Semiconductor device manufacturing: process – Voltage variable capacitance device manufacture
Reexamination Certificate
2000-03-29
2003-05-06
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Voltage variable capacitance device manufacture
C438S397000, C438S048000, C257S528000, C257S052000
Reexamination Certificate
active
06559024
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to varactor diodes and, more particularly, to a method of forming a varactor diode having a hyperabrupt junction profile.
2. Description of the Related Art
In a semiconductor, charge carriers (electrons or holes) diffuse from a high carrier-density region to a low carrier-density region. For this reason, charge carriers diffuse across the junction of an unbiased semiconductor diode to create a depletion region of ionized atoms, i.e., atoms which have lost their mobile carriers. Once this built-in potential V
bi
has been established by the initial diffusion, it acts as a barrier to further diffusion.
If a reverse bias is imposed across the diode, the depletion region widens to expose a region of negative charges (due to acceptor atoms) on one side of the junction and a region of positive charges (due to donor atoms) on the other. The width of the depletion region is a function of the impurity doping levels of the diode junction. For example, if both sides of the junction are equally doped, the depletion region will extend an equal distance from the junction. With unequal doping levels, the depletion region will extend farther into the side which has the smaller impurity concentration.
The electric field is found by integrating the negative and positive charges. In contrast, the potential drop across the junction is found by a second charge integration or, equivalently, an integration of the electric field. If the doping concentration is constant, the electric field in the depletion region peaks at the junction and decreases linearly to the edges of the depletion region. In this case, the potential drop across the depletion region has a quadratic form.
If the reverse bias is increased to a breakdown voltage V
BR
, a large reverse current results because the electric field at the junction exceeds the dielectric strength of the diode's semiconductor material. Covalent atomic bonds are ruptured, a large number of minority carriers are released and the diode is said to avalanche. The electric field and potential drop of depletion regions has been discussed by many authors (e.g., Singh, Jasprit., Semiconductor Devices, McGraw-Hill, Inc., New York, 1994, pp. 192-208). In contrast with diodes that are purposely intended to operate in breakdown, varactor diodes are generally configured to avoid breakdown over an operational reverse-bias range.
In a varactor diode, each side of the diode junction is conductive and the depletion region acts as a dielectric so that a reverse-biased semiconductor junction has the structure of a capacitor, i.e., two conducting regions separated by a dielectric. The capacitance depends directly on the junction area and inversely on the width of the depletion region, i.e., C=(∈A)/d in which c is the dielectric constant, A is the junction's cross-sectional area and d is the width of the depletion region. The diode capacitance decreases with increased reverse bias because this change in bias causes the depletion width to increase. The capacitance ratio over a specified reverse bias range is generally referred to as the tuning ratio.
Varactors find utility in a variety of electronic circuits. For example, a varactor diode in a resonant circuit can control the frequency of a voltage-controlled oscillator (VCO) or the amplifier frequency in a receiver. Typically, VCOs and receiver amplifiers are tuned smoothly across their operating bands. Varactor diodes for these applications usually exhibit a junction capacitance that is proportional to an exponential power of the reverse-bias voltage V
r
, e.g., C varies as (1/V
r
)
½
. Abrupt-junction varactor diodes have uniform doping on each side of the junction with an abrupt transition at the junction. It will be appreciated from the aforementioned inverse square root relationship, as the applied voltage is increased by a factor of 4, the junction capacitance in an abrupt junction varactor diode will decrease by a factor of 2:1. Another way of expressing this relationship is that over the range of 1 to 4 volts, the capacitance-tuning ratio of an abrupt junction varactor diode is 2:1.
As more and more applications have been identified which require either faster tuning/frequency hopping speeds or low voltage operation, recent emphasis has been placed on the finding ways to increase the tuning ratios of varactor diodes so as to enable larger swings of capacitance or reactance with the same or reduced applied voltages. Such emphasis has culminated in the development of so-called hyperabrupt junction varactor diodes. In a conventional hyperabrupt-junction diode, the doping level increases as the junction is approached from either side, yielding an exponential relationship in which C varies as (1/(V
r
+h)
k
, in which V
r
is the biasing voltage, h is the height of the Schottky barrier, the parameter k defines the doping variation as a function of the distance with respect to the surface. Thus, as opposed to the abrupt junction case described above where the exponent k is exactly equal to one-half, a hyperabrupt profile is obtained when the exponent k is greater than ½.
In
FIG. 1A
, there is shown one type of conventional hyperabrupt junction varactor diode generally indicated at
10
and comprising a cathode defined by an epitaxial layer
14
of semiconductor material (e.g., Si or GaAs) doped N type on an N+ substrate
12
. A variably doped hyperabrupt region
16
is defined beneath an anode region
18
, the latter being provided with an ohmic contact
20
of, for example, PtSi, to an overlying anode metalization layer
22
.
While the conventional structure depicted in
FIG. 1A
is capable of producing hyperabrupt varactor diodes having capacitance-voltage tuning ratios of up to 12:1, these structures have demonstrated a high degree of variability on both a unit-to-unit and a lot-to-lot basis—even from the same vendor. Moreover, it has proved difficult to move past a tuning factor of 12:1. The capacitance-voltage response of a conventional hyperabrupt varactor structure, as exemplified by
FIG. 1A
, is depicted in FIG.
1
B. For purposes of comparison, the capacitance voltage response of a conventional varactor is also shown in FIG.
1
B.
The inventors herein are the first to appreciate that the aforementioned limitations in repeatability and tuning ratio are directly attributable to the processes which have heretofore been employed in the fabrication of hyperabrupt structures. Returning briefly to the conventional structure shown in
FIG. 1A
, it will be recalled that the variably doped hyperabrupt cathode region
16
directly underlies the anode region
18
. In the illustrative structure of
FIG. 1
, the hyperabrupt region
16
is N-type while the anode is P-type. High energy ion implantation of phosphorous, either singly or multiply charged, is the generally accepted mode of creating hyperabrupt region
16
in the N-type cathode layer
14
. By way of illustrative example, this ion implantation may be accomplished either through the already existing P+ anode region
18
or, alternatively, directly into the cathode layer
14
with a subsequent P+ implant or diffusion to form the anode structure. In either case, the implantation of hyperabrupt region
16
must generally be performed at very high energy levels—either on the order of 150 keV to 350 keV in conjunction with multiply charged phosphorous ions (P
++
) to result in an effective singly charged level of 300 keV to 700 keV, or substantially higher implantation energy levels using a MeV implanter with single charged phosphorous ions. The implant(s), either the hyperabrupt cathode implant region
16
alone or in conjunction with the anode implant region
18
, are then thermally activated via a furnace or a Rapid Thermal Annealing (RTA) heat cycle. The resulting hyperabrupt doping profile typically realized in the exemplary structure of
FIG. 1A
is depicted in FIG.
1
C.
A principal disadvantage associated with
Boles Timothy Edward
Goodrich Joel Lee
Lally Thomas Robert
Loring, Jr. James Garfield
Jr. Carl Whitehead
Schillinger Laura M
Tyco Electronics Corporation
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