Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
2000-04-13
2002-11-19
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C702S099000
Reexamination Certificate
active
06484117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to integrated circuits and in particular to a temperature control system for an integrated circuit.
2. Description of Related Art
In many applications it is important for transistors forming logic circuit to switch with constant, predictable switching speeds. However, the switching speed of a transistor, particularly a metal oxide semiconductor (MOS) transistor, is highly temperature sensitive; as a transistor warms up, it reduces the speed with which it turns on an off. A transistor's temperature is influenced, for example, by its ambient temperature, the size, number and proximity of heat sources and sinks in its immediate vicinity, and by the heat transfer efficiency of its surrounding media. Since a transistor itself is a variable heat source, generating substantial heat when it is on and very little heat when it is off, the manner at which the transistor is operated can also influence its temperature. As the duty cycle of a transistor increases, it tends to generate more heat, warm itself up, and therefore slow down. This effect is tempered to some extent in complementary metal oxide semiconductor (CMOS) integrated circuits where n-channel (nMOS) and p-channel (pMOS) transistors forming logic gates are paired and arranged such that the nMOS and pMOS transistors of each pair have opposite switching states. When one transistor of a pair is on, the other transistor of the pair is off. Thus the amount of heat the pair generates tends to be independent of the switching states of the transistors, provided the transistors are well matched.
Nonetheless, the temperature of a CMOS circuit can fluctuate not only due to variations in its ambient temperature but also due to changes in its frequency of operation. When a CMOS transistor pair changes state, the pMOS transistor turns on (or off) at the same time the nMOS transistor turns on (or off). But the two transistors do not turn on or off instantly; there is a period of time during a state change when both transistors are in their active regions, between fully off and fully on. During that time both transistors generate heat, and the total amount of heat they collectively generate per unit time is greater during state changes than between them. CMOS logic gates change state when their input signals change state. Thus as the frequency of a CMOS logic circuit input signal increases, the rate at which the logic gates forming that circuit change state also increases, and so too does the heat the logic gates generate. That is why CMOS circuits get hotter when they operate at higher frequencies. Therefore, in applications requiring a high degree of switching speed stability, it is helpful to control the temperature of an integrated circuit (IC), particularly when a circuit's inputs change state at varying frequencies.
FIG. 1
illustrates a well-known feedback system for controlling the temperature of a CMOS or other kind of IC
10
so as to stabilize the switching speed of its transistors. A sensor
12
monitors the temperature of IC
10
and generates an indicating signal (IND) having a parameter indicative of the IC's temperature. Sensor
12
may be, for example a thermistor in contact with IC
10
. A control circuit
14
compares the IND signal to a reference signal (REF) and supplies an output control signal (CONT) to an external heater
16
near IC
10
. When the magnitude of the IND signal exceeds the magnitude of the REF signal, indicating for example that IC
10
is hotter than desired, control circuit
14
signals heater
16
to reduce the rate at which it generates heat, thereby allowing IC
10
to cool down. Conversely when the magnitude of the IND signal falls below the magnitude of the REF signal, indicating that IC
10
is cooler than desired, control circuit
14
signals heater
16
to increase the rate at which it generates heat, thereby warming IC
10
. The magnitude of REF therefore controls the temperature of IC
10
.
One limitation on the ability of the feedback system of
FIG. 1
to closely control IC temperature is that it takes time for a change in temperature of IC
10
to be detected by sensor
12
, and additional time for a change in heat generated by the external heater
16
to influence the temperature of IC
10
. This feedback delay limits the accuracy with which the feedback system can control the temperature of IC
10
when the rate at IC
10
generates internal heat changes rapidly, as when there is an abrupt change in input signal frequency. The temperature of circuits in integrated circuit testers and other applications that change abruptly from inactive to full speed operation can undergo a rapid swing before the feedback system of
FIG. 1
has time to compensate for the changed operating conditions.
FIG. 2
illustrates an improved prior art temperature control system for an IC
18
that is topologically similar to that of
FIG. 1
except that a sensor
20
, control circuit
22
and heater
24
are implemented within IC
18
itself, with sensor
20
and heater
24
positioned as close as possible to the logic circuits
26
being temperature controlled. Sensor
20
monitors the temperature of logic circuit
26
and generates an indicating signal (IND) having a parameter indicative of the IC's temperature. For example sensor
20
may be a diode and the IND signal may be the diode's threshold voltage, a parameter that is highly sensitive to temperature. Or, as another example, sensor
12
may be a ring oscillator formed by gates implemented on IC
18
having a temperature sensitive frequency. Heater
24
can be implemented as a set of transistors that are turned on and off by the control signals CONT. This design reduces the feedback lag between temperature changes in logic circuit
26
and compensating changes the heat it receives from heater
24
. However while sensor
20
may directly or indirectly sense the temperature of a particular part of IC
18
, the sensed temperature may not be representative of the temperature or switching speed of all transistors on IC
18
. The transistors forming logic circuit
26
are distributed in space and some of those transistors may be nearer sensor
20
or heater
24
than others. Also at various times the IC input signals may increase the frequency of operation of some portions of logic circuit
26
while decreasing the frequency of others. Thus the rate of internal heat generation within logic circuit
26
can vary from area-to-area of IC
18
. When sensor
20
senses one area of IC
18
growing colder, controller
22
tells heater
24
to generate more heat, even though some parts of logic circuit
26
may already be growing warmer.
FIG. 3
illustrates a prior art temperature compensation system wherein the logic of an IC
30
is organized into a set of logic blocks
32
, with each logic block
32
being provided with its own sensor
34
, controller
36
and heater
38
. Since this system localizes sensing and heating, the various logic blocks
32
are better controlled than in the centralized systems of
FIGS. 1 and 2
. While the system of
FIG. 3
can be generally quite effective in controlling temperature of an IC subject to a slowly changing thermal environment, its ability to control the temperature of the transistors forming a given logic block
32
is still limited to some extent by lags in the feedback loop provided by sensor
34
, controller
36
and heater
38
. This feedback system is particularly vulnerable when a sudden change in the input signal frequency of a logic block
32
results in a sudden change in the amount of heat the logic block generates. Such an event can cause the transistor switching speed to briefly go out of its acceptable range before the feedback system has had time to detect the temperature change and to adjust the heat flow into the logic block.
What is needed is an improved temperature compensation system that quickly responds to changes in input signal frequency.
SUMMARY OF THE INVENTION
In accordance with o
Bedell Daniel J.
Credence Systems Corporation
Hilten John S.
Smith-Hill and Bedell
Washburn Douglas N
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