Lateral IGBT in an SOI configuration and method for its fabrication

A lateral IGBT in an SOI configuration having a top side and an underside is proposed. The lateral IGBT has a drain zone extending to the top side and is of a first conductivity type. The underside of the LIGBT forms a substrate of a second conductivity type. A lateral insulation layer is situated between the substrate and the drain zone. At least one laterally formed region of the second conductivity type is situated in the drain zone, in the vicinity of the lateral insulation layer. These laterally formed regions being spaced apart from one another lying in one plane.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The invention relates to a lateral insulated gate bipolar transistor
 (LIGBT) in a silicon on insulator (SOI) configuration having a top side
 and an underside, and a drain zone of a first conductivity type extending
 to the top side. An anode zone of a second conductivity type is
 incorporated in the drain zone and the anode zone extends to the top side.
 Furthermore, a base zone of the second conductivity type is incorporated
 in the drain zone, the base zone extending to the top side and there being
 incorporated in the base zone a source zone of the first conductivity
 type, which source zone extends to the top side. The underside of the
 lateral IGBT forms a substrate of the second conductivity type. A lateral
 insulation layer is provided between the drain zone and the substrate.
 Circuit configurations having power semiconductor switches are used in the
 automotive sector, in the telecommunications sector, in the consumer
 sector and also for the purpose of load control and many other
 applications. The LIGBT is one of the components used most in circuit
 configurations of this type. A high blocking voltage can be obtained with
 LIGBTs, on account of the long drift zone, but the current-carrying
 capacity is unsatisfactory in comparison with a vertical IGBT.
 In order to be able to ensure a required reverse voltage for a
 predetermined thickness of the drain zone, suitable doping of the drain
 zone is performed. In this case, the blocking voltage is taken up not only
 by the drain zone but also, to a significant proportion, by the insulation
 layer situated underneath.
 SUMMARY OF THE INVENTION
 It is accordingly an object of the invention to provide a lateral IGBT in
 an SOI configuration and a method for its fabrication which overcome the
 above-mentioned disadvantages of the prior art methods and devices of this
 general type, which has lower turn-off losses in comparison with the prior
 art without simultaneously decreasing the reverse voltage in the process.
 With the foregoing and other objects in view there is provided, in
 accordance with the invention, a lateral IGBT in an SOI configuration
 having a top side and an underside. The lateral IGBT has a drain zone of a
 first conductivity type, which drain zone extends to the top side. An
 anode zone of a second conductivity type is incorporated in the drain zone
 and the anode zone runs to the top side. Furthermore, a base zone of the
 second conductivity type is incorporated in the drain zone and the base
 zone extends to the top side. There being incorporated in the base zone a
 source zone of the first conductivity type, which source zone extends to
 the top side. The underside of the LIGBT forms a substrate of the second
 conductivity type. The LIGBT has a source electrode that is in contact
 with the source zone and the base zone. Furthermore, it contains a drain
 electrode that is in contact with the anode zone. A gate insulating layer
 disposed on the top side is situated between the source zone and the anode
 zone. A gate electrode is disposed on the gate insulating layer. A lateral
 insulation layer is provided between the drain zone and the substrate.
 The invention is based on the concept that at least one laterally formed
 region of the second conductivity type is provided in the drain zone, in
 the vicinity of the insulation layer. In this case, the laterally formed
 region has a smaller area than the lateral insulation layer that is
 provided.
 This configuration has the advantage that the terrace-shaped profile of the
 voltage rise during the turn-off of the LIGBT is shifted toward a lower
 voltage value and, at the same time, the terrace phase is shortened in the
 process. This results in a smaller power loss. At the same time, however,
 the maximum reverse voltage is maintained.
 The reason why the turn-off losses can be reduced without the maximum
 reverse voltage being simultaneously decreased in the process is that the
 laterally formed regions of the second conductivity type provided in the
 vicinity of the lateral insulation layer are formed in the source-drain
 direction and, at the same time, have interruptions and, consequently, do
 not areally cover the entire lateral insulation layer. At least partial
 regions of the drain zone are thus in contact with the lateral insulation
 layer. These interruptions enable the electric field to penetrate the
 insulation oxide in the event of the IGBT being subjected to reverse
 voltage loading. The penetration of the electric field into the insulation
 oxide is of relevance, primarily in respect to the reverse voltage
 endurance. On the other hand, the laterally formed regions of the second
 conductivity type enable the stored charge to be depleted more rapidly
 during the switching-off operation of the LIGBT and, consequently, the
 switch-off losses to be reduced.
 In one development, the LIGBT has a vertical and insulating boundary in the
 form of a trench, which boundary extends from the top side to as far as
 the lateral insulating layer. The vertical insulation region (in the form
 of a trench) enables the LIGBT to be integrated with any other
 semiconductor components (e.g. logic elements) on a semiconductor
 substrate.
 In a further refinement of the LIGBT, the anode zone lies in a drain
 extension of the first conductivity type and the drain extension adjoins
 the top side. The drain extension has a higher doping than the drain zone
 and serves to keep the space charge zone away from the anode zone during
 the switching-off operation of the LIGBT.
 In one development, the LIGBT has a field plate covering the drain
 extension on the gate insulating layer. Favorable influencing of the
 electric field within the LIGBT is obtained as a result of this.
 In a further refinement, the LIGBT has a vertically running region of the
 second conductivity type, which region abuts the insulation region in the
 form of a trench. In this case, the vertically running region of the
 second conductivity type is connected to the base zone. The latter serves
 to control the inversion layer on the underside of the LIGBT. If the LIGBT
 is in the blocking state, the transition between the vertically running
 region of the second conductivity type and the drain zone of the first
 conductivity type is controlled in the reverse direction and, in this way,
 prevents the formation of an inversion layer between the drain zone and
 the lateral insulation layer.
 Furthermore, in an advantageous refinement, the LIGBT is characterized in
 that the substrate is at a fixed potential. The latter may be a fixed
 voltage or, in an advantageous manner, ground. However, it is also
 conceivable for the potential to float. In this case, the substrate can
 take up a higher reverse voltage. The insulation. region in the form of a
 trench should be at the lowest potential of the component. It is
 preferable for the insulation region in the form of a trench to be
 connected to the ground potential.
 In a further refinement, the LIGBT is constructed mirror-symmetrically in a
 lateral orientation. In this case, the anode zone and the drain electrode
 situated thereon lie in the center of the semiconductor component. The
 insulation regions in trench form in this case form the lateral outer
 boundary of the LIGBT. It is advantageous if the LIGBT has a finger-shaped
 form, since a utilization that is particularly efficient in respect of
 area can thereby be obtained in the event of a plurality of LIGBTs being
 connected in parallel on a semiconductor substrate.
 In a further refinement, the lateral regions provided in the drain zone
 have a polygonal form and are spaced apart from one another regularly
 lying in one plane. In this case, the lateral regions of the second
 conductivity type may have any desired form, provided that it is ensured
 that two lateral regions lying next to one another are respectively spaced
 apart from one another by a certain distance. It is advantageous if the
 configuration is effected in a regular sequence. The lateral regions may
 in this case have a square, rectangular, octagonal or else round form.
 In another refinement, the lateral regions provided in the drain zone have
 a form that is adapted to the circumference of the LIGBT, and are spaced
 apart from one another identically lying in one plane. If the LIGBT has
 e.g. a finger-shaped border, then lateral regions provided in the drain
 zone are likewise finger-shaped. The lateral region of the second
 conductivity type which lies nearest the vertical insulation region in the
 form of a trench has the largest circumference, while the lateral region
 of the second conductivity type which lies nearest the axis of symmetry
 has the smallest circumference. In this configuration, too, it must again
 be ensured that the lateral regions are at a certain distance from one
 another, so that the electric field can penetrate the lateral insulation
 layer. It is advantageous for the spacing of the individual lateral
 regions with respect to one another to be regularly disposed.
 The LIGBT according to the invention can be fabricated particularly easily
 if the lateral regions of the second conductivity type which are provided
 in the drain zone adjoin the lateral insulation layer. This is not
 absolutely necessary, however.
 It is likewise conceivable, in one refinement of the LIGBT according to the
 invention, for the lateral regions lying nearest the edge in the drain
 zone to be in contact with the insulation region in the form of a trench.
 The lateral form of the LIGBT may be round or finger-shaped. The
 finger-shaped form is advantageous on account of effective utilization of
 area in the event of a plurality of IGBTs being connected in parallel.
 However, the IGBT is not restricted to these lateral forms. Other forms
 are also conceivable.
 It is advantageous for both the lateral insulation layer and the insulation
 region in the form of a trench to be fabricated from SiO.sub.2, since, in
 this case, known fabrication methods can be used and particularly simple
 fabrication is possible.
 It has turned out that it is particularly advantageous for the charge
 carrier density of the lateral regions provided in the drain zone to be
 configured between one times 10.sup.17 and one times 10.sup.19.
 The fabrication of the LIGBT according to the invention can be carried out
 in a simple manner by the following steps, without the fabrication steps
 having to be greatly modified relative to a conventional lateral IGBT. It
 becomes evident from the following fabrication steps that the invention is
 fabricated in a wafer-bonded process.
 In the first step, at least one laterally formed region of the second
 conductivity type is produced in that side of the drain zone that
 subsequently faces the substrate. If only one lateral region is provided,
 then it must be configured to be smaller than the subsequent dielectric
 well. If a plurality of lateral regions are provided, then they are spaced
 apart from one another. In this case, the lateral regions provided in the
 drain zone may have e.g. a polygonal form or a form adapted to the form of
 the lateral IGBT and are advantageously spaced apart from one another
 regularly lying in one plane. In this case, the lateral regions that are
 provided can be fabricated not only by diffusion but also by
 photoresist-masked implantation. The use of boron is expedient.
 It is more advantageous to select a polygonal form for the lateral regions
 that are provided in the drain zone, and to space them apart from one
 another regularly, since this fabrication step can be carried out
 independently of the subsequent emplacement of the IGBT structure produced
 on the opposite side of the drain zone. If the lateral regions have a form
 adapted to the form of the LIGBT, it must be ensured that they are aligned
 with the drain zone, the anode zone and also the base and source zones in
 such a way that the lateral regions of the second conductivity type are
 disposed mirror-symmetrically in the finished LIGBT. In the case where the
 bottom doping of the lateral regions in the drain zone has a polygonal
 form and the size of the individual doped polygons is considerably smaller
 than the lateral extent of the component, the dielectric wells with the
 components situated on the top side need not, by contrast, be aligned with
 the pattern of the doped lateral regions.
 In a second step, the lateral insulation layer is applied to a
 semiconductor wafer of the second conductivity type, which semiconductor
 wafer forms the substrate. It is advantageous to apply the lateral
 insulation layer to the semiconductor wafer forming the substrate since,
 on account of the differently doped surface regions of the drain zone, an
 oxide layer of varying thickness would be produced and would make it more
 difficult to apply the substrate to the lateral insulation layer. After
 the side of the drain zone which forms the subsequent top side of the
 LIGBT with the electrode has been thinned back, the composite of the
 lateral insulation layer and the substrate is connected to the side
 opposite to the top side of the drain zone. The anode zone, the base zone
 and also the source zone are subsequently produced in a known manner on
 that side of the drain zone which is remote from the lateral insulation
 layer. The further steps correspond to the fabrication of a conventional
 lateral IGBT, e.g. the production of an insulation region in the form of a
 trench and also the provision of the necessary electrodes and/or field
 plates.
 Other features which are considered as characteristic for the invention are
 set forth in the appended claims.
 Although the invention is illustrated and described herein as embodied in a
 lateral IGBT in an SOI configuration and a method for its fabrication, it
 is nevertheless not intended to be limited to the details shown, since
 various modifications and structural changes may be made therein without
 departing from the spirit of the invention and within the scope and range
 of equivalents of the claims.
 The construction and method of operation of the invention, however,
 together with additional objects and advantages thereof will be best
 understood from the following description of specific embodiments when
 read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In all the figures of the drawing, sub-features and integral parts that
 correspond to one another bear the same reference symbol in each case.
 Referring now to the figures of the drawing in detail and first,
 particularly, to FIG. 1 thereof, there is shown a structure of an LIGBT
 according to the prior art. A configuration of this type is described for
 example in Proceedings of 1995 International Symposium on Power
 Semiconductor Devices & ICs, Yokohama, Pages 325-329.
 FIG. 1 shows the lateral IGBT in an SOI configuration with a top side and
 an underside, a weakly n-doped drain zone 1 extending to the top side. A
 drain extension 13 extending to the top side is incorporated in the drain
 zone 1, the drain extension being heavily n-doped. A p-doped anode zone 2
 lies in the drain extension 13 and likewise extends to the top side.
 Furthermore, a base zone 3, which extends to the top side and is likewise
 p-doped, is incorporated in the drain zone 1, in which base zone 3, for
 its part, an n-type source zone 4 extending to the top side is
 incorporated. The underside of the semiconductor component is formed of a
 substrate 5, which is heavily p-doped. Furthermore, a source electrode 6
 that is in contact with the source zone 4 and also the base zone 3 is
 provided. A drain electrode 7 is in contact with the anode zone 2. A gate
 insulating layer 9 is disposed on the top side of the semiconductor
 component and the gate insulating layer 9 is situated between the source
 zone 4 and the anode zone 2. A gate electrode 8 and also a field plate 14
 are situated on the gate insulating layer 9. Furthermore, a lateral
 insulation layer 10 is provided between the drain zone 1 and the substrate
 5. The semiconductor component furthermore has, on its side, an insulation
 region 12 in the form of a trench. The insulation region 12 extends from
 the top side to as far as the lateral insulation layer 10. The LIGBT
 furthermore has p-type diffusion at the side of the trench insulation 12.
 The base zone 3, the source zone 4 and also the region situated under the
 gate electrode 8 are connected to a vertically running region 15 (p-type
 diffusion). The anode zone 2 and also the drain electrode 7 situated
 thereon are disposed in the center of the LIGBT. The LIGBT, shown in cross
 section in FIG. 1, generally has a finger-shaped form in plan view.
 The method of operation of the LIGBT of FIG. 1 is explained below. If a
 voltage is applied to the gate electrode, then electrons pass from the
 source zone 4 into the drain zone 1. The lateral PNP transistor 2, 1, 3 is
 thus driven and, for its part, injects holes into the drain zone 1.
 In order to switch the component off, the mobile charge carriers must be
 removed from the drain zone 1. This depletion operation becomes apparent
 by virtue of a current flow after the gate voltage has been switched off.
 FIG. 2 illustrates a typical profile of drain voltage and drain current
 against time during turn-off. What is characteristic of a component of
 this type is that, during the switching-off operation, the voltage across
 the component does not rise monotonically to its reverse voltage value but
 rather firstly rises rapidly to an average level, then continues for a
 time with a significantly lower rate of voltage rise, so that it then
 ultimately rises to the value of the reverse voltage. Not until the full
 reverse voltage is present across the semiconductor component can the
 current fall to 0. The terrace-shaped voltage rise is caused by the
 initially lateral depletion of the stored charge and then by the vertical
 depletion of the stored charge during the switching-off operation. The
 consequence of the terrace-shaped voltage profile is an unfavorable power
 loss balance of the LIGBT during turn-off.
 A solution to this problem can be obtained by introducing a heavily doped
 p-diffused zone at the bottom of the dielectrically insulated well. The
 heavily doped p-diffused zone would consequently lie between the drain
 zone 1 and the lateral insulation layer 10 and be in contact with the
 vertically running p-doped region 15. An LIGBT having a p-diffused layer
 of this type is disclosed in Proceedings of 1997 International Symposium
 on Power Semiconductor Devices & ICs, pages 313-316. The consequence of
 introducing such a heavily doped p-diffused zone is that the terrace phase
 occurs at a significantly lower drain voltage and the power loss is thus
 significantly reduced. As is known, the power loss during the turn-off
 operation is calculated according to the formula
EQU P.sub.off =1.multidot..intg.U.multidot.dt
 The disadvantage of this configuration, in comparison with that shown in
 FIG. 1, is that, given the same thickness of the drain zone 1, the
 blocking ability of the LIGBT is greatly reduced. The reduction in the
 blocking ability is caused by the fact that the electric field cannot
 penetrate the oxide insulation of the LIGBT subjected to reverse voltage
 loading because of the heavily doped layer at the bottom of the
 dielectrically insulated well.
 Published, European Patent Application EP 0 338 312 A2 discloses an IGBT in
 which an oxide insulation layer in the form of a well is provided in the
 substrate. An anode region is provided within the oxide well, the anode
 region being disposed, proceeding from the surface, along the oxide well,
 and the anode region is perforated at the bottom of the oxide well. In
 that case, regions of the opposite conductivity type to that of the anode
 region are provided and form a further well on the side remote from the
 oxide insulation layer along the anode region. A high and uniform current
 flow is thereby obtained, the main current predominantly flowing in the
 vertical direction.
 FIG. 3 shows the lateral IGBT according to the invention, in cross section.
 The IGBT has the drain zone 1 of the first conductivity type, which drain
 zone 1 extends to the top side.
 The anode zone 2 of the second conductivity type is incorporated in the
 drain zone 1 and the anode zone 2 runs to the top side. Furthermore, the
 base zone 3 of the second conductivity type is incorporated in the drain
 zone 1 and the base zone 3 extends to the top side. There being
 incorporated in the base zone 3 the source zone 4 of the first
 conductivity type and the source zone 4 extends to the top side. The
 underside of the LIGBT forms the substrate 5 of the second conductivity
 type. The source electrode 6 is in contact with the source zone 4 and the
 base zone 3. The drain electrode 7 is in contact with the anode zone 2.
 The gate insulating layer 9 is disposed on the top side of the LIGBT and
 the gate insulating layer 9 is situated between the source zone 4 and the
 anode zone 2. The gate electrode 8 is disposed on the gate insulating
 layer 9. The lateral insulation layer 10 is provided between the drain
 zone 1 and the substrate 5. Furthermore, the LIGBT has the vertical
 insulation layer 12 in the form of a trench and the insulation layer 12
 extends from the top side of the LIGBT to as far as the lateral insulation
 layer 10. The latter has the lowest potential of the semiconductor
 component; it is preferably at ground potential. The region of the second
 conductivity type 15 adjoins the insulation layer 12 in the form of a
 trench and is also connected to the base zone 3. The anode zone 2 lies in
 the drain extension 13 of the first conductivity type, which drain
 extension 13 adjoins the top side. The doping of the drain extension 13 is
 significantly higher than that of the drain zone 1. Furthermore, a field
 plate 14 covering the drain extension 13 is provided on the gate
 insulating layer 9.
 The LIGBT furthermore has laterally formed regions 11 of the second
 conductivity type which are identically spaced apart from one another
 lying in one plane. The laterally formed regions 11 of the second
 conductivity type adjoin the lateral insulation layer 10. The consequence
 of this is that the laterally formed regions 11 are at the same potential
 as the lateral insulation layer 10.
 It is also conceivable for the laterally formed regions 11 to be situated
 within the drain zone 1 such that they lie in one plane and are spaced
 apart from one another identically. However, it is necessary that the
 laterally formed regions 11 lie in the vicinity of the lateral insulation
 layer 10. The LIGBT has a structure that is mirror-symmetrical with
 respect to an axis I--I. Three lateral regions 11 of the second
 conductivity type that lie next to one another are illustrated in FIG. 3.
 Consequently, a total of six laterally formed regions 11 lie next to one
 another in one plane in the drain zone. However, it is also conceivable
 for more or fewer of the laterally formed regions 11 to be provided in the
 drain zone 1. In this case, it must only be ensured that the lateral
 insulation layer 10 is in contact with the drain zone 1 at some points.
 These interruptions are necessary in order to enable the electric field to
 penetrate the lateral insulation layer 10 in the event of the IGBT being
 subjected to reverse voltage loading, since otherwise the blocking ability
 of the component would be reduced. The laterally provided regions 11
 contribute, on the other hand, to depleting the stored charge from the
 space charge zone more rapidly, during the turn-off of the L-IGBT, and,
 consequently, to reducing the switch-off losses.
 FIG. 4a and FIG. 4b show the lateral IGBT according to the invention in
 plan view in accordance with FIG. 3 along the line II--II. FIG. 4a shows
 the lateral regions 11 of the second conductivity type which are
 incorporated in the drain zone 1. In this case, the lateral regions 11
 have a form that is adapted to the form of the LIGBT, that is to say they
 are finger-shaped. In FIG. 4a, four finger-shaped, laterally formed
 regions 11 are identically spaced apart from one another and lying in one
 plane. The mirror-symmetrical structure of the LIGBT emerges in this view.
 FIG. 4b shows the lateral regions 11 lying in the drain zone 1. The
 laterally formed regions 11 have a rectangular form and are spaced apart
 from one another identically. The laterally formed regions 11 could also
 have a round, triangular or else polygonal form.
 In both cases, the lateral regions 11 of the second conductivity type which
 are spaced apart from one another enable the electric field to penetrate
 the lateral insulation layer 10. The advantage of the configuration of the
 lateral regions 11 in accordance with FIG. 4b by comparison with FIG. 4a
 is that if the size of the individual laterally formed regions is
 considerably smaller than the lateral extent of the LIGBT, the lateral
 regions do not have to be aligned with the configuration of the base zone,
 the anode zone, etc. The pattern of laterally formed regions 11 can be
 applied over the entire surface as early as during the production of the
 wafer. The consequence of this is that the laterally formed regions 11 are
 also present on the underside, at locations where a logic circuit
 configuration is provided. They do not, however, have any effects on the
 logic circuit since the function thereof is principally determined by
 effects near the surface.
 If the laterally formed regions 11 have a form adapted to the lateral
 extent of the IGBT, it must be ensured that the form is aligned with the
 regions disposed on the top side, since the requirement of a
 mirror-symmetrical structure must be fulfilled. This requirement increases
 the complexity in the course of fabricating the LIGBT according to the
 invention. FIG. 3 illustrates the LIGBT with an n-doped drain zone 1. The
 dopings of the other regions are specified in a corresponding form.
 However, it is also conceivable for the doping of the drain zone to be of
 the opposite conduction type, that is to say of the p-doped type. The
 remaining regions are doped correspondingly.