Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1998-11-06
2001-07-17
Whitehead, Jr., Carl (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S370000, C257S374000, C257S378000, C257S525000, C257S526000, C257S616000, C257S639000, C257S640000, C257S649000
Reexamination Certificate
active
06262456
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an integrated circuit (IC) and the fabrication of an integrated circuit. More particularly, the present invention relates to an integrated circuit having transistors with multiple threshold voltage values.
BACKGROUND OF THE INVENTION
Ultra-large-scale integrated (ULSI) circuits generally include a multitude of transistors, such as, more than one million transistors and even several million transistors, that cooperate to perform various functions for an electronic component. Some transistors on the integrated circuit (IC) or chip are part of circuits which perform different operations than other circuits.
Some transistors perform functions for circuits in the critical signal path of the IC, where speed is crucial to the proper operation of the IC. In contrast, other transistors perform functions for circuits in the non-critical signal path of the IC, where speed is not as important. Transistors in the non-critical signal path are preferably designed to consume less power than transistors in the critical signal path. Still, other transistors may perform functions for a signal path having a criticality somewhere between the critical signal path and the non-critical signal path and accordingly have different speed and power consumption requirements.
Generally, transistors which have higher threshold voltages (Vth) consume less power than transistors which have low threshold voltages due to smaller off-state current leakage. Threshold voltage refers to the minimum gate voltage necessary for the onset of current flow between the source and the drain of a transistor. Transistors which have lower threshold voltages are faster (e.g., have quicker switching speeds) than transistors which have higher threshold voltages.
In ULSI circuits, transistors, such as, metal oxide semiconductor field effect transistors (MOSFETs), with low threshold voltages can be used in logic paths which have high speed requirements. In contrast, transistors, such as, MOSFETs, with higher threshold voltages can be used in the non-critical signal path (e.g. storage devices), thereby reducing the off-state leakage current and hence reducing the standby power consumption of the entire IC.
ULSI circuits are generally manufactured in accordance with complementary metal oxide semiconductor (CMOS) technology and design criteria which utilize N-channel MOSFETs and P-channel MOSFETs. The N-channel and P-channel MOSFETs generally include a polysilicon gate structure disposed between a drain and a source. The polysilicon gate structure controls charge carriers in a channel region to turn the transistor on and off.
According to conventional designs, multiple threshold voltages for transistors on a single IC are obtained by selectively providing channel implants for the transistors. Additional channel implants (e.g., doping the channel region to change the work function difference between the gate and the channel) are used for those transistors with higher threshold voltage requirements (e.g., Vth>0.3V). The transistors which have lower voltage threshold requirements (e.g., Vth=0.3V or less) do not receive the additional channel implants.
Utilizing channel implants to adjust the threshold voltages of transistors can be problematic because transistor short channel performance is very susceptible to process variations. In particular, short channel performance is extremely sensitive to channel implants or additional doping steps. Accordingly, the modification of the channel with implants can result in significantly different short channel performance between transistors, which adversely affects the predictability of the design and operability of the IC. This characteristic is particularly problematic as transistors become smaller and packing densities increase. Additionally, providing channel implants adds additional steps to the fabrication process and makes the IC more difficult to manufacture.
Thus, there is a need for an integrated circuit or electronic device that includes transistors having different threshold voltage levels which can be manufactured according to a simpler process. Further still, there is a need for a ULSI circuit which does not utilize channel implants to adjust threshold voltages among transistors. Even further still, there is a need for a ULSI circuit with transistors having multiple threshold voltages that is higher in density and can be more efficiently manufactured.
SUMMARY OF THE INVENTION
The present invention relates to an integrated circuit including, in one exemplary embodiment, at least 1,000,000 field effect transistors. The transistors include a first group of the transistors that have a first threshold voltage and a second group of the transistors that have a second threshold voltage. The first threshold voltage is different than the second threshold voltage. The transistors have a doped semiconductor gate material comprised of a first semiconductor material provided with a second semiconductor material. The doped semiconductor gate material of the first group of transistors has a first concentration of the second semiconductor material. The doped semiconductor gate material of the second group of transistors has a second concentration of the second semiconductor material. The first concentration is different than the second concentration. The difference between the first threshold voltage and the second threshold voltage is related to a difference between the first concentration and the second concentration.
The present invention further relates to a large-scale integrated circuit including a plurality of first transistors having a first threshold voltage and a plurality of second transistors having a second threshold voltage. The first threshold voltage is less than the second threshold voltage. The first transistors have a first doped semiconductor gate material comprised of a first semiconductor material implanted with a second semiconductor material in a first concentration. The second transistors have a second doped semiconductor gate material comprised of the first semiconductor gate material implanted with the second semiconductor material in a second concentration. The first concentration is more than the second concentration. The first transistors and the second transistors have substantially the same channel characteristics.
The present invention further still relates to an integrated circuit having a first group of transistors having a first threshold voltage and a second group of transistors having a second threshold voltage. The first threshold voltage is less than the second threshold voltage. The integrated circuit is manufactured by providing a silicon gate material over a substrate, selectively implanting the silicon gate material with germanium, and selectively etching the silicon gate material to form gates for the first group of transistors and the second group of transistors. The gate material associated with the first group of transistors has a lower concentration of germanium than the gate material associated with the second group of transistors.
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Lin Ming-Ren
Yu Bin
Advanced Micro Devices , Inc.
Foley & Lardner
Jr. Carl Whitehead
Warren Matthew E.
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