# Patent

## Title
Circuits to reduce threshold voltage tolerance and skew in multi-threshold voltage applications

## Abstract

A circuit and a method for adjusting the performance of an integrated circuit, the circuit includes: first and second sets of FETs having respective first and second threshold voltages, the first threshold voltage different from the second threshold voltage; a first monitor circuit containing at least one FET of the first set of FETs and a second monitor circuit containing at least one FET of the second set of FETs; a compare circuit adapted to generate a compare signal based on a performance measurement of the first monitor circuit and a performance measurement of the second monitor circuit; and a control unit adapted to generate a control signal to a voltage regulator based on the compare signal, the voltage regulator adapted to supply a bias voltage to wells of FETs of the second set of FETs, the value of the bias voltage based on the control signal.

## Background
FIELD OF THE INVENTION
The present invention relates to the field of integrated circuits; more specifically, it relates to methods and circuits to reduce threshold voltage tolerance and skew in integrated circuits utilizing devices having multiple different threshold voltages.
BACKGROUND OF THE INVENTION
In order to reduce power consumption and increase performance, circuits having devices with different threshold voltages have been used in different portions of the integrated circuit. Devices with low threshold voltages are faster, but have greater sub-threshold voltage leakage (consume more power) compared with devices having high threshold voltages but low sub-threshold voltage leakage. Using a mix of high threshold voltage devices on non-performance critical circuit paths and low threshold voltage devices on performance critical circuit paths can result in lower overall power consumption and higher performance than using devices having the same threshold voltages.
However, it is critical that the designed relationship between the different threshold voltage values of different-threshold voltage devices be maintained in the fabricated integrated circuit in order to ease timing closure during design and avoid signal propagation timing issues. Therefore, there is a need for methods and circuits for maintaining the design values and/or relationships between the different threshold voltage values of multiple threshold voltage devices.
SUMMARY OF THE INVENTION
A first aspect of the present invention is a circuit, comprising: a first set of field effect transistors (FETs) having a designed first threshold voltage and a second set of FETs having a designed second threshold voltage, the first threshold voltage different from the second threshold voltage; a first monitor circuit containing at least one FET of the first set of FETs and a second monitor circuit containing at least one FET of the second set of FETs; a compare circuit adapted to generate a compare signal based on a performance measurement of the first monitor circuit and a performance measurement of the second monitor circuit; and a control unit adapted to generate a control signal to a voltage regulator based on the compare signal, the voltage regulator adapted to supply a bias voltage to wells of FETs of the second set of FETs, the value of the bias voltage based on the control signal.
A second aspect of the present invention is the first aspect, wherein the compare circuit includes a first edge counter connected between the first monitor circuit and a first comparator and a second edge counter connected between a reference clock and the first comparator; and further including: an additional compare circuit including a third edge counter connected between the second monitor circuit and a second comparator and a fourth edge counter connected between the reference clock and the second comparator; and a first memory device containing a first performance specification for the first monitor circuit coupled to the first comparator and a second memory device memory containing a second performance specification for the second monitor circuit coupled to the second comparator.
A third aspect of the present invention is a method, comprising: providing a first set of field effect transistors (FETs) having a designed first threshold voltage and a second set of FETs having a designed second threshold voltage, the first threshold voltage different from the second threshold voltage; providing a first monitor circuit containing at least one FET of the first set of FETs and a second monitor circuit containing at least one FET of the second set of FETs; providing a compare circuit adapted to generate a compare signal based on a performance measurement of the first monitor circuit and a performance measurement of the second monitor circuit; and providing a control unit adapted to generate a control signal to a voltage regulator based on the compare signal, the voltage regulator adapted to supply a bias voltage to wells of FETs of the second set of FETs, the value of the bias voltage based on the control signal
A fourth aspect of the present invention is the third aspect wherein the compare circuit includes a first edge counter connected between the first monitor circuit and a first comparator and a second edge counter connected between a reference clock and the first comparator; and further including: providing an additional compare circuit including a third edge counter connected between the second monitor circuit and a second comparator and a fourth edge counter connected between the reference clock and the second comparator; and providing a first p[device containing a first performance specification for the first monitor circuit coupled to the first comparator and a second memory device containing a second performance specification for the second monitor circuit coupled to the second comparator.
A fifth aspect of the present invention is a method, comprising: (a) measuring the performance of a first monitor circuit having at least one field effect transistor (FET) of a first set of FETs, each FET of the first set of FETs having a designed first threshold voltage; (b) measuring the performance of a second monitor circuit having at least one field effect transistor (FET) of a second set of FETs, each FET of the second set of FETs having a designed second threshold voltage, the second threshold voltage different from the first threshold voltage; and (c) applying a bias voltage to wells of the FETs of the second set of FETs based on comparing a measured performance of the first and second monitor circuits to specified performances of the first and second monitor circuits.
A sixth aspect of the present invention is the fifth aspect further including: (d) applying an additional bias voltage to wells of FETs of the first set of FETs based on the comparing the performances of the first and second monitor circuits measured in steps (a) and (b) to specified performances of the first and second monitor circuits.



BRIEF DESCRIPTION OF DRAWINGS
The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
FIG. 1 is an exemplary circuit illustrating the principle for threshold voltage control according to the embodiments of the present invention;
FIG. 2 is a cross-sectional view illustrating the physical structure of the inverter of FIG. 1;
FIG. 3A is a schematic block circuit diagram of a circuit for threshold voltage control according to a first embodiment of the present invention;
FIG. 3B is a schematic block circuit diagram of a circuit for threshold voltage control according to a second embodiment of the present invention
FIG. 4A is a block circuit diagram of an exemplary individual compare circuit of the compare unit of FIG. 3A;
FIG. 4B is a block circuit diagram of an exemplary individual compare circuit of the compare unit of FIG. 3B;
FIG. 5A is a diagram illustrating the control signal generated by the control unit of FIG. 3A according to the first embodiment of the present invention;
FIG. 5B is a diagram illustrating the control signal generated by the control unit of FIG. 3B according to the second embodiment of the present invention;
FIG. 6 is a schematic block diagram of a typical voltage regulator used to adjust well bias according to embodiments of the present invention;
FIG. 7A is a diagram illustrating an exemplary floor plan of an integrated circuit chip according to the embodiments of the present invention;
FIG. 7B is an exemplary cross-sectional diagram of multiple field effect transistors formed in a common well; and
FIG. 8 is a flowchart of the methods of controlling threshold voltages of multiple-threshold voltage devices according to the embodiments of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The performance of a field effect transistor (FET) is defined as the switching speed of the FET and the performance of a circuit is defined as the time delay of a signal propagated from an input to an output of the circuit.
FIG. 1 is an exemplary circuit illustrating the principle for threshold voltage control according to the embodiments of the present invention. In FIG. 1, an inverter 100 includes a P-channel field effect transistor (PFET) P1 and an N-channel field effect transistor (NFET) N1. The source of PFET P1 is connected to VDD, the source of NFET N1 is connected to ground (or VSS), the drains of PFET P1 and NFET N1 are connected to the output of the inverter and the gates of PFET P1 and NFET N1 are connected to the input of the inverter. PFET P1 is fabricated in an N-well and NFET N1 is fabricated in a P-well as is well known in the art. VDD is defined as the most positive voltage level of the power supply and ground (or VSS) is defined as the most negative voltage level of the power supply.
FIG. 2 is a cross-sectional view illustrating the physical structure of the inverter of FIG. 1. PFET P1 includes a gate 105, a source 110, a drain 115, an N-well 120 and an N-well contact 125. NFET N1 includes a gate 130, a source 135, a drain 140, a P-well 145 and a P-well contact 150. The electrical connections described supra in reference to FIG. 1 are illustrated in FIG. 2. The crosshatched regions indicate dielectric material.
The threshold voltages (Vts) of PFET P1 and NFET N1 are determined during fabrication and are a function of the various doping levels of the source/drains, channel region and gate electrode. The Vts of PFET P1 and NFET N1 may be adjusted (after fabrication of the integrated circuit containing PFET P1 and NFET N1 is complete) by applying bias voltages VWBP and VWBN respectively to the N-well of PFET P1 and the P-well of NFET N1. Applying a positive (reverse) bias to the N-well of PFET P1 decreases its Vt (makes its Vt more negative) thus slowing down the PFET, while applying a negative (forward) bias to the N-well of PFET P1 increases its Vt (makes its Vt less negative) thus speeding up the PFET. Applying a positive (forward) bias to the P-well of NFET N1 decreases its Vt (makes its Vt less positive) thus speeding up the NFET, while applying a negative (reverse) bias to the P-well of NFET N1 increases its Vt (makes its Vt more positive) thus slowing down the NFET.
When integrated circuits having multiple Vt FETs are designed, the Vts of FETs are designed to be positive or negative fractions of VDD. For example, a first FET may be designed to have a Vt=VDD/3, a second FET may be designed to have a Vt=VDD/4 and a third FET may be designed to have a Vt=VDD/5. If VDD=1.0 volt, then the Vts are 0.33, 0.25 and 0.20 volts respectively. All other parameters being equal, a FET with a Vt of 0.20 volt will be faster than FETs having Vts of 0.25 volt and 0.33 volt. Further, the switching speeds of the different Vt devices are designed to be ratios of each other However, due to process variations, there is a +/− tolerance on the actual Vt and therefore the switching speed obtained when the FETs are manufactured. This is called Vt tolerance. Further, since each FETs Vt is typically set during independent manufacturing steps, the tolerances of different FET types do not necessarily track. For example, a first Vt FET could be manufactured faster than design while a second Vt FET on the same integrated circuit chip could be manufactured slower than design. This is called Vt skew Table I illustrates this problem.







TABLE I







THRESHOLD VOLTAGES VERSUS SWITCHING SPEED












Threshold
Relative
Switching
Switching



Voltage
Switching
Speed
Speed



in volts
Speed
Tolerance
Range







0.20
0.75
+/−0.15
0.90–0.60



0.25
1.00
+/−0.15
1.15–0.85



0.30
1.25
+/−0.15
1.40–1.10










In table I, the middle speed FET (Vt=0.25 volt) is the reference FET (Switching Speed=1.0). Thus, the actual switching speed of the fastest device (Vt=0.20 volt) can overlap the actual switching speed of the middle speed device (Vt=0.25 volt) and the actual switching speed of the middle speed device (Vt=0.25 volt) can overlap the actual switching speed of the slowest device (Vt=0.30 volt). This can complicate timing closure of the integrated circuit design because all combinations of Vt skew and tolerance must be accounted for, or it could upset the timing of the circuit design if the switching speed ratios are assumed to be the designed values.
In a first embodiment of the present invention, the performance (switching speed) of the different Vt devices is monitored. One Vt device is chosen as a reference and the well bias of the other Vt devices adjusted (which adjusts the Vt which in turn adjusts the switching speed) until the design performance ratio (design switching speed ratio) of the other Vt devices versus the reference Vt device are brought to or near to design values. PFETs and NFETs are monitored and adjusted separately. This removes Vt skew, but does not adjust for Vt tolerance.
In a second embodiment of the present invention the performance (switching speed) of the different Vt devices is monitored. The well bias of all different Vt devices is adjusted (which adjusts the Vt which in turn adjusts the switching speed) of the different Vt devices until the design performance specification (design switching speed specification) of all the Vt devices are brought to or near to design values. PFETs and NFETs are monitored and adjusted separately. This removes Vt skew and adjusts for Vt tolerance.
FIG. 3A is a schematic block circuit diagram of a circuit for threshold voltage control according to a first embodiment of the present invention. In FIG. 3A, a threshold voltage adjustment circuit 155A includes an adjustable voltage regulator unit 160A, a set of PFET monitor circuits P1 through PN, a set of NFET monitor circuits N1 through NM, a compare unit 165A and a control unit 170A. In the example of FIG. 3A, reference monitor P1 monitors a reference PFET having a design Vt of P1 and reference monitor N1 monitors a reference NFET having a design Vt of N1. Adjustable voltage regulator unit 160A includes a separate N-well bias voltage regulator for each of the other different Vt PFETs having respective design Vts of P2 through PN and a different P-well bias voltage regulator for each of the other different Vt NFETs having design Vts of N2 through NM. Compare unit 165A includes a separate compare circuit for each different Vt PFET (Vts P2 through PN) and for each different Vt NFET (Vts N2 through NM) that compares the performance of the monitored FET to the performance of the reference FET. Adjustable voltage regulator unit 160A is coupled to a power supply (ultimately to an external power supply).
Each N-well and P-well bias voltage regulator generates a well bias voltage that is distributed throughout the integrated circuit chip to respective Vt devices as well as to respective monitors P2 through PN. The output of each monitor P2 through PN is connected to a respective compare circuit within compare unit 165A as is the output of reference monitor P1. The output of each monitor N2 through NM is connected to a respective compare circuit within compare unit 165A as is the output of reference monitor N1. The output of compare unit 165A is connected to control unit 170A Control unit 170A generates control signals supplied to respective well bias voltage regulators in adjustable voltage regulator unit 160A.
The number (n) of PFET monitor circuits is the number of different Vt PFETs in the integrated circuit that are to be monitored and there is one monitor for each. The number (m) of NFET monitor circuits is the number of different Vt NFETs in the integrated circuit that are to be monitored and there is one monitor for each. Reference monitor P1 monitors the performance of first (and reference) Vt PFETs having a reference design threshold voltage of P1. Monitor P2 monitors the performance of second Vt PFETs having a design threshold voltage of P2. Monitor circuit PN monitors the performance of nth Vt PFETs having a design threshold voltage of PN. Reference monitor N1 monitors the performance of first (and reference) Vt NFETs having a reference design threshold voltage of N1. Monitor N2 monitors the performance of second Vt NFETs having a design threshold voltage of N2. Monitor NM monitors the performance of mth Vt NFETs having a design threshold voltage of NM.
The value of n need not be the same as the value of m. Not every different Vt NFET or different Vt PFET on the integrated circuit need be monitored or connected to threshold voltage adjustment circuit 155A. In one example, adjustable voltage regulator unit 160A, PFET monitor circuits P1 through PN, NFET monitor circuits N1 through NM, compare unit 165A and control unit 170A are physically located on the integrated circuit chip. In one example, PFET monitor circuits P1 through PN, NFET monitor circuits N1 through NM, compare unit 165A and control unit 170A are physically located on the integrated circuit chip while adjustable voltage regulator unit 160A is physically located off chip.
FIG. 3B is a schematic block circuit diagram of a circuit for threshold voltage control according to a second embodiment of the present invention. FIG. 3B is similar to FIG. 3A except as noted. In FIG. 3B, a threshold voltage adjustment circuit 155B includes an adjustable voltage regulator unit 160B, a set of PFET monitor circuits P1 through PN, a set of NFET monitor circuits N1 through NM, a compare unit 165B and a control unit 170B. Adjustable voltage regulator unit 160B includes a separate N-well bias voltage regulator for each of the different Vt PFETs (being monitored) and a separate P-well bias voltage regulator for each of the different Vt NFETs (being monitored).
Each voltage regulator unit generates a well bias voltage that is distributed throughout the integrated circuit chip to respective Vt devices as well as to respective monitors P1 through PN and N1 through NM. The output of each monitor P1 through PN and N1 through NM is connected to its respective compare circuit within compare unit 165B where it is compared to a performance target for that Vt FET (instead of to a performance of a reference Vt FET as in the first embodiment of the present invention) The output of compare unit 165B is connected to control unit 170B. Control unit 170B generates control signals supplied to respective well bias voltage regulators in adjustable voltage regulator unit 160B.
In one example, adjustable voltage regulator unit 160B, PFET monitor circuits P1 through PN, NFET monitor circuits N1 through NM, compare unit 165B and control unit 170B are physically located on the integrated circuit chip. In one example, PFET monitor circuits P1 through PN, NFET monitor circuits N1 through NM, compare unit 165B and control unit 170B are physically located on the integrated circuit chip while adjustable voltage regulator unit 160B is physically located off chip.
FIG. 4A is a block circuit diagram of an exemplary individual compare circuit of compare unit 165A of FIG. 3A. In FIG. 4A, a compare circuit 180A includes a first edge counter 185, a second edge counter 190, a third edge counter 186, a comparator 195A, a programmable memory 200A and a voltage and temperature sense circuit 205. The output of a monitor ring oscillator 210 (monitoring an FET having a design Vt=P2 through PN or N2 through NM) is connected to the count input of first edge counter 185 Ring oscillators are well known in the art and comprise an odd number of sequential inverters with the feedback of the output to the input of the first inverter. The output of first edge counter 185 (a signal indicating the latest actual edge count) is connected to first compare input of a comparator 195A. The output of a reference monitor ring oscillator 211 (monitoring an FET having a design Vt=P1 or N1) is connected to the count input of third edge counter 186. A precise reference clock (external or internal to the integrated circuit chip) is connected to the count input of second edge counter 190 Second edge counter 190 supplies three control signals (reset, start and stop) to first and third edge counters 185 and 186, and a count valid signal to comparator 195A. A third compare input of comparator 195A is coupled to programmable memory 200A, which contains pre-determined target performance monitor ring oscillator 210 to reference monitor oscillators 211 ratios. Voltage and temperature sense circuit 205 is connected to programmable memory 200A and provides lookup values for selecting the appropriate target performance ratio based the temperature and the voltage supplied to the monitor ring oscillator.
While each compare circuit may include its own programmable memory and a voltage and temperature sense circuit, a common programmable memory may be shared between two or more of the compare circuits, a common voltage and temperature sense circuit may be shared between two or more of the compare circuits, or both a common programmable memory and a common voltage and temperature sense circuit may be shared between two or more of the compare circuits.
Monitor ring oscillator 210 and reference monitor ring oscillator 211 are exemplary monitor circuits. Monitor ring oscillator 210 represents any of PFET monitors P2 through PN or NFET monitors N2 through NM. If ring oscillator 210 represents PFET monitor P2, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P2. If ring oscillator 210 represents PFET monitor PN, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage PN. If ring oscillator 210 represents NFET monitor N2, then the NFETs in the signal delay path of the ring oscillator is NFETs designed to have threshold voltage N2. If ring oscillator 210 represents NFET monitor NM, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage NM. If reference ring oscillator 211 represents PFET reference monitor P1, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P1. If reference ring oscillator 211 represents NFET reference monitor circuit N1, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N1. The delays through monitor ring oscillator 210 and reference ring oscillator 211 varies as a function of Vt and well bias voltage (and also temperature and supply voltages as described infra).
In operation, second edge counter 190 issues a reset to first and third edge counters 185 and 186 to reset its count to zero. Then second edge counter 190 issues a start to first and third edge counters 185 and the first edge counter starts counting edges of the signal from monitor ring oscillator 210, second edge counter starts counting clock edges from the reference clock and third edge counter 186 starts counting edges of the signal from reference ring oscillator 211. When second edge counter 190 reaches a preset count value it issues a stop signal to first and second edge counters 185 and 186 and a count valid signal to comparator 195A. Comparator 195A compares the ratio of the actual count from first and second edge counters 185 and 186 to a value of a design ratio stored in programmable memory 200A and then issues a digital signal to control unit 170A (see FIG. 3A). The digital signal indicates the difference between the actual ratio and the design ratio. For example, if after 100 reference clock edges, the monitor actual count is 80 and the reference actual count is 20 then the ratio is 8:2. However, if the target ratio is 8:3, then the monitored FETs Vt needs to be changed to give 30 actual counts.
Since the design count is influenced by the actual (as opposed to the designed) VDD/VSS voltage levels (which affects the voltages on the source, drain and gates of FETs) and PFET/NFET temperature, programmable memory 200A includes a lookup table which comprises a two dimensional matrix of count ratios indexed in a first axis by voltage level increments and in a second axis by temperature increments. Voltage and temperature sensor circuit 205 measures the supply voltage to and temperature of the monitor circuit and passes the information to programmable memory 200A so a temperature and voltage compensated design ratio can be passed on to comparator 195A There is some rounding error, dependent upon the granularity of matrix. Ratios of count values in the matrix may be obtained by simulation of the design, for example by using a software program such as SPICE (simulation program for integrated circuits emphasis) SPICE is a circuit simulator that was originally developed at the Electronics Research Laboratory of the University of California, Berkeley (1975) and now has many commercial variations. The user inputs circuit topology in spice netlist format. The simulator calculates and plots nodal voltages and currents in both time and frequency domains.
FIG. 4B is a block circuit diagram of an exemplary individual compare circuit of compare unit 165B of FIG. 3B. In FIG. 4B, a compare circuit 180B includes first edge counter 185, second edge counter 190, a comparator 195B, a programmable memory 200B and voltage and temperature sense circuit 205. The output of a monitor ring oscillator 210 (monitoring an PET having a design Vt=P1 through PN or N1 through NM) is connected to the count input of first edge counter 185. The output of first edge counter 185 (a signal indicating the latest actual edge count) is connected to first compare input of a comparator 195B. A precise reference clock (external or internal to the integrated circuit chip) is connected to the count input of second edge counter 190 Second edge counter 190 supplies three control signals (reset, start and stop) to first edge counter 185 and a count valid signal to comparator 195B. A second compare input of comparator 195B is coupled to programmable memory 200B, which contains pre-determined target performance values for the monitor ring oscillator 210. Voltage and temperature sense circuit 205 is connected to programmable memory 200B and provides lookup values for selecting the appropriate performance target based the temperature of monitor ring oscillator 210 and the voltage supplied to the monitor ring oscillator.
While each compare circuit may include its own programmable memory and a voltage and temperature sense circuit, a common programmable memory may be shared between two or more of the compare circuits, a common voltage and temperature sense circuit may be shared between two or more of the compare circuits, or both a common programmable memory and a common voltage and temperature sense circuit may be shared between two or more of the compare circuits.
Monitor ring oscillator 210 has been described supra, however monitor ring oscillator 210 in FIG. 4B may represent any of PFET monitors P1 through PN or NFET monitors N1 through NM. If ring oscillator 210 represents PFET monitor circuit P1, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P1. If ring oscillator 210 represents PFET monitor P2, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P2. If ring oscillator 210 represents PFET monitor circuit PN, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage PN. If ring oscillator 210 represents NFET monitor circuit N1, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N1 If ring oscillator 210 represents NFET monitor circuit N2, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N2. If ring oscillator 210 represents NFET monitor circuit NM, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage NM. The delay through ring oscillator 210 varies as a function of Vt and well bias voltage (and also temperature and supply voltages as described infra).
In operation, second edge counter 190 issues a reset to first edge counter 185 to reset its count to zero. Then second edge counter 190 issues a start to first edge counter 185 and the first edge counter starts counting edges of the signal from ring oscillator 210 and second edge counter starts counting clock edges from the reference clock. When second edge counter 190 reaches a preset count value it issues a stop signal to first edge counter 185 and a count valid signal to comparator 195B. Comparator 195B compares the value of the actual count from first edge counter 185 to a value of a design count stored in programmable memory 200B and then issues a digital signal to control unit 170B (see FIG. 3B). The digital signal indicates the difference between the actual edge count and the design edge count. For example, if after 100 reference clock edges, the actual count is 95 and the design count is 90 then the difference is 5 counts, indicating the monitored FETs Vt needs to changed to give 90 actual counts.
Since the design count is influenced by the actual (as opposed to the designed) VDD/VSS voltage levels (which affects the voltages on the source, drain and gates of FETs) and PFET/NFET temperature, programmable memory 200B includes a lookup table which comprises a two dimensional matrix of counts indexed in a first axis by voltage level increments and in a second axis by temperature increments. Voltage and temperature sensor circuit 205 measures the supply voltage to and temperature of the monitor circuit and passes the information to programmable memory 200B so a temperature and voltage compensated design count can be passed on to comparator 195B There is some rounding error, dependent upon the granularity of matrix. Count values in the matrix may be obtained by simulation of the design, for example by using a software program such as SPICE described supra.
It should be understood that in FIGS. 4A and 4B, a ring oscillator is only one example of a circuit that may be used to monitor the performance of FETs and other monitor circuits and if appropriate, other types of compare circuits may be employed to compare the actual performance/switching speed of a PFET or and NFET to the designed performance/switching speed of the PFET or NFET.
FIG. 5A is a diagram illustrating the control signal generated by control unit 170A of FIG. 3A according to the first embodiment of the present invention. In FIG. 5A, control unit 170A generates control signals VREGP2 to VREGPN and VREGN2 to VREGNM that are sent to respective voltage regulators in adjustable voltage regulator unit 160A. VREGP2 adjusts N-well bias voltage on PFETs designed to have threshold voltage P2. VREGPN adjusts N-well bias voltage on PFETs designed to have threshold voltage PN. VREGN2 adjusts P-well bias voltage on NFETs designed to have threshold voltage N2. VREGNM adjusts P-well bias voltage on NFETs designed to have threshold voltage NM. There is no VREGP1 or VREGN1 signal because PFETs designed to have threshold voltage P1 and NFETs designed to have threshold voltage N1 are reference devices.
Control unit 170A includes logic circuits that “calculate” or interface with on-chip stored software instructions to calculate an adjustment of well bias voltage. The adjustments reflect changes (if any) to be made in the actual threshold voltages of PFETs having design threshold voltages P2 through PN so that, when changed threshold voltages P2′ through PN′ are divided by the actual threshold voltage P1′, the design threshold ratios discussed supra (if not the actual Vt values) are restored to within an acceptable tolerance range. For example (P2′/P1′)=(P2/P1). The adjustments reflect changes (if any) to be made in the actual threshold voltages of NFETs having design threshold voltages N2 through NM so that, when the changed threshold voltages N2′ through NM′ are divided by the actual threshold voltage N1′, the design threshold ratios discussed supra (if not the actual Vt values) are restored to within an acceptable tolerance range. For example (N2′/N1′)=(N2/N1).
The signals VREGP2 to VREGPN and VREGN2 to VREGNM may be two-bit or multiple-bit words indicating an magnitude of increase, magnitude of decrease or no change in the well bias to be applied voltage regulators of adjustable voltage regulator unit 160A. Control unit 170A also generates an out of range signal, when it is not possible to adjust an individual voltage regulators output voltage any further.
FIG. 5B is a diagram illustrating the control signal generated by control unit 170B of FIG. 3B according to the second embodiment of the present invention. In FIG. 5B, control unit 170B generates control signals VREGP1 to VREGPN and VREGN1 to VREGNM that are sent to respective voltage regulators in adjustable voltage regulator unit 160B. VREGP1 adjusts N-well bias voltage on PFETs designed to have threshold voltage P1. VREGPN adjusts N-well bias voltage on PFETs designed to have threshold voltage PN. VREGN1 adjusts P-well bias voltage on NFETs designed to have threshold voltage N1. VREGNM adjusts P-well bias voltage on NFETs designed to have threshold voltage NM.
Control unit 170B includes logic circuits that “calculate” or interface with on-chip stored software instructions to calculate an adjustment of well bias voltage. The adjustments reflect changes (if any) to be made in the actual threshold voltages of PFETs having design threshold voltages P1 through PN so that, when changed, the changed threshold voltages P1′ through PN′ should result in the performance (switching speeds) of PFETs having the design threshold voltages P1 through PN being the designed performance values or within an acceptable tolerance range of the designed performance values, although the actual threshold voltages may not be the designed values. The adjustments reflect changes (if any) to be made in the actual threshold voltages of NFETs having design threshold voltages N1 through NM so that, when changed, the changed threshold voltages N1′ through NM′ should result in the performance (switching speeds) of NFETS having design threshold voltages N1 through NM being the designed performance values or within an acceptable tolerance range of the designed performance values, although the actual threshold voltages may not be the designed values.
The signals VREGP1 to VREGPN and VREGN1 to VREGNM may be two-bit or multiple-bit words indicating a magnitude of increase, magnitude of decrease or no change in the well bias to be applied to voltage regulators of adjustable voltage regulator unit 160B. Control unit 170B also generates an out of range signal, when it is not possible to adjust an individual voltage regulators output voltage any further.
FIG. 6 is a schematic block diagram of a typical voltage regulator used to adjust well bias according to embodiments of the present invention. In FIG. 6, a voltage regulator 215 includes a bandgap voltage reference 220, a digital to analog converter 225 an operational amplifier 230, a PFET 235 and a resistor 240. The output of bandgap voltage reference 220 is connected to an input of digital to analog converter 225 and controls signals VREGPX or VREGNY (where X=1 to N and Y=1 to M) are connected to control pins of the digital to analog converter. The output of digital to analog converter 225 is connected to a first input of operational amplifier 230 and the output of the operational amplifier is connected to the gate of PFET 235. The source of PFET 235 is connected to VDD and the drain of PFET 235 is connected to ground though resistor 240, to a second input of operational amplifier 230 and to the output of voltage regulator 215. The output voltage regulator 215 is connected to well PX or NY corresponding to the VREG signal applied to the input of the voltage regulator.
FIG. 7A is a diagram illustrating an exemplary floor plan of an integrated circuit chip according to the embodiments of the present invention. While the different Vt NFETs and PFETs may be placed anywhere on the integrated circuit chip this requires extensive wiring to distribute the well bias voltages. In one option, illustrated ion FIG. 7A, PFETs of the same Vt are formed in common N wells 245A1 to 245AN and NFETs of the same Vt are formed in common P wells 245B1 to 245BM on integrated circuit chip 250. This reduces the amount of well bias wiring from threshold voltage control circuit 155B. Notice that if threshold voltage control circuit 155B is replaced by threshold voltage control circuit 155A (see FIG. 3A) then there are no wires to P-well 245A1 and N-well 245B1 and in fact, those transistors need not be in a common well, but may be dispersed throughout the integrated circuit chip.
FIG. 7B is an exemplary cross-sectional diagram of multiple field effect transistors formed in a common well. In FIG. 7B, a group of PFETs P2 are formed in a common N-well 255, which is connected to VREGP2. The crosshatched regions indicate dielectric material.
FIG. 8 is a flowchart of the methods of controlling threshold voltages of multiple-threshold voltage devices according to the embodiments of the present invention. In FIG. 8, after power-up, in step 300, the performance monitor circuits monitor the performance of the various Vt NFETs and PFETs and in step 305 two options are presented. The first option is to adjust performance ratios of different Vt PFETs/NFETs relative to a reference threshold PFET/NFET performance (the reference PFET/NFET is one of the different Vt PFETs/NFETs) and the second option is to adjust the performance of all different Vt PFETs/NFETs to design values. If the first option is chosen, the method proceeds to step 310, in which the first embodiment of the present invention described supra is employed. If the second option is chosen the method proceeds to step 315, in which the second embodiment of the present invention described supra is employed. After step 310 or 315 is performed two additional options are presented in step 320. The first additional option is to perform a one-time performance correction, and the second additional option is to perform continuous performance corrections. If the first additional option is chosen, the well bias voltages are adjusted by a single amount and the method terminates. Of course, the method using the first additional option may be repeated periodically. If the second additional option is chosen, the well bias voltages are adjusted incrementally by small amounts, and the monitors re-sampled and the well bias voltages adjusted incrementally in a continuous loop. If no adjustment is required (the monitor performance is already acceptable), the voltage regulators are not changed.
Thus, the embodiments of the present invention provide methods and circuits for maintaining the design values and/or relationships between the different threshold voltage values of multi-threshold voltage devices.
The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.

## Claims

1. A circuit, comprising:
a first set of field effect transistors (FETs) having a designed first threshold voltage and a second set of FETs having a designed second threshold voltage, said first threshold voltage different from said second threshold voltage;
a first monitor circuit containing at least one FET of said first set of FETs and a second monitor circuit containing at least one FET of said second set of FETs:
a compare circuit adapted to generate a compare signal based on a performance measurement of said first monitor circuit and a performance measurement of said second monitor circuit, said compare circuit including a first edge counter connected between said first monitor circuit and a first comparator and a second edge counter connected between a reference clock and said first comparator;
a control unit adapted to generate a control signal to a voltage regulator based on said compare signal, said voltage regulator adapted to supply a bias voltage to wells of FETs of said second set of FETs, the value of said bias voltage based on said control signal;
an additional compare circuit including a third edge counter connected between said second monitor circuit and a second comparator and a fourth edge counter connected between said reference clock and said second comparator;
a first memory device containing a first performance specification for said first monitor circuit coupled to said first comparator and a second memory device containing a second performance specification for said second monitor circuit coupled to said second comparator;
wherein said control unit is adapted to generate an additional control signal based on an additional compare signal generated by said additional compare circuit; and
an additional voltage regulator, said additional voltage regulator adapted to supply an additional bias voltage to wells of FETs of said first set of FETs, the value of said additional bias voltage based on said additional control signal.

2. The circuit of claim 1, wherein said compare circuit includes a first edge counter connected between said first monitor circuit and a first comparator, a second edge counter connected between a reference clock and said first comparator and third edge counter connected between said second monitor circuit and said comparator; and further including:
a memory device containing a ratio of said first performance specifications of said first monitor circuit to a second performance specification of said second monitor device, said memory device coupled to said comparator.

3. The circuit of claim 2, wherein said control unit is adapted to generate said control signal such that a ratio of a performance of FETs of said first set of FETs and a performance of FETs of said second set of FETs is equal to a performance ratio specification or is within an acceptable tolerance range of said performance ratio specification.

4. The circuit of claim 1, wherein said control unit is adapted to generate said control signal and said additional control signal such that a performance of FETs of said first set of FETs is equal to a first performance specification or is within an acceptable tolerance range of said first performance specification and such that a performance of FETs of said second set of FETs is equal to a second performance specification or is within an acceptable tolerance range of said second performance specification.

5. The circuit of claim 1, further including:
means for adjusting expected values of said performance measurements of said first and second monitor circuits based on a temperature of and supply voltage to said first and second monitor circuits.