Manufacturing process for a monolithic semiconductor device comprising at least one transistor of an integrated control circuit and one power transistor integrated on the same chip

Abstract

The device uses the horizontal insulating region and the buried layer as the power transistor base and emitter respectively. An epitaxial growth is interposed between the two diffusions needed to form the aforesaid regions and those needed to create the base and the emitter of the transistor of the integrated control circuit.

Description

In the known type of integrated circuit of FIG. 1, C, E and B represent the collector, emitter and base electrodes of a low-voltage npn transistor of the integrated control circuit, while Cp, Ep and Bp represent the collector, emitter and base electrodes of a high-voltage npn power transistor.

The device is capable of operating correctly if the p type insulating region, which is made up of the regions 1 and 3, is connected to the point of the device with the lowest potential.

According to this hypothesis, on observing FIG. 2 (that is to say, the electrical equivalent of the structure) it can be seen that the diode having as its anode the insulating regions 1 and 3 and as its cathode the collector region 7 of the power transistor is reverse biased and consequently the components of the integrated control circuit are insulated from each other and from the power stage. This circumstance enables the device to operate. On the other hand, the distance d between the lower margin of the insulating region 1 and the substrate 4 is smaller than the distance 1 between the lower margin of the base region 9 of the power transistor and the substrate itself: the result of this is that the break-down voltage of the aforementioned diode is lower than the break-down voltage of the power transistor and consequently the peak operating voltage of the device is established by the first of them rather than by the second.

When designing a device, the thickness d must be fixed so that it enables the required peak voltage to be stood. Vice versa, the current rating of the power stage depends on the collector thickness (and therefore on 1) according to the law of reverse proportion. Since said current rating is penalized by the difference s (=1-d) between the junction depth of the base of the power transistor and that of the horizontal insulating region 1, it is necessary to reduce this difference to a minimum (and if possible to zero). In order to do so, it could be considered expedient to increase the junction depth of the base of the power transistor by increasing the diffusion times and/or temperatures; however this gives rise to two problems:

(1) the p type doping agent contained in the horizontal insulating region 1 could rise to the surface 8 faster than the n type doping agent present in the buried layer 2 and could therefore create "phantom" layers of a conductivity opposite to that which should exist in the collector region of an npn transistor;

(2) the n type doping agent contained in the buried layer 2 could rise to the surface 8 and alter the profile of the base region 5 of the low voltage npn transistor and, in the worst hypothesis, short circuit the emitter 6 and collector 2 regions.

It could also be considered expedient to reduce the thickness of the last epitaxial layer 20: this solution involves the same risk described in the aforegoing paragraph 2) and, in any case, would mean reducing the ultimate working voltage for the components of the integrated control circuit.

Another attempt to solve this problem is the one described in the U.S. Pat. No. 4,239,558, which refers to a structure of the type illustrated in FIG. 3, wherein, however, a number of problems can be identified, the most serious of which concerns the limitation of the voltage at which the integrated control circuit can operate.

In fact, the concentration profile of the various types of doping agent along the emitter 35, base 31 and epitaxial collector 33 regions of a power transistor made according to the process relating to the structure of FIG. 3, is schematically illustrated in FIG. 4. The concentrations, in logarithmic scale, are measured on the axis Co of the figure, whereas the distances (depth) of the points of the various regions from the upper surface 38 of the chip are measured on the axis Pr.

Curves EP, BP and CP refer to the doping agent used for the emitter, the base and the collector epitaxial region of the transistor respectively. The surface concentration of the curve EP must be approximately two orders of magnitude greater (or in other words approximately 10² times greater) than that of the curve BP in order to achieve a reasonable degree of emitter efficiency and in turn BP must be approximately three orders of magnitude greater (or in other words approximately 10³ times greater) than that of the curve CP so that the drain region essentially extends to the collector epitaxial region (necessary conditions for achieving a high early voltage and avoiding punch-through).

If then the collector epitaxial region has a concentration of impurities equal to 10¹⁴ atoms/cm³ (the value needed to achieve break-downs not less than approximately 1000 volts), the surface concentrations for the curves EP and BP must be in the order of 10¹⁹ and 10¹⁷ atoms/cm³ respectively.

Referring to FIG. 3 again, the base region 36 of the transistor of the integrated control circuit must have a concentration higher than that of the collector region 34 and lower than that of the emitter region 37. Since the collector region of the transistor of the integrated control circuit is made at the same time as the emitter region of the power transistor, the doping of the base and emitter regions of the transistor of the integrated control circuit must be very high, close to the solid solubility limits of the various impurities in question. In FIG. 5 the curves E, B, C, I and CP show the resulting concentration profile of the various doping impurities along the emitter, base and collector regions of the transistor of the integrated control circuit, along the horizontal insulation region 32 and the collector epitaxial region 33.

The break-down values of the emitter-base and base-collector junctions of the transistor of the integrated control circuit, as well as of the junction between the collector of the latter and the horizontal insulation region are extremely low as a result of the aforesaid high doping. They can be increased only if the concentrations are reduced, which would immediately downgrade performance.

The solution according to this invention makes it possible to overcome these problems by means of a structure which uses the horizontal insulation region and the buried layer as the base and the emitter of the power transistor respectively, as shown in FIG. 6 (where the symbols have the same meaning as the same symbols used in FIG. 1), interposing however an epitaxial growth of the n type (see layer marked 17 in FIG. 6) between the two said diffusions and those needed to create the base and the emitter of the transistors of the integrated control circuit.

One of the peculiarities of the structure is that 1=d, since the base of the power transistor and the horizontal insulation region are made by means of the same diffusion process: the current rating of the device can therefore be kept at its maximum value once the operating voltage has been fixed. In order to analyze how the structure according to the invention overcomes the problems of the known art (in particular those found in the structure shown in FIG. 3), reference must be made to FIGS. 7 and 8.

The first shows the profile of the concentration of doping agent along one section of the structure of FIG. 6 passing through the emitter region 16, base region 15 and collector region 26 of the transistor of the integrated control circuit, the horizontal insulation region 24 and finally the epitaxial collector region 22 of the power transistor, while the second refers to a cross section through regions 13 and 25 of the emitter, region 23 of the base and epitaxial collector of the power transistor.

The ratios between the concentrations of doping agent in the emitter, base and collector regions of the power transistor and the transistor of the integrated control circuit are satisfactory inasmuch as an epitaxial growth is realized following the creation of the horizontal insulation region and the buried layer and prior to the diffusion of the base and emitter regions of the transistor of the integrated control circuit. Said epitaxial layer separates the two pair of highly doped regions (see FIG. 7) and thus allows the integrated control circuit to operate at sufficiently high voltages.

Since the emitter and base regions of the power transistor are "buried", they must be connected to the surface by means of high concentration "enrichment" regions so as to reduce the series-resistances of the base and the emitter: to this end the insulation (region 19 of the base) and sinker (region 13 of the emitter) regions are used without modifying the manufacturing process, simply by using a different photomask layout.

One method of manufacturing the invention is described below.

An epitaxial layer 22 of the type n- with a concentration of impurities of approximately 10¹⁴ atoms/cm³ is grown on a substrate 11 of monocrystalline silicon type n+ with a high concentration of impurities (concentration>10¹⁹ atoms/cm³).

Using the normal oxidation, photomasking, implantation and diffusion techniques two regions 23 and 24 of the type p+ with a surface concentration of approximately 5·10¹⁶ atoms/cm³ are created to form the base of the npn power transistor and the horizontal insulation region of the components of the integrated control circuit (FIG. 9) respectively.

Using normal oxidation, photomasking, implantation and diffusion techniques, two regions 25 and 26 of n+ type are realized inside regions 23 and 24 respectively, with dopant surface concentrations of 2·10¹⁹ atoms/cm³, which are destined to become the emitter region of the power transistor and the buried layer of an NPN transistor of the integrate control circuit (FIG. 10).

At this point, by means of a new epitaxial growth, a layer 17 of the n type is obtained, which has a constant dopant concentration of 10¹⁴ atoms/cm³, and extends on all the surface of the chip.

Still using normal oxidation, photomasking, implantation and diffusion techniques regions 19 and 21 of the p+ type with surface concentrations of 10¹⁸ atoms/cm³ are made, stretching from the surface 12 as far as regions 23 and 24 (FIG. 11).

It should be noted that whereas regions 21 constitute the vertical insulation regions needed to insulate the various components of the integrated control circuit from each other and to insulate the circuit itself from the power stage, the "enrichment" regions 19 enable the base region 23 of the power transistor to be connected electrically to the surface.

Next the "enrichment" regions 13 and 14 of type n+ are formed with surface concentrations of 2·10¹⁹ atoms/cm³, the purpose of which is to reduce the series-resistance of for the emitter of the power transistor and the collector of the low voltage transistor respectively (FIG. 12).

At this point the known techniques (see FIG. 13) are used to form the base 15 and emitter 16 region of the npn low voltage transistor, the aperture of the contacts (see aperture present on the insulating layer 10) and the interconnection of the various elements of the device by means of a process of metal-coating and photomasking (see metal-coatings 18): all the terminal electrodes of the various components are situated on the front of the chip with the exception of the collector of the power transistor which is on the back.

Although only one form of manufacturing process has been illustrated and described, it is clear that numerous variations and modifications can be made to the invention without moving away from its scope.

For example, the epitaxial region 17 can be achieved through a double growth, as described in Italian patent application No. 6613A/86.

The border regions of the base of the power transistor and of the insulation region can be surrounded by other regions having the same type of conductivity.

Claims

1. A manufacturing process for manufacturing a monolithic semiconductor device comprising that includes at least one transistor of an integrated control circuit and one power transistor integrated on the same chip, the phases of which envisage steps comprising:

the epitaxial growth (a) epitaxially growing on a semiconductor substrate (11) of a first type of conductivity, of a semiconductor layer (22) which is also of the aforesaid first type the same type of conductivity as said semiconductor substrate;

the simultaneous formation, (b) simultaneously forming by diffusion of doping agent within the surface of the layer (22), of a first semiconductor region (23) of a second type of conductivity and a second semiconductor region (24) of a said second type of conductivity, said first region (23) constituting the base region of the power transistor, said second region (24) constituting the horizontal insulating region of the integrated control circuit with respect to the power transistor;

the simultaneous formation, (c) simultaneously forming by diffusion within the surface of the aforesaid regions (23) and (24) said first and second regions, of two other regions (25) and (26) , respectively, of the first type of conductivity, constituting, respectively, the emitter region of the power transistor and the buried layer of the collector region of the integrated control circuit transistor;

the formation (d) forming, by further subsequent superficial diffusions, of the a base (15) and the an emitter (16) of the aforesaid said transistor of the integrated control circuit;

the formation (e) forming, by simultaneous diffusion, of connecting regions (21) and (19), at the a surface (12) of the horizontal insulating region (24) and base region (23) of the power transistor, respectively;

the formation (f) forming, by simultaneous diffusion, of enrichment regions (13) and (14) for the said emitter (25) of the power transistor and the buried layer (26) of the power transistor and said base of the integrated control circuit transistor, respectively ; and

characterized by the fact that the formation of the regions (21), (19), (13) and (14) of the base (15) and of the emitter (16) of the transistor of the integrated control circuit is preceded by the epitaxial growth of a semiconductor layer (17) of the first type of conductivity.

(g) epitaxially growing another semiconductor layer of the first type of conductivity before the performing the steps of forming the regions in steps (d), (e) and (f).

2. A manufacturing process as claimed in claim 1 characterized by the fact that the region (15) of the base of aforesaid integrated control circuit transistor extends including the step of extending from the surface (12) of aforesaid said another semiconductor layer (17) as far as the buried layer (26) of the said collector region of said integrated control circuit transistor.

3. A process for manufacturing a monolithic semiconductor device that includes a three region vertical structure and a power transistor integrated on the same chip, the steps comprising:

(a) epitaxially growing on a semiconductor substrate of a first type of conductivity a semiconductor layer which is the same type of conductivity as said semiconductor substrate;

(b) simultaneously forming within the surface of said semiconductor layer a first semiconductor region of a second type of conductivity and a second semiconductor region of said second type of conductivity, said first region constituting a base region of the power transistor, said second region constituting a horizontal insulating region for a portion of the integrated circuit;

(c) simultaneously forming within the surface of said first and second regions two other regions of the first type of conductivity, constituting, respectively, the emitter region of the power transistor and a buried layer;

(d) epitaxially growing a second semiconductor layer of said first type of conductivity, at least a portion of said second layer being above and next to said buried layer thereby becoming a bottom region of said three region vertical structure;

(e) forming a third region of said second conductivity type in said second layer, at least a portion of said third region being above and next to said bottom region of the three region vertical structure, said third region thereby becoming a middle layer of said three region vertical structure;

(f) forming a fourth region of said first conductivity type in said middle region, said fourth region thereby becoming a top region of the three region vertical structure; and

(g) simultaneously forming connecting regions from the top surface of said second layer to said horizontal insulating region and to said base region of the power transistor respectively.

4. A manufacturing process as claimed in claim 3 including the step of forming a connecting region from the top surface of said second layer to said buried layer.