Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2000-08-08
2003-04-08
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S448000, C428S704000, C428S213000, C423S348000
Reexamination Certificate
active
06544655
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related generally to semiconductor manufacturing and Micro Electro Mechanical Systems (MEMS). More specifically, the invention relates to methods for reducing the curvature of a boron-doped silicon layer.
BACKGROUND OF THE INVENTION
Micro Electro Mechanical Systems (MEMS) often utilize micromachined structures such as beams, slabs, combs, and fingers. These structures can exhibit curvature due to internal stresses and doping gradients. The curvature can be a significant source of error in inertial sensors such as accelerometers and gyroscopes. Many desired structures have a flatness design criteria that is difficult or impossible to achieve using current processes. In particular, silicon layers heavily doped with boron can have a significant curvature when used in suspended structures.
The aforementioned structures are often made starting with a silicon wafer substrate. A boron-doped silicon epitaxial layer is then grown on the silicon wafer substrate and is subsequently patterned in the desired shape. As is further described below, the boron is used as an etch stop in later processing to allow the easy removal of the silicon substrate, leaving only the thin boron-doped epitaxial layer.
At the interface between the boron-doped epitaxial layer and the silicon substrate, the boron tends to diffuse out of the epitaxial layer and into the silicon substrate. This depletes the epitaxial layer of some boron, and enriches the silicon substrate with boron. The epitaxial layer thus often has a reduced concentration of boron near the interface, which is sometimes called the “boron tail.”
After the boron-doped silicon epitaxial layer has been grown to the desired thickness, or at some later point of processing, the silicon substrate is removed often using an etchant that is boron selective. Specifically, the etchant will etch away the silicon substrate, but not the boron-doped silicon epitaxial layer. One such etchant is a solution of ethylene diamine, pyrocatechol, and water (EDP). The etchant typically etches the silicon at a fast rate up to a certain high level boron concentration, at which point the etch rate significantly slows. This high boron concentration level is termed the etch stop level.
The boron concentration near the epitaxial layer surface having the boron tail may be lower than the etch stop level, allowing the etching to remove some of the epitaxial layer surface at a reasonable rate, stopping at the etch stop level of boron concentration beneath the initial surface. The resulting boron-doped structure, such as a beam, thus has two surfaces, the silicon side surface that has the boron tail and the air side surface that has a boron surface layer concentration substantially equal to the concentration in the bulk of the beam away from either surface. Thus, the opposing surfaces have different boron surface layer concentrations.
Boron occupies substitutional lattice sites in silicon, the boron having a Pauling's covalent radius roughly 25% smaller than that of silicon. The size difference causes the boron-doped layers to shrink relative to the undoped or lower doped layers. This size difference leads to an initial tensile stress, with higher boron concentrations leading to higher tensile stresses and lower boron concentrations leading to lower tensile stresses. After release from the substrate, the lower boron concentrations in the tail results in a relatively lower tensile stress than the tensile stress in the air side layer having a higher boron concentration. The tensile stress can transition to a compressive stress after further process steps, such as oxidation and annealing at high temperatures. Regardless of the exact mechanism, an unequal surface layer boron concentration in silicon can lead to an unequal application of stress by those layers in the structure which can lead to the cupping or out-of-plane bending and curvature of a structure where flatness is desired.
What would be desirable, therefore, is a process for reducing the unequal surface layer concentrations of boron in boron-doped silicon to produce substantially flat or planar boron-doped silicon microstructures.
SUMMARY OF THE INVENTION
The present invention provides methods for forming relatively planar boron-doped silicon layers having reduced out-of-plane curvature by providing substantially balanced doping profiles of boron near each of the layer surfaces. A boron-doped silicon epitaxial layer is first grown on a silicon substrate, causing the boron near the silicon substrate to diffuse out of the epitaxial layer into the silicon substrate. As in the prior art, this depletes the boron concentration near the interface between the epitaxial layer and the silicon substrate. However, and in a first illustrative embodiment of the present invention, a second epitaxial layer is grown on the first boron-doped silicon epitaxial layer. The second epitaxial layer preferably has a boron concentration that is less than the boron concentration in the first grown epitaxial layer. Thus, boron in the first boron-doped epitaxial layer tends to diffuse into both the silicon substrate and the second epitaxial layer. This creates substantially similar “boron tails” at both surfaces of the first epitaxial layer. A boron selective etch can be used to remove both the silicon substrate and the second epitaxial layer. Since the remaining first epitaxial layer has substantially similar “boron tails” at both top and bottom surfaces, the compressive stresses are substantially balanced leaving a relatively planar layer.
It is contemplated that any suitable material can be used to deplete the boron concentration near the top surface of the first boron-doped epitaxial layer. For example, rather than growing a silicon based second epitaxial layer, it is contemplated that an oxide layer may be used. Preferably, the oxide layer is selected such that the boron segregated into the oxide layer, depleting the surface silicon layer of boron. One suitable oxide layer is silicon oxide that can be formed through the oxidation of the silicon in the expitaxial layer.
Rather than growing a boron-doped first epitaxial layer on the silicon substrate, it is contemplated that the top surface of a silicon wafer may be directly doped with boron by, for example, diffusion, ion implantation, or any other suitable method. Then, the second epitaxial layer may be grown directly on the top surface of the silicon wafer. As described above, the boron may tend to diffuse both into the substrate and into the second epitaxial layer, leaving substantially similar “boron tails” on both sides of the heavily doped silicon layer. A boron selective etch can then be used to remove both the low-boron-doped silicon substrate and the second epitaxial layer.
Instead of forming substantially similar “boron tails” on either side of a heavily boron-doped layer to reduce the curvature of the layer, the present invention also contemplates providing a layer with a boron tail near one surface, and then substantially removing the boron tail. In this embodiment, a first boron-doped silicon epitaxial layer may be grown on a silicon substrate. Alternatively, and as indicated above, boron may be provided directly in the top surface of the silicon substrate. In either case, the boron tends to diffuse into the silicon substrate, thereby creating a boron tail. The silicon substrate can be etched using a first etchant for a first period of time, such that the silicon substrate and at least part of the boron tail are removed at a first etch rate. The silicon substrate can then be further etched using a second etchant for a second period of time, such that more of the boron tail is removed at a second etch rate. The second etchant can be the same as the first etchant or a different etchant that is less inhibited by boron.
In a related method, it is contemplated that the second etchant may be non-boron selective etchant, such as a dry etch (RIE). In this embodiment, the first etchant, which is boron selective, can be used to remove the sil
Cabuz Cleopatra
Erdmann Francis M.
Glenn Max C.
Horning Robert D.
Fredrick Kris T.
Honeywell International , Inc.
Jones Deborah
Stein Stephen
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