epitaxial layer may be covered with a thin oxide layer, if so desired, which offers advantages in me subsequent doping steps. This oxide layer is not shown in me drawing. In a next step shown in Fig. 5, the surface of me epitaxial layer is provided with an n-type layer 24 through implantation 23 of phosphorus ions. The doping level of mis implantation step is, for example, approximately 1.5*10 12 atoms per cm 2 . The implantation is carried out over die entire surface wi out a mask in mis embodiment. It is possible in other embodiments for mis implantation to be carried out locally only widi the use of a mask. In a next step shown in Fig. 6, the surface is provided with a mask 25 with openings at the areas of die p-type zones 4a to be provided. The p-type regions 27 are provided in die epitaxial layer through implantation of boron ions widi a higher doping concentration than me preceding n-type implantation 23, i.e. me zone 24 is locally redoped. The mask 25 is then removed again, whereupon die n-type impurities in the zones 20 and 24 are diffused in opposite directions into the epitaxial layer 3 at high temperature, where they will form the sub-layers 3a and 3b. Simultaneously, the p-type impurities in the zones 22 and 27 diffuse towards one anomer and form the insulation regions 4a, 4b (Fig. 7) in the epitaxial layer. The transistors may be manufactured in usual manner in a next series of steps, whereby the configuration shown in Fig. 1 is obtained. The use of the mediod described above renders it possible to adjust die doping of the surface layer 3 widi accuracy. The total doping is substantially equal to the sum of the dopings of the implantations 19 and 23 and is accordingly approximately 2*10 12 atoms per cm 2 , in accordance wid the resurf condition. The fact that the layer 3 is doped at least part from a buried layer moreover achieves that the doping below p-type 2»nes of high voltage, such as zone 5, is so high that breakdown between these zones and die substrate is prevented. The process remains comparatively simple because mis buried layer is provided without a mask. It will be obvious diat die invention is not limited to the embodiment described here, but that many more variations are possible to those skilled in die art within the scope of the invention. Thus a p-type epitaxial layer may be deposited on die substrate instead of an n-type epitaxial layer. The invention is not limited to the sequence of the various steps according to the embodiment. Thus, for example, implantation 23 may alternatively take place after implantation 26, and possibly even after d e back-gate regions 5 and 12 have been provided. It is also possible for die boron implantation 22 to extend partly below active transistor regions, for example below the back-gate region 12 of transistor T 2 Instead of or in addition to the LDMOST structures described, other (high-voltage) circuit elements such as bipolar transistors or p-channel MOS transistors may be provided in the surface layer 3.

Claims: Claims: 1. A method of manufacturing a high-voltage semiconductor device of die resurf type whereby a silicon body with a substrate of a first conductivity type is provided at a surface widi a surface layer of the opposite, second conductivity type whose thickness and doping concentration are chosen such diat die surface layer can be depleted at least locally d roughout its thickness widiout breakdown, whereby die layer is subdivided into a number of mutually separated islands by means of insulation zones of the first conductivity type which are diffused into die surface layer, and whereby a circuit element with a surface zone of the first conductivity type which extends from the surface into the surface layer over part of die thickness of the surface layer is provided in at least one of said islands, characterized in d at the surface layer of d e second conductivity type is formed partly through diffusion of impurities which are introduced into die semiconductor body from the surface and partly through diffusion of impurities from a buried layer which is provided over the entire surface of the surface layer with a doping concentt-ation which is lower than die doping concentration of die insulation zones of me first conductivity type and which is redoped by die buried zones of the first conductivity type at the areas of the insulation regions. 2. A method as claimed in Claim 1, characterized in that the two doping steps are carried out such d at the total doping in the surface layer has a value equal to or substantially equal to 2*10 12 atoms per cm 2 . 3. A method as claimed in Claim 1 or 2, characterized in that the surface layer is formed in a comparatively weakly doped epitaxial layer of the first or second conductivity type which is deposited on the substrate after the substrate has been provided at its surface with the buried zones of the first and the buried layer of the second conductivity type. 4. A method as claimed in any one of the preceding Claims, characterized in that the doping concentration of the buried layer is chosen such that punch-through between die substrate and the surface zone is prevented when a voltage is applied between the surface zone of the first conductivity type of die circuit element and the substrate of d e first conductivity type. 5. A semiconductor device manufactured by a method as claimed in any one of the preceding Claims.

Inventors: LUDIKHUIZE ADRIANUS WILLEM, LUDIKHUIZE, ADRIANUS, WILLEM
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29