Voltage controlled slow wave transmission line

A slow wave transmission line is compatible for fabrication on a substrate having an epitaxial layer by creating under the transmission line a buried layer, thus allowing voltage control of the electrical length of the line.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of transmission lines forming 
part of a monolithic integrated circuit chip and, more particularly, to 
such chips having an epitaxial layer formed thereon. 
Transmission lines formed by metallized strips deposited on the passivated 
surface of a substrate are well known, and are used for many purposes 
beyond mere connection of one point on an IC chip with another point. Such 
lines may operate in one of three general modes. When the product of the 
frequency and the resistivity of the substrate is large enough to produce 
a small dielectric loss angle, the substrate acts as a dielectric, and the 
line operates in a mode closely resembling the TEM mode, which may be 
termed the dielectric "quasi-TEM" mode. The dielectric loss in the 
SiO.sub.2 layer can be ignored and almost all of the energy is transmitted 
through the silicon layer at nearly the velocity of light in a vacuum. 
When the product of the frequency and the conductivity (1/R) is large, the 
substrate appears to be a lossy conductor wall. With a very thin SiO.sub.2 
layer, the dispersion effect is controlled by skin effect in the substrate 
and the device operates in a "skin effect" mode. 
However, when the frequency is not as high and the resistivity is in the 
moderate range; e.g., 10.sup.-4 to 10.sup.+2 ohms-cm, the propagation 
velocity may be slowed down to a few hundredths of the velocity of light 
in a vacuum. This mode of operation has been termed the "slow wave" mode. 
A complete description of the theoretical basis of the three modes of 
operation may be found in "Properties of Microstrip Line on Si-SiO.sub.2 
System" published in the IEEE "Transactions on Microwave Theory and 
Techniques", Vol. MTT-19, No. 11, November 1971, pp. 869-881. 
The development of the epitaxial process provided many advantages for 
integrated circuit designs and many monolithic integrated circuits are 
presently made with an epitaxial layer formed on the substrate. To provide 
a slow-wave transmission line has heretofore required a Schottky metal 
which was not compatible with bipolar processes. It is highly desirable to 
be able to provide a slow wave transmission line by using the normal 
process steps in epitaxial integrated circuit fabrication. 
Another slow wave transmission line, filed as of even date with the present 
application, and bearing U.S. Ser. No. 944,059, is also compatible with 
bipolar integrated circuit construction. This other transmission line has 
no buried layer, but has a p+ diffusion beneath the transmission line. It 
is, therefore, not voltage controllable as is the present invention, but 
has better slow wave performance. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a voltage 
controllable transmission line on an integrated circuit having an 
epitaxial layer thereon, for operation in the slow wave mode. 
It is a particular object to provide a slow wave line by using only the 
normal fabrication processes. 
These objects and others are provided in accordance with the invention by 
enhancing the epi-substrate diode capacitance to a value still allowing 
slow wave operation, but diffusing a low resistivity or "buried" lawyer 
(n.sup.+ in an npn structure) between the epitaxial layer and the 
substrate in the area underneath the metallized strip of the transmission 
line. With this type of construction, the line may be made considerably 
shorter than would be the case without the voltage control.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The transmission line of the present invention will be best understood with 
respect to the drawing figures wherein like parts have like reference 
numerals throughout. 
The chart of FIG. 1 displays in general terms the three operational modes 
of a transmission line. The area 10 is the usual mode of operation, the 
quasi-TEM mode and results from a high value of the frequency-resistivity 
product. The area 11 denotes the frequency-resistivity product providing 
skin effect mode operation. It will be noted that both of the above modes 
operate at the higher frequency ranges, e.g. upwards of 100 MHz. The area 
12 is the area of particular interest in the present invention, and is 
denoted the "slow wave" mode domain. As will be seen, the slow wave mode 
is possible at generally lower frequencies than the other two modes and 
with a moderate range of resistivity of the silicon layer. A transition 
region 14 is shown separating the practical areas of operation in the 
three modes. 
In FIG. 2 a preferred embodiment of the invention is shown as part of an 
epitaxial integrated circuit. For purposes of reference an area designated 
by the bracket 16 denotes a standard npn transistor with collector contact 
18, emitter contact 19 and base contact 20 shown extending through etched 
portions in a silicon dioxide or silicon nitride layer 22. As in the usual 
method of construction, a semiconductor substrate 24 has been provided by 
slicing a very thin wafer from a single grown crystal of silicon. An 
n-epitaxial layer 26 has been grown on the upper surface of the substrate 
24. The P+ area 28 adjacent the transistor 16 is present for isolation 
purposes. On the surface of the insulating layer 22, a metallized strip 30 
forms a portion of the transmission line of the invention. As is 
customary, the lower surface of the substrate 24 is also metallized, 
forming a ground plane 32. Under the transmission strip 30, there is 
formed a "buried layer" of n+ material 34 by techniques known in the art. 
Adjacent the cross-section is shown a schematic representation of the 
equivalent circuit of the transmission line area. A capacitor C1 
represents the capacity between the strip 30 and the oxide or nitride 
layer 22. The epitaxial layer is represented by a capacitor C2 and a small 
resistance R1, and the substrate layer 24 is represented by capacitor C3 
and resistance R2. The capacitor C4 is the capacity across the buried 
layer substrate interface. Since it will be seen that coupling a voltage 
source 36 from the epitaxial layer 26 to ground would vary the capacitance 
of C4, there is provided a means for controlling the capacitance by 
external means, and likewise the electrical length of the transmission 
line becomes voltage variable. 
The theoretical basis of the slow wave mode of transmission lies in the 
fact that the velocity of transmission is proportional to 1/.sqroot.lc 
where l is proportional to .mu..sub.o d and c is proportional to 
.epsilon./d, where d is the distance between the two elements, .epsilon. 
is the dielectric constant of the material between the plates, and 
.mu..sub.o is a function of the material of the elements. Let d.sub.1 
represent the thickness of the insulating layer, such as SiO.sub.2 and D 
represent the effective distance between the conductive elements of the 
transmission line. Then, if the resistance R of the substrate approaches 
zero, D approaches d.sub.1, the inductance of the line decreases, and the 
velocity of propagation is near the velocity of light in a vacuum. This is 
the so-called "skin effect" mode. 
If R of the substrate approaches infinity, D approaches the actual d, and 
the capacitance of the line decreases. This is the quasi-TEM mode. 
If R of the substrate is not as large, D approaches d for l, but d.sub.1 
for c. The inductance remains approximately the same, but c increases. The 
velocity then is 1/.sqroot..mu..sub.o .epsilon.d/d.sub.1, which may be as 
low as a few hundredths (e.g. 0.03) the velocity of light in a vacuum. 
Conductors for coupling the transmission line to other circuitry of the 
integrated circuit are not visible in the cross-section of FIG. 2 but 
would be constructed as required by the total circuitry. Many of the steps 
of the diffusion and photolithography processes are omitted in the above 
description since they are well known in the art. It will also be seen 
that FIG. 2 has been drawn for maximum clarity and is not to scale. 
Thus, there has been provided, in accordance with the invention a means, 
compatible with epitaxial processes, for providing a slow wave 
transmission line on an integrated circuit chip, with the capability for 
voltage control of the line length. Other variations and modifications 
will be apparent to those skilled in the art and is intended to cover all 
such as fall within the spirit and scope of the appended claims.