1. Field of Invention
The techniques described herein relate generally to Chemical Mechanical Polishing (CMP), and more particularly to techniques for planarizing a semiconductor wafer having overlapping layers.
2. Discussion of the Related Art
Chemical Mechanical Polishing (CMP) is a technique commonly used for polishing semiconductor wafers to obtain a substantially planar surface. Achieving a planar surface can be important to achieve uniformity and high yield, and obtain suitable depth of focus for subsequent photolithographic alignment steps. Conventional CMP techniques use a polishing pad that polishes the semiconductor wafer using an abrasive slurry and chemical mixture.
Some embodiments relate to a method of forming an overlapping layer on a wafer and removing portions of the overlapping layer. For example, a first layer may be formed above a substrate, and a second layer may be formed at least partially on the first layer. The second layer may be isotropically etched to remove a region of the second layer over the first layer. The wafer may be polished to remove at least a portion of the second layer remaining after isotropically etching the second layer.
Some embodiments relate to a method of forming an overlapping layer on a wafer and removing portions of the overlapping layer. For example, a first layer may be formed above a substrate, and a second layer may be formed at least partially on the first layer. The second layer may be etched to remove a region of the second layer over the first layer. The wafer may be polished to remove at least a portion of the second layer remaining after etching the second layer. The polishing rate of the second layer may be greater than a polishing rate of the first layer.
Some embodiments relate to a semiconductor chip that includes a first layer formed over a region of a substrate comprising a PMOS transistor. The top of the first layer has a substantially planar surface. The semiconductor chip also includes a second layer formed over a region of a substrate comprising an NMOS transistor. The top of the second layer has a substantially planar surface that is substantially co-planar with a top of the first layer. The first layer may be formed of a material having a lower polishing rate than that of the second layer.
This summary is presented by way of illustration and is not intended to be limiting.
It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein.
In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like reference character. For purposes of clarity, not every component may be labeled in every drawing. The drawings are not necessarily drawn to scale, with emphasis instead being placed on illustrating various aspects of the invention. In the drawings:
Applicants have recognized and appreciated problems caused by the protruding step-height of a patterned overlapping layer on a semiconductor wafer, and the surface irregularities that can be caused by using CMP to remove the overlapping layer. Examples of these problems will be discussed with reference to
According to one conventional technique shown in
Another conventional technique is to polish the wafer to a greater degree during CMP such that the stepped sidewalls 4 and remaining portions of silicon nitride layer C on silicon nitride layer B are removed. However, this technique may result in severe wafer non-uniformity during CMP. For example, when the polishing rate of silicon nitride layer B is greater than the polishing rate of silicon nitride layer C, a significant portion of silicon nitride layer B can be removed unintentionally during CMP, causing severe non-uniformity, as shown in
Various inventive techniques described herein can provide improved uniformity by reducing or eliminating overlapping layer step-heights. In some embodiments, an overlapping layer can be isotropically etched to form an opening with reduced sidewall height. Reducing the sidewall height prior to CMP can lower the burden of planarizing the wafer during CMP.
In some of the inventive techniques described herein, a CMP process may be used to polish an overlapping layer to reduce or eliminate its sidewall step-height. The overlapping layer may be selected to have a higher CMP polish rate than the layer upon which it is formed. In some embodiments, an overlapping layer B is formed partially on top of layer C. A sacrificial layer A may be formed on layers B and C that has a CMP polishing rate of greater than or equal to the CMP polishing rate of the overlapping layer B. The rates of polishing of the layers A, B and C may satisfy the relationship RA≧RB>RC, where RA is the polishing rate of layer A, RB is the polishing rate of layer B, and RC is the polishing rate of layer C. The wafer may be polished down to the underlying layer C using CMP by removing portions of overlapping layer B and sacrificial layer A.
As shown in
The wafer 40 may then be polished using CMP to planarize its surface. A standard CMP process may be used to polish layer A, B and C, as known in the art, although a specialized or optimized process may be used in some circumstances. In some embodiments, the polishing rates of the layers A, B and C satisfy the relationship RA≧RB>RC, where RA is the polishing rate of layer A, RB is the polishing rate of layer B, and RC is the polishing rate of layer C. As used herein, the term “polishing rate” means the rate at which the height of a layer is reduced by removal of material in the polishing process.
In some embodiments, overlapping layer B may be formed to have a higher polishing rate than underlying layer C, which may allow removing the overlapping region of layer B without removing a substantial thickness of layer C. For example, layer C may be formed of compressively-strained silicon nitride and layer B may be formed of tensilely-strained silicon nitride, as the polishing rate of tensilely-strained silicon nitride may be greater than that of compressively-strained silicon nitride. As an example, the polishing rate of compressively-strained silicon nitride may be about 7.2 Å/s and the polishing rate of tensilely-strained silicon nitride may be about 10.6 Å/s in a standard CMP process. However, the invention is not limited to the use of silicon nitride for layers B and C, as any other suitable materials may be used with suitable polishing rates. The desired relative polishing rates may be achieved by using different materials instead of, or in conjunction with, layers of different strain. For example, layer B may be formed of a first material with a relatively high polishing rate and layer C may be formed of a different material with a relatively low polishing rate. Various techniques may be used to achieve the desired relative polishing rates of layers B and C. In some embodiments it may be preferable, though not necessary, that the polishing rate of layer B be much higher than the polishing rate of layer C (i.e., Rb>>Rc) to improve the selectivity of removing the overlapping portion layer B while keeping layer C substantially intact. However, suitable results can be achieved when the polishing rate of layer B is only slightly or moderately higher than that of layer C.
In some embodiments, the polishing rate of sacrificial layer A may be greater than or approximately equal to the polishing rate of layer B. For example, layer A may be formed of silicon dioxide, and the polishing rate of silicon dioxide may be about 22 Å/s in a standard CMP process, which may be greater than that of tensilely-strained silicon nitride layer B. However, the invention is not limited to the use of silicon dioxide for layer A, as other suitable materials can be used. In some embodiments it may be preferable, though not necessary, that the polishing rate of layer A be approximately equal to that of layer B. However, suitable results can be achieved when the polishing rate of layer A is greater than that of layer B.
This invention is not limited in its application to the details of construction and the arrangement of components set forth in the foregoing description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.