Silicon-on insulator (SOI) substrate having dual surface crystallographic orientations and method of forming same
A method is provided of forming a silicon-on-insulator (SOI) substrate having at least two exposed surface crystal orientations. The method begins by providing an SOI substrate having a first silicon layer with a surface having a first crystal orientation located on a first buried oxide layer. The buried oxide layer is located on a silicon substrate having a surface with a second crystal orientation. The first silicon layer and the first buried oxide layer are selectively removed from a first portion of the SOI substrate to expose a first surface portion of the silicon substrate. A second silicon layer is epitaxially grown over the first surface portion of the silicon substrate. The second silicon layer has a surface with a second crystal orientation. A second buried oxide layer is formed in the second silicon layer. Subsequent to the fabrication of the SOI substrate, N and P type MOSFETS may be formed on the surfaces with different crystal orientations.
This application is a divisional and claims the benefit of priority of co-pending U.S. patent application Ser. No. 10/800,348, filed Mar. 12, 2004, entitled “Silicon-On Insulator (SOI) Substrate Having Dual Surface Crystallographic Orientations And Method Of Forming Same,” which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTIONThe present invention relates generally to a silicon-on-insulator (SOI) substrate on which a semiconductor device such as a MOSFET can be fabricated, and more particularly to a silicon-on-insulator (SOI) substrate having portions with different surface crystallographic orientations on which a P-MOSFET and an N-MOSFET can be fabricated.
BACKGROUND OF THE INVENTIONAccording to current processes known in the microelectronics industry, the substrate of integrated devices is typically wafers of monocrystalline silicon. In the last few years, as an alternative to wafers consisting of silicon alone, composite wafers, so-called “SOI” (Silicon-on-Insulator) wafers have been proposed, comprising two silicon layers, one of which is thinner than the other, separated by a silicon oxide layer. SOI structures are becoming widely utilized for construction of electronic devices. For example, such structures can be employed to produce semiconductor devices, such as VLSI devices, micro-electro-mechanical systems (MEMS), and optical devices. One method of producing an SOI structure, known by the acronym SIMOX (separation by implanted oxygen) forms a buried oxide layer (BOX) in a semiconductor substrate by implanting oxygen ions into the substrate followed by a high temperature annealing step. The insulating layer provides electrical isolation of devices that are built in the superficial silicon layer.
Considerable attention has recently been paid to SOI wafers, since integrated circuits having a substrate formed from wafers of this type have considerable advantages compared with similar circuits formed on conventional substrates, formed by monocrystalline silicon alone. These advantages include, faster switching speed, greater immunity to noise, smaller loss currents, elimination of parasitic component activation phenomena, reduction of parasitic capacitance, greater resistance to radiation effects, and greater component packing density.
One particular device formed on an SOI is a MOSFET. In order to meet an increasing demand for high-performance portable equipment, demand for SOI-MOSFETs offering the above-mentioned advantages is also expected to increase. As SOI-MOSFETs continue to be reduced in size, one problem that arises concerns the need to maintain high electron/hole mobility in their channels. Unfortunately, increased MOSFET scaling can degrade mobility in very short channels because of the high impurity levels that are employed to suppress short channel effects and because the parasitic resistance becomes more sensitive. Additionally, mobility saturates at very short channel lengths.
MOSFETs may be classified as P-type, in which the channel is doped P-type, or N-type, in which the channel is doped N-type. For a variety of reasons it is often desirable to incorporate both N-MOSFETs and P-MOSFETs in the same circuit. For example, RF analog circuits such as a low noise amplifier using both types of MOSFETS can be fabricated with enhanced performance characteristics such as higher gain and lower current. It is well known that the hole mobility for a P-MOSFET is much higher when it is formed on a silicon substrate with a top surface having a (110) crystal orientation (an “Si(110) surface or layer”) than when it is formed on a silicon substrate with a top surface having a (100) crystal orientation (an “Si(100) surface or layer”). On the other hand, it is also well known that the electron mobility for an N-MOSFET is degraded when it is formed on a Si(110) surface of a substrate in comparison to when it is formed on a Si(100) surface of a substrate. Because of this opposite behavior of electron and hole mobility, it is difficult to integrate an N-MOSFET and a P-MOSFET on the same SOI substrate while maintaining satisfactory performance from both devices.
SUMMARY OF THE INVENTIONIn accordance with the present invention, a method is provided of forming an SOI substrate having at least two exposed surface crystal orientations. The method begins by providing an SOI substrate having a first silicon layer with a surface having a first crystal orientation located on a first buried oxide layer. The buried oxide layer is located on a silicon substrate having a surface with a second crystal orientation. The first silicon layer and the first buried oxide layer are selectively removed from a first portion of the SOI substrate to expose a first surface portion of the silicon substrate. A second silicon layer is epitaxially grown over the first surface portion of the silicon substrate. The second silicon layer has a surface with a second crystal orientation. A second buried oxide layer is formed in the second silicon layer.
In accordance with one aspect of the invention, the first silicon layer and the first buried oxide layer are removed by providing a hard mask over the first silicon layer, providing a photoresist pattern on the hard mask, and etching portions of the first silicon layer and the buried oxide layer that are not covered by the photoresist. Finally, the photoresist is removed
In accordance with another aspect of the invention, the hard mask comprises Si3N4.
In accordance with another aspect of the invention, the step of forming the second buried oxide layer includes the steps of implanting oxygen ions into the second silicon layer and annealing the SOI substrate.
In accordance with another aspect of the invention, the first crystal orientation is a (110) orientation and the second crystal orientation is a (100) orientation.
In accordance with another aspect of the invention, the first crystal orientation is a (100) orientation and the second crystal orientation is a (110) orientation.
In accordance with another aspect of the invention, an SOI substrate is provided. The SOI substrate includes a silicon substrate having a surface with a first crystal orientation and first and second buried oxide layers each extending over and in contact with different portions of the silicon substrate surface. First and second silicon layers are located over the first and second buried oxide layers, respectively. The first and second silicon layers have surfaces with different crystal orientations, one which is the first crystal orientation.
BRIEF DESCRIPTION OF THE DRAWINGS
As shown in
Next, in
Next, as shown in
An annealing step follows the oxygen implantation step. The annealing step can be performed at a temperature in a range between approximately 1100 C. The annealing step redistributes the implanted oxygen ions and chemically bonds them to silicon to form a continuous buried layer 118 of silicon dioxide (SiO2), i.e., BOX region, thereby separating an upper silicon layer 116, on the surface of which semiconductor devices are to be manufactured, from the remaining bulk silicon region 106 below. The BOX region has a thickness in a range of approximately 100 to 150 nm. As
Finally, hard mask 112 is removed to expose the Si(110) surface on which the P-MOSFET device is fabricated.
The resulting dual plane SOI substrate has two exposed silicon surfaces, one with a (110) surface orientation and the other with a (100) surface orientation. The exposed silicon surfaces 102 and 116 are formed on respective BOX layers 104 and 118 that are located on the Si(100) support substrate 106.
In one alternative embodiment of the invention, the SOI substrate 100 may be replaced with SOI substrate 600 shown in
Claims
1. An SOI substrate, comprising:
- a silicon substrate having a surface with a first crystal orientation;
- first and second buried oxide layers each extending over and in contact with different portions of the silicon substrate surface;
- first and second silicon layers located over said first and second buried oxide layers, respectively, said first and second silicon layers having surfaces with different crystal orientations, one of said different orientations being said first crystal orientation.
2. The SOI substrate of claim 1 wherein the first crystal orientation is a (110) orientation and the second crystal orientation is a (100) orientation.
3. The SOI substrate of claim 1 wherein the first crystal orientation is a (100) orientation and the second crystal orientation is a (110) orientation.
Type: Application
Filed: Aug 29, 2005
Publication Date: Jan 5, 2006
Inventor: Tenko Yamashita (Somers, NY)
Application Number: 11/214,140
International Classification: H01L 27/01 (20060101);