BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the elimination of premature avalanche breakdown and subsequent breakdown voltage walkout in high voltage semiconductor devices, in particular, to eliminating premature initial breakdown and subsequent breakdown voltage walkout in trench isolated high voltage semiconductor devices by applying voltage directly to silicon adjacent to the trench isolating the device.
2. Description of the Related Art
The phenomenon of breakdown voltage “walkout” can present a serious problem to semiconductor designs that must exhibit a fixed initial breakdown voltage. Defined simply, “walkout” is the alteration in the breakdown voltage of a semiconducting device that results from at least one prior occurrence of avalanche breakdown.
FIGS. 1A-1F illustrate the phenomenon of walkout at the base-collector junction of a conventional LOCOS isolated NPN bipolar transistor.
FIG. 1A illustrates a cross-sectional view of the base/collector PN junction 100 of a conventional NPN bipolar transistor. The NPN bipolar transistor includes field oxide 102, a heavily doped p type base region 104, having base contact 106, and a lightly doped n type collector region 108 having a collector contact 110. For ease of illustration, collector contact 110 is shown as being on the side of collector 108, rather than at its actual location at the silicon surface.
As shown in FIG. 1B, when the base/collector PN junction is forward bias, a positive potential difference is applied between the base contact 106 and the collector contact 110. When the base/collector junction is biased in this manner, with the base contact at a more positive potential than the collector contact, holes 112 flow from base 104 to collector 108, and electrons 113 flow from collector 108 to base 104.
As shown in FIG. 1C, when the base/collector PN junction is reverse biased, a negative potential difference is applied between the base contact 106 and collector contact 110. When the base/collector junction is biased in this manner, with the collector contact at a more positive potential than the base contact, depletion region 114 in collector 108 and depletion region 115 in base 104 are formed. The potential difference between collector contact 110 and base contact 106 is dropped across the adjacent depletion regions 114-115.
The thickness of depletion regions 114 and 115 depends upon 1) the magnitude of the potential difference between collector contact 110 and base contact 106; and 2) the dopant concentrations of collector 108 and base 104. Because the dopant concentration of the collector (typically 1014-1017 atoms/cm3), is generally lower than the dopant concentration of the base (typically 5×1017-1019 atoms/cm3) depletion region 114 formed in collector 108 is generally wider than depletion region 115 formed in base 104.
Under the reverse biased conditions described above, only a very small reverse current flows through the NPN transistor. However, if a large enough positive voltage difference is applied between collector contact 110 and base contact 106, avalanche breakdown of the PN junction will occur and substantial current will flow across the base/collector PN junction 100. FIG. 1D illustrates avalanche breakdown of base/collector PN junction 100.
Avalanche breakdown as shown in FIG. 1D is triggered when the electric field at any point within the depletion regions 114-115 exceeds a certain critical value. The dopant concentrations of collector 108 and base 104, and the junction depth of base 104 are among the factors which determine how large the potential difference between collector contact 110 and base contact 106 can be before the critical electric field is reached in any part of depletion regions 114-115.
During avalanche breakdown, electrical charge carriers can acquire high energy and be injected into the field oxide. These injected charge carriers can remain embedded and affect the conductivity of surrounding silicon regions.
FIG. 1E shows a cross-sectional view of the base/collector PN junction 100 following an avalanche breakdown event in which electrons 116 have been injected into the region 118 of field oxide 102 immediately adjacent to base/collector PN junction 100. FIG. 1F illustrates that upon reverse-biasing base/collector PN junction 100 of FIG. 1E, embedded electrons 116 distort the shape of depletion region 114. Their effect is to widen depletion region 114, and therefore, for a given potential difference between collector contact 110 and base contact 106, to reduce the peak electric field in the portion of depletion regions 114-115 adjacent to field oxide 102. Since the highest electric field in depletion regions 114-115 usually is located close to where PN junction 100 intersects the interface between silicon and field oxide 102, the embedded charge 116 has the effect of increasing the breakdown voltage of base/collector PN junction 100.
To summarize, following an initial avalanche breakdown that injects high energy carriers into an oxide, the breakdown voltage of a device can increase as the embedded carriers reduce the peak electric field near the field oxide.
The aforementioned increase in breakdown voltage is commonly known as “walkout”. Walkout is illustrated graphically in FIG. 2. FIG. 2 plots voltage versus current in a semiconductor device that is experiencing walkout. FIG. 2 illustrates that following an initial avalanche breakdown event triggered by application of a first initial breakdown voltage (BVinit), the voltage required to recreate avalanche breakdown is increased, or “walked out”, to a higher voltage (BV1). FIG. 2 shows that subsequent breakdown events can result in a cumulative increase in breakdown voltage (i.e. BVinit<BV2<BV3).
Field plates have conventionally been used to increase PN junction breakdown voltages by reducing electric field where a PN junction intersects an oxide/silicon interface. The use of a field plate for this purpose is shown in FIGS. 3A-3B.
FIG. 3A shows a cross-sectional view of the base/collector PN junction 300 of an NPN bipolar transistor equipped with a conductive field plate 301 having a field plate contact 301 a. Conductive field plate 301 is in contact with surface portion 319 of field oxide 302 proximate to the field oxide/silicon interface 318. FIG. 3B shows that if field plate contact 301A is held at the same potential as base contact 306, while base/collector PN junction 300 is reverse biased, the width of depletion region 314 proximate to field oxide/silicon interface 318 is increased. This reduces the peak electric field in the portion of depletion region 314 proximate to field oxide/silicon interface 318, and therefore increases the breakdown voltage of base/collector PN junction 300.
The effect of the field plate is very similar to the effect of embedded negative charge shown in FIG. -F. Though the field plate is usually electrically connected to base contact 306, it may be independently biased. As long as the bias on the field plate is less positive than the bias on the collector contact 310, the effect will be to increase the breakdown voltage of base/collector PN junction 300.
FIGS 1A-1F and FIGS. 3A-3B depict only the base/collector junction of an NPN bipolar transistor. However, if trench isolation is used to isolate individual transistors in an integrated circuit, the avalanche breakdown determining the highest collector voltage may occur initially not at the base/collector PN junction, but proximate to the trenches which isolate the individual transistors.
FIG. 4 illustrates a plan view of a high voltage trench isolated NPN bipolar transistor 400. High voltage trench isolated transistor 400 includes a heavily doped n type emitter region 402 having an emitter contact 404, formed within p type base region 406 having base contact 408. P type base region 406 is itself formed within n type collector region 410 including n+ collector contact region 411 having collector contact 412. High voltage NPN transistor 400 is electrically isolated from surrounding silicon 421 by the trench isolation structure 422.
If the surrounding silicon region 421 is at an electrical potential lower than the potential applied to collector contact 412, the potential difference between surrounding silicon 421 and collector contact 412 will be dropped partly across trench isolation structure 422, and partly across space-charge regions formed in the silicon on either side of the trench (depleted or accumulated silicon present either in collector region 410 adjacent to the trench, or in surrounding silicon region 421 adjacent to the trench). The electric field will be strongest in the corners 408 a.
The precise mechanism causing premature breakdown at trench corners and subsequent breakdown voltage walkout in trench isolated high voltage NPN transistor 400 can be explained by reference to FIGS. 5A-5D and 6A-6E.
FIGS. 5A-5D show cross-sectional views of a portion of a generic trench isolated device that includes a border region 500 between an n type doped area 510, a trench isolation structure 522, and surrounding silicon 521.
FIG. 5A shows border region 500 with no potential difference applied between n doped area 510 or surrounding silicon 521. FIG. 5B shows the application of a positive voltage to n doped area 510 while surrounding silicon 521 remains at ground potential. Under these conditions, a depletion region 530 is formed in n type region 510 proximate to insulating trench 522. The potential difference applied between the n-type region 510 and surrounding silicon region 521 is dropped partly across the trench isolation structure 522, and partly across the depletion region 530, which has a positive space charge because it is depleted of free electrons. Electron/hole pairs 532 b-532 a are thermally generated in depletion region 530.
FIG. 5C shows that under the influence of the electric field in depletion region 530, thermally generated holes 532 a are swept towards the grounded surrounding silicon 521, while thermally generated electrons 532 b are swept towards contact 512, which is at a positive potential. Blocked by the insulating trench 522, the thermally generated holes 532 a gather in an inversion layer 534 adjacent to insulating trench 522. As more holes gather in inversion layer 534, more of the applied voltage between contact 512 and surrounding silicon region 521 is dropped across insulating trench 522, and correspondingly less of the applied voltage is dropped across depletion region 530. The width of depletion region 530 decreases with the increased number of holes in inversion layer 534.
FIG. 5D shows that eventually, when a steady state has been reached, enough holes gather in inversion layer 534 so that essentially all of the applied voltage difference between contact 512 and surrounding silicon 521 drops across trench isolation structure 522, and depletion layer 530 is very narrow. The time required after application of a potential difference between contact 512 and surrounding region 521 to reach a steady state depends on the thermal generation rate of electron/hole pairs in depletion layer 530. If the initial width of depletion layer 530 is large enough to cause a field higher than the critical field for avalanche breakdown, the initial current transient will be fast. However, after the peak field in depletion layer 530 drops below the critical field for avalanche breakdown, the current created by generated electron/hole pairs is low, meaning that the transient current which builds up inversion layer 534 is slow. After this transient, no steady-state current flows.
However, such is not necessarily the case for a trench isolated high voltage NPN bipolar transistor. FIGS. 6A-6E show cross-sectional views of a portion of a trench isolated NPN bipolar transistor device that includes a border region 600 between an n type doped area 610 having a p type base 602, a trench isolation structure 622, and surrounding silicon 621.
FIG. 6A shows a cross-section of part of the transistor 600, with a p type base 604 contacted by a base contact 602. In FIG. 6A no potential is applied to the base contact 604, the collector contact 612, or the surrounding region 621. FIG. 6B shows a positive voltage applied to collector contact 612, surrounding region 621 at ground potential, and base contact 604 at a potential very close to ground. Two depletion layers are formed within the n type collector region 610: a depletion layer 630 adjacent to the sidewall of trench isolation structure 622, and a depletion region 631 adjacent to the p type base 602.
FIG. 6C shows that if there is a positive potential difference between collector contact 612 and base contact 604, and between collector contact 612 and surrounding region 621, electron/hole pairs 632 b/a and 633 b/a are thermally generated in the depletion regions 630 and 631. In depletion region 630 adjacent to insulating trench 622, generated electrons 632 b are swept towards the collector contact 612 and generated holes are swept toward the surrounding region 621. In depletion region 631, generated electrons are swept toward the collector contact 612 and generated holes are swept towards the base region 602. If, for any given voltage on the collector contact 612, the peak electric field in depletion region 630 is higher than the peak electric field in depletion region 631, then avalanche breakdown will occur in region 630 before it occurs in region 631.
FIG. 6D illustrates the undesirable situation in which avalanche breakdown occurs in depletion region 630 adjacent to insulating trench 622, before it occurs in depletion region 631 adjacent to base region 602. Although it is not shown on FIG. 6D, on sides of the transistor region which do not have a heavily doped collector contact region adjacent to them, depletion regions 630 and 631 actually merge. Holes 632 a which are swept to the insulating trench 622 build up into an inversion layer 634 because they cannot pass through the insulating trench 622. This inversion layer is distributed over all four trench sidewalls. On sidewalls close to base 602, inversion layer 634 cannot build up to the extent that it did in FIG. 5D. This is because holes are continually swept into base 602 since depletion region 631 is continuous with depletion layer 630 where no heavily doped collector contact region is interposed between base 602 trench isolation structure 622. In this situation there is a large continuous avalanche current between collector 612 and base 604, because an inversion layer 634 cannot build up to the extent necessary to drop most of the potential difference between collector 612 and surrounding region 621 across trench 622, rather than across depletion layer 630. The electric field in depletion region 630 remains high enough to cause continuing avalanche breakdown.
In fact, the situation of FIG. 6D will not continue indefinitely. As shown in FIG. 6E, some avalanche generated holes 632 a will have enough energy to become embedded in the oxide of insulating trench 622. This fixed positive charge 636 will increase the electric field in insulating trench 622 and decrease the electric field in depletion region 630. The effect of this is a slow increase in breakdown voltage, which appears as a “walkout”, as shown in FIG. 2.
Given that the avalanche current described in FIGS. 6A-6E lasts until the breakdown voltage walks out to the applied collector voltage, a process that can take several seconds or minutes, the premature breakdown described in FIGS. 6A-6E can be a serious problem in high voltage trench isolated NPN bipolar transistors.
The above-described mechanism of walkout in high voltage trench isolated NPN bipolar transistors is confirmed by experimental evidence. One by-product of avalanche breakdown is the emission of infrared light. During breakdown of high voltage trench isolated NPN bipolar transistors as described in FIG. 4 and FIGS. 6A-6E, particularly high infrared intensities have been observed in the corners at the interface with the trench sidewalls.
Therefore, there is a need in the art for methods and structures for eliminating breakdown voltage walkout in high voltage trench isolated semiconductor devices.
SUMMARY OF THE INVENTION
The present invention relates to preventing walkout in trench isolated semiconductor devices by biasing silicon adjacent to the trench isolation structure of the device. As a result of the direct application of voltage to adjacent silicon, premature breakdown at trench corners is prevented.
Voltage applied to the surrounding epitaxial silicon elevates the initial breakdown voltage of the device and eliminates walkout by reducing the strength of the electric field between the silicon of the device and the surrounding silicon. Specifically, application of a positive voltage to surrounding silicon that is equal to or more positive than the most positive potential occurring at the collector during normal operation of a high voltage trench isolated NPN bipolar transistor, ensures that no walkout will occur in the device.
An isolation structure for a semiconductor device in accordance with one embodiment of the present invention includes a silicon material having a surface, a semiconductor device formed within the silicon material, and a substantially vertical trench isolation structure circumscribing the semiconductor device and extending from the surface of the silicon material to a depth into the silicon material. An electrical contact is positioned in the silicon material surrounding the device at a location outside of the trench isolation structure. The contact for the surrounding silicon is disposed so as to permit the application of a bias voltage to semiconductor material outside of the trench isolation structure. This voltage bias signal is of sufficient magnitude to maintain an electric field across the trench isolation structure below the critical field strength for avalanche breakdown.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.