Electrical resistors – Resistance value temperature-compensated
Reexamination Certificate
1998-12-08
2001-04-03
Gellner, Michael L. (Department: 2832)
Electrical resistors
Resistance value temperature-compensated
C338S009000, C338S309000
Reexamination Certificate
active
06211769
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system for fabricating passive resistors in integrated circuits that display minimal change in resistance over a wide range of operating temperatures.
2. Brief Description of the Prior Art
Integrated circuits are generally fabricated with polysilicon resistors that are formed on the semiconductor substrate. Such resistors generally have a resistor body, generally formed of doped polysilicon and generally having metallic leads coupled to opposing ends of the resistor body, generally through contacts in field oxide. The contacts are connected to metal interconnect. The resistor body can be formed concurrently with polysilicon transistor gate electrodes, such resistor body generally doped and generally resting on the field oxide.
Integrated circuits that require passive resistors often have tight tolerances on the resistance value of these resistors. However, these prior art semiconductor resistors are subject to variations in resistance value. Sources of variation in the resistance value of these resistors include process fluctuations that result in physical changes to the resistor properties such as physical dimension or resistivity and changes in temperature. The sources of change in temperature can be either external to the resistor itself or internal due to the self-heating effects associated with power dissipation. As the resistor temperature changes, the value of resistance of the resistor also changes.
The general prior art method utilized for minimizing the resistance alteration effects due to the temperature coefficient of resistance (TCR) of a semiconducting resistor (a resistor formed of semiconductor material) is to increase the doping concentration in the resistor body to a sufficiently high level such that the TCR of the resistor body is at a minimum. Then the resistors are built with dimensions that make the resistor head resistance a small percentage of the resistor body resistance. As a result, the resistor head TCR has little effect on the overall resistor temperature characteristics.
To reach sufficiently low TCR conditions in the resistor body alone, the impurity or doping concentration must be very high, about 3×10
20
atoms/cm
3
for polysilicon resistors. Extra processing steps are often required to reach this level of impurity concentration. These steps add cost to the production of the circuit and limit the range of resistance values obtainable in that the sheet resistance (R
s
) is low (about 70 to about 100 ohms/square). The simple expression for resistance is R=R
s
(L/W), where L is length and W is width of the resistor body. This equation shows that to achieve the desired value of R when R
s
is low, W must be minimized (which increases the head component of resistance and increases process variability) and/or L must be increased, the latter increasing capacitance and area consumed on the chip. The increase in length is also detrimental at high frequency where these resistors are sometimes used.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described inadequacies of prior art resistors are minimized.
The change in resistance with temperature of a semiconducting material can be modeled by a numerical fit that takes the form:
R(T)/R(T
0
)=1+TCR1×(T−T
0
)+TCR2×(T−T
0
)
2
where the resistance at temperature T is R(T), T
0
is the initial or reference temperature and TCR
1
and TCR
2
are the fitting coefficients for the resistor body and the two resistor heads which make connection at opposite ends of the resistor body respectively. The above equation applies to the body or the head separately. In other words, the above equation can be used for either the body or for the head of the resistor individually. Then the coefficients TCR
1
, TCR
2
which are the numerical fitting coefficients are found by fitting the equation to the data. It can be thought of as fitting the expression y=1+ax+bx
2
to some data where a and b are the fitting coefficients. For every case (level of doping concentration) that has been observed for polysilicon resistors, the second fitting coefficient TCR
2
is several orders of magnitude lower than TCR
1
, so TCR
2
has almost no effect on the equation over the temperature range of interest, this being from about −55° C. to about 140° C. This equation represents the best fit to the resistance data taken over the above range of temperatures. The temperature coefficients of resistance TCR
1
and TCR
2
for the resistor body can be either negative for low to mid levels of doping concentration or positive for very high levels of doping concentration. The same statement applies to the head TCR
1
and TCR
2
, but the head TCR
1
and TCR
2
generally do not change from negative to positive at the same doping concentration as the body.
When a semiconducting material is used for the resistor body, electrical contacts are made to the resistor body in a region known as the resistor head. Typically, the electrical path to the resistor body is made through metal leads and contacts and possibly a metal or metallic compound in contact with the resistor body. For the present discussion, the combination of all of these components is considered to be the resistor head. The total resistance of the resistor structure can be written as R=R
b
+2×R
h
, where R
h
is the resistance of the resistor body and R
b
is the resistance of each resistor head. Both R
h
and R
b
will have temperature characteristics as described by the above equation in R(T)/R(T
0
). The temperature coefficients of the resistor heads, which include the interface resistance (or contact resistance) between the resistor head material and the resistor body, can be different from the temperature coefficients of resistance of the resistor body. Metals and metallic compounds typically have positive coefficients or resistance. The head-to-resistor body interfaces may have either positive or negative temperature coefficients of resistance, depending upon the doping concentration in the body.
When the resistor body is built with an overall negative coefficient of resistance, the above equation in R(T)/R(T
0
) when applied to R
b
becomes less than 1 for temperatures greater than T
0
and the value of R
b
decreases with increasing temperature. When the resistor head is built with an overall positive coefficient of resistance, the value of R
h
increases with increasing temperature or the R(T)/R(T
0
) equation becomes greater than 1 when applied to R
h
. With proper design of the physical dimensions of the resistor heads and resistor body, the magnitude of the increase in resistance of the resistor heads can offset the magnitude of the decrease in resistance of the resistor body, resulting in very low overall change in resistance for the entire resistor structure.
The above described method of reducing changes in resistance due to temperature allows precision passive resistor structures to be built with materials that are already available in the process flow. No extra processing steps are generally required unless the polysilicon resistor is part of a self-aligned silicide process flow in which case one additional process step is required. The silicide must be blocked from the resistor body, usually by a patterned oxide or nitride. Extra processing can be provided to change the resistivity of the resistor body or resistor heads, however the above solution provides the desired result, whether or not extra processing is utilized.
The procedure in accordance with the present invention generally does not require additional processing steps as required in the prior art to obtain the high doping levels since lower impurity concentrations in the semiconductor which do not require the additional processing are generally adequate. In addition, the method in accordance with the present invention can take advantage of the positive TCR associated with metal leads and contacts that are commonly used
Baldwin Greg C.
Tsao Alwin J.
Brady III Wade James
Gellner Michael L.
Lee Kyung S.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
LandOfFree
System to minimize the temperature coefficient of resistance... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System to minimize the temperature coefficient of resistance..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System to minimize the temperature coefficient of resistance... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2465095