Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-10-20
2001-05-08
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S397000, C257S401000, C257S513000, C257S520000
Reexamination Certificate
active
06229187
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to silicon on insulator (SOI) field effect transistor structures, and more specifically to SOI substrate structures advantageous in the fabrication of such transistors.
BACKGROUND OF THE INVENTION
Conventional or bulk semiconductor devices are formed in semiconductive material by implanting a well of either P-type or N-type conductivity silicon in a silicon substrate wafer of the opposite conductivity. Gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors (FETs). When a given chip uses both P-type and N-type, it is known as a complimentary metal oxide semiconductor (CMOS). Each of these transistors must be electrically isolated from the others in order to avoid shorting the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various transistors. This is undesirable for the current industry goals for size reduction. Additionally, junction capacitance between the source/drain and the bulk substrate and “off” state leakage from the drain to the source both increase power consumption. Junction capacitance also slows the speed at which a device using such transistors can operate. These problems result in difficulties in reducing the size, power consumption, and voltage of CMOS technology devices.
In order to deal with the junction capacitance and “off state” leakage problem as well as obtain reduced size, silicon on insulator technology (SOI) has been gaining popularity. A SOI wafer is formed from a bulk silicon wafer by using conventional oxygen implantation techniques to create a buried oxide layer at a predetermined depth below the surface. The implanted oxygen oxidizes the silicon into insulating silicon dioxide in a guassian distribution pattern centered at the predetermined depth to form the buried oxide layer. The problem with forming field effect transistors on an SOI wafer is the floating body effect. The floating body effect occurs because the buried oxide layer isolates the channel, or body, of the transistor from the fixed potential silicon substrate and therefore the body takes on charge based on recent operation of the transistor. The floating body effect causes the current-to-voltage curve for the transistor to distort or kink, which in turn causes the threshold voltage for operating the transistor to fluctuate. This problem is particularly apparent for passgate devices such as those used in dynamic random access memory (DRAM) wherein it is critical that the threshold voltage remain fixed such that the transistor remains in the off position to prevent charge leakage from the storage capacitor. Another problem associated with SOI technology is heat build up. The insulating silicon dioxide in the buried oxide layer is a poor heat conductor and prevents effective heat dissipation into bulk silicon below the buried oxide layer.
Accordingly, there is a strong need in the art for a semiconductor field effect transistor structure, and a method for forming such structure, that includes the low junction capacitance and low “off” state leakage characteristics of the SOI FET but does not suffer the disadvantages of a floating body potential and heat build up.
SUMMARY OF THE INVENTION
A first object of this invention is to provide a method of forming a silicon on insulator wafer with a perforated buried oxide layer by masking a portion of the surface of a silicon substrate to form a masked region and an unmasked region and performing an oxygen implant to oxidize the silicon substrate to form an insulating layer of silicon dioxide beneath the unmasked region and an unoxidized perforation beneath the masked region.
In the preferred embodiment, the masking step includes applying a layer of photoresist to the top surface of the wafer, exposing the photoresist in the masked region to UV illumination, and developing the photoresist to harden the photoresist in the masked region and remove the photoresist from the unmasked region. After performing the oxygen implantation, the photoresist mask is removed.
To eliminate the floating body effect, the unoxidized perforation corresponds to an active region of a field effect transistor to be fabricated on said wafer. The active region includes a central channel region and a source region and drain region on opposing sides of the central channel region. To reduce or eliminate junction capacitance, the unoxidized perforation preferably has a cross sectional area that is less than or equal to the cross sectional area of the of the channel region.
A second object of this invention is to provide a method of fabricating a field effect transistor by: a) masking the surface of a silicon substrate to create a masked perforation region and an unmasked insulating region; b) performing an oxygen implant to create a planar layer of insulating silicon dioxide beneath the surface of the silicon substrate in the unmasked insulating region; c) masking the surface of the silicon substrate to create a masked active region and an unmasked trench region; d)etching an insulating trench in the unmasked trench area; e) masking the surface of the silicon substrate to create a masked channel region and each of an unmasked source region and drain region; and f) doping each of the unmasked source region and drain region to silicon of the opposite conductivity as the silicon substrate.
Preferably the step of masking the surface of a silicon substrate to create a masked perforation region and an unmasked insulating region includes applying a layer of photoresist to the top surface of the wafer, exposing the photoresist in the masked region to UV illumination, and developing the photoresist to harden the photoresist in the masked region and remove the photoresist from the unmasked region.
Preferably the step of masking the surface of the silicon substrate to create a masked channel region and each of an unmasked source region and drain region includes growing a layer of silicon dioxide on the surface of the silicon substrate and depositing a layer of polysilicon on the surface of the silicon dioxide and patterning and etching the polysilicon to remove the polysilicon from the unmasked source region and drain region.
Yet a third objective of this invention is to provide a method of fabricating a field effect transistor including: a) forming a planar layer of insulating silicon dioxide beneath the surface of the silicon substrate, wherein the planar layer includes a perforated region of semiconductor silicon; b) forming an insulating trench extending between the surface of the silicon substrate and the planar layer to define an active region electrically coupled to the perforated region; c) forming a gate oxide and polysilicon gate on a portion of the active region to define a channel region, a source region, and a drain region; and d) doping each of source region and the drain region to silicon of the opposite conductivity as the channel region.
Preferably the step of forming a planar layer of insulating silicon dioxide includes applying a mask to the top surface of the wafer to define a masked region over the perforated region and an unmasked region wherein the mask is impervious to an ION beam of oxygen and implanting a high dose of oxygen into the silicon substrate in the unmasked region by exposing the wafer to an ION beam of oxygen. The substrate is heated to bond the implanted oxygen with the silicon substrate to form the planar layer of insulating silicon dioxide.
As previously discussed, the step of applying a mask includes applying a layer of photoresist to the top surface of the wafer, exposing the masked region to UV illumination; and developing the photoresist to harden the photoresist in the masked region and remove the photoresist in the unmasked region;
A fourth objective of this invention is to provide a field effect transistor formed on a semiconductor substrate including an active region, including a central channel region and a source re
Advanced Micro Devices , Inc.
Renner , Otto, Boisselle & Sklar, LLP
Wojciechowicz Edward
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