Bipolar transistor and method for producing same

Details Bipolar transistor and method for producing same Bipolar transistor and method for producing same

2001-09-17

2004-05-25

Nguyen, Tuan H. (Department: 2813)

Semiconductor device manufacturing: process

Forming bipolar transistor by formation or alteration of...

C438S312000, C438S368000, C438S350000, C257S197000, C257S200000

Reexamination Certificate

active

06740560

ABSTRACT:

The invention relates to a bi-polar transistor and a procedure, for its manufacture.
BACKGROUND OF THE ART
The implementation of epitaxially manufactured silicon-germanium hetero-bi-polar transistors (SiGe HBT) and the resulting cost-reducing simplification of the technological processes have lately provided a new impetus for a further development of Si bi-polar transistors. In this respect, the combination of an epitaxially produced base with the process-simplifying possibilities of the single polysilicon technology offers an attractive direction of development.
In comparison with conventional base profiles produced by implantation or diffusion, silicon-germanium base layers made by epitaxy allow producing, simultaneously, smaller base widths and base layer resistance without unusable small current gains or high leakage currents. The technology allows implementation of a concentration of the active doping agent of up to 1×10
20
cm
−3
, as is described—for example—in A. Schüppen, A. Gruhle, U. Erben, H. Kibbel und U. König: 90
GHz fmax SiGe
-
HBTs, DRC
94, page IIA-2, 1994. However, in order to prevent leakage currents due to tunnel processes, a low-doped region is required between the high-concentration zones of the emitter and the base. As a matter of fact, if the base doping exceeds the value of 5×10
18
cm
−3
, and if the high concentration of the emitter reaches down to the base—as is usual with implanted base profiles—the consequence is the existence of unacceptably high tunnel currents. As opposed to implanted base profiles, the application of epitaxy allows, simultaneously and without any problems, the production of narrow base profiles and a low-doped region (cap layer).
FIG. 1
illustrates the emitter zone of a SiGe HBT. This transistor design reflects typical characteristics of a single poly-silicon process. An SiGe base
12
and subsequently a cap layer
13
were deposited over a monocrystal collector zone
11
.
FIG. 1
does not show a lateral insulation of the transistor zone. If semiconductor material grows both on the monocrystal substrate
11
and on the insulator zone—not shown in the picture—(i.e., differential epitaxy), it is possible to utilize the grown semiconductor layers as a connection between a contact on the insulation zone and the inner transistor. Such a connection should be, designed with as low impedance as possible. This is why it would be advantageous if the epitaxial layer thickness could be set up independently from the base width. A poly-silicon or an &agr;-silicon layer
15
is deposited on an insulation layer
14
, in which emitter windows were etched by means of a wet-chemical etching process. During the deposition or subsequently, the &agr;-silicon layer
15
obtains—by implantation—a doping of the emitter's conductivity type and serves as diffusion source for the emitter doping
16
in the monocrystal substrate. Insulator layer
14
is applied in order to prevent damage to cap layer
13
during the structuring of the polycrystal &agr;-silicon layer
15
performed later. In the overlapping region
17
—a zone between the edge of the emitter window and the outer delimitation of the structured poly-silicon or &agr;-silicon layer
15
, a layer, sequence arises consisting of semiconductor material, insulator material and semiconductor material. Depending on the doping of the cap layer
13
, the interfacial charges and the recombination properties of the surface as well as on the operation conditions of the transistor, this design can cause—analogous to a MOS capacity—an enhancement but also a depletion of mobile charge carriers on the surface of the cap layer
13
. With a forward-current base-emitter diode, this can affect both the ideal nature of the base current and the low-frequency noise properties. Under certain circumstances, generation currents and breakdown voltage in the non-conducting direction can be affected. The condition that—due to the tunnel (currents) danger—the doping agent concentration should not exceed the value of 5×10
18
cm
−3
leads to the question, by means of which procedure this zone should be suitably doped. The following text discusses the variants for SiGe HBT so far known: homogeneous n-doping or p-doping near the tunnel limit or quasi undoped zones (i-zones). A. Chantre, M. Marty, J. L. Regolini, M. Mouis, J. de Pontcharra, D. Dutrtre, C. Morin, D. Gloria, S. Jouan, R. Pantel, M. Laurens and A. Monroy:
A high performance low complexity SiGe HBT for BiCMOS integration
, BCTM '98, 1998, pages 93-96 uses a p-doping of about 5×10
18
cm
−3
. This results in a decisive disadvantage in that the thickness of, the cap layer must be set up within a tolerance range of a few nanometers from the penetration depth of the doping agent diffusing from the poly-silicon emitter layer. Greater cap layer thickness values (which would be advantageous for a low-impedance connection between the inner base and a connector in the insulation zone) are not possible since it would negatively affect the effect of the germanium profile. A. Gruhle, C. Mähner:
Low l/f noise SiGe HBTs with application to low phase noise microwave oscillators, Electronics Letters
, Vol. 33, No. 24, 1997, pages 2050-2052 uses a cap layer 100 nm thick with an n-concentration of 1-2×10
18
cm
−3
. EP-A-0 795 899 indicates similar conditions, where preferably a cap layer of a thickness of 70 nm with a n-doping concentration of 2×10
18
cm
−3
is used. Although this variant eliminates the problem of the thickness tolerance, and avoids the danger of tunnel currents by reducing the doping agent concentration in the cap layer, it still does not take full advantage of the possibilities of reducing the base-emitter capacity.
This disadvantage can be eliminated by not doping the cap layer as is described, for example, in B. Heinemann, F. Herzel and U. Zillmann:
Influence of low doped emitter and collector regions on high
-
frequency performance SiGe
-
base HBTs, Solid
-
St. Electron,
1995, Volume 38(6), pages 1183-1189. However, it can easily lead to a depletion of the aforementioned overlapping region
17
. These connections are explained in further text by means of a two-dimension design element simulation.
FIG. 2
shows the simplified transistor design used in the simulation. The electrical effect of the oxide semiconductor surface in the overlapping region is modeled by means of a positive surface charge density of 1×10
11
cm
−2
and a surface recombination speed of 1000 cm/s.
FIG. 3
illustrates vertical profiles along a section line horizontal to the overlapping region. The profiles show three doping variants in the cap layer
13
and a p-doped SiGe base
12
identical for all three cases. The following cap doping cases are compared: a quasi undoped cap layer
13
(profile i) and two homogeneous n-dopings (profile n
1
with 1×10
18
cm
−3
and profile n
2
with 2×10
17
cm
−3
).
FIG. 4
shows the transition frequency as a function of the collector current for various doping variants. Especially with small collector currents, an increase in transition frequency with a falling doping level in the cap layer
13
can be noticed. While profile i provides relatively best transition frequencies, it has, however, the disadvantage that the ideal nature of the base current (
FIG. 5
) is noticeably affected in comparison with the other profiles.
SUMMARY OF THE INVENTION
The task of this invention is to indicate a bi-polar transistor and a procedure for its manufacture that eliminates the described disadvantages of conventional arrangements, in order to achieve especially minimal base-emitter capacities and best high-frequency properties without noticeably affecting the static properties of the bi-polar transistor with a low-doped cap layer—above all the ideal nature of the base current and low-frequency noise—and without increasing the process complexity.
This invention fulfills this task by introducing a special doping profile into an epitaxially

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