Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2002-02-15
2004-12-07
Alanko, Anita (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S725000, C438S726000, C438S744000, C216S067000, C216S079000
Reexamination Certificate
active
06828251
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to a method for etching vias and hole contact structures for semiconductor device manufacturing and more particularly to methods for plasma etching with improved etching selectivity for low-k materials and nitride containing dielectric anti-reflective coatings.
BACKGROUND OF THE INVENTION
During the formation of semiconductor devices it is often required that the conductive layers be interconnected through holes in the insulating layer. Such holes are commonly referred to as contact holes, i.e., when the hole extends through an insulating layer to an active device area, or vias, i.e., when the hole extends through an insulating layer between two conductive layers. The profile of a hole is of particular importance since that it exhibits specific electrical characteristics when the contact hole or via is filled with a conductive material. Typically, the holes are high aspect ratio holes, meaning that the ratio of length to width is at least greater than about 1. Such holes are typically formed by a plasma etch process where complex chemical processes result in relatively higher etching rates in one direction versus another, known as anisotropic etching. The relative anisotropicity of the etching process will in turn determine the etching profile of a etched hole. As semiconductor structures are inevitably driven to smaller sizes, successful etching of higher aspect ratio holes is becoming more difficult.
Plasmas in a plasma etch process containing fluorocarbons or hydrofluorocarbons selectively etch oxides relative to underlying silicon containing layers. For example, plasmas containing fluorocarbons or hydrofluorocarbons such as CF
4
and CHF
3
have been used to perform such an etch. Using fluorocarbon or hydrofluorocarbon containing plasmas provides a means of selectively etching oxide films against an underlying silicon containing layer, i.e., the etching of the oxide film down to the underlying silicon layer without significantly etching the underlying silicon containing layer. Accordingly, a high oxide to silicon etch rate ratio is required.
The key mechanisms responsible for high silicon dioxide to silicon etch rate selectivity in fluorocarbon plasmas involve the combination of at least two factors. First, the deposition of nonvolatile residue, e.g., a polymeric carbon containing residue, on various surfaces during the etching process acting to slow the relative etching rate on those surfaces, and second, oxygen from the etching of an oxide which acts to reduce residue deposition thereby increasing an etching rate on that surface. While carbon containing residues are found to deposit on all surfaces inside an etch chamber containing fluorocarbon or hydrofluorocarbon plasmas, less accumulation is observed to occur on oxide surfaces, e.g., doped silicon dioxide, than on non-oxide surfaces, e.g., silicon containing surfaces such as silicon nitride, doped silicon, or polysilicon.
Carbon containing residues or polymeric residues deposit on surfaces in a hole (e.g., sidewalls, hole bottom) when fluorocarbon plasmas are present due to a variety of mechanisms. For instance, fluorocarbon radicals dissociate upon being absorbed on a surface. Less residue accumulates on silicon dioxide surfaces because some of the carbon combines with the oxygen of the oxide being etched to form volatile carbon monoxide or carbon dioxide. The relatively clean surface allows the silicon dioxide layer to be etched at a faster rate compared to surfaces where polymeric carbon residues have deposited.
Further, when using fluorocarbon or hydrofluorocarbon containing plasmas, if the etching mechanism proceeds strictly by the reaction of silicon with fluorine atoms generated by the plasma to form SiF
4
, isotropic rather than anisotropic etching occurs thereby providing no advantage over wet etching methods in forming contact holes or vias. Plasmas, however, generated using fluorocarbons or hydrofluorocarbons proceed by an anisotropic etch mechanism which is believed to depend on the manner of the bombardment of the etched surface with energetic ions.
For example, in a typical silicon dioxide etching process, to provide a contact hole or via on a wafer, incident energetic particles generally arrive in a direction perpendicular to the wafer surface, striking the bottom surfaces of the etched features. In anisotropic etching processes, such as those using fluorocarbon or hydrofluorocarbon containing plasmas, polymer deposition on the sidewalls and bottom surface of the contact hole or via being etched occurs simultaneously with the etching of the oxide. Surfaces struck by the ions at a lower rate tend to remove the nonvolatile polymeric residual layer at a lower rate, thereby at steady state, leaving a layer of nonvolatile polymeric residue on surfaces such as the sidewalls of the etched opening, thereby protecting such surfaces against etching by the reactive gas. As such, etching is performed preferentially in a direction perpendicular to the wafer surface since the bottom surfaces etch at a higher rate than the polymeric residue containing sidewalls (i.e., anisotropic etching).
However, an “etch stop” phenomenon with respect to high aspect ratio features, such as contact holes and vias, is problematic. For example, during the etching of a contact hole or via, a nonvolatile polymeric residual layer may be formed on the sidewalls and bottom surface of the contact hole or via from carbon containing neutral species resulting from the etch process. Deposition of the polymeric residual layer and etching of the oxide layer occur simultaneously. When high aspect ratio features are etched, the etch rate and etch chemistry vary with the aspect ratio and etching depth of the feature. Often the etching process begins normally until the etching depth reaches a particular depth or aspect ratio at which point the etching process undesirably stops, i.e., “etch stop” phenomenon.
Therefore, a major problem in etching high aspect ratio contact holes and vias in oxides is that the etch chemistry changes with changing aspect ratio (depth) of the etched hole resulting in premature etch stop. This effect is most severe in the oxide contact hole and via etch processes because of the need to use a chemistry in which the etching of the oxide and the deposition of a polymeric residual material are taking place simultaneously. Because of the polymer deposition, the etch process may stop spontaneously well before the desired oxide is etched to a desired depth, i.e., etch stop.
Another issue compounding the problem of etching of small high aspect ratio holes is the use of low-k (low dielectric constant) materials in various parts of semiconductor devices. For instance, in a damascene structure with an inter-metal dielectric (IMD), it may be advantageous to use lower dielectric constant materials in order to reduce signal delay times as semiconductor structures become smaller and smaller, for instance below 0.13 micron. The problem of signal delay increases as semiconductor dimensions decline requiring the use of lower k materials. Some of the newer low-k materials (or ultra low-k materials) frequently include carbon based materials such as Si—C, Si—CH
3
, etc., as a result of methods used to make low-k materials by increasing the material porosity. The term low-k materials herein refers to materials with a dielectric constant of between about 2.0 and 3.0. Due to the presence of carbon containing materials in low-k materials, polymeric residual material tends to form at a higher rates during the etching process on the sidewalls and hole bottoms compared to higher k carbon-free materials such as silicon dioxide, thereby changing the etching chemistry and leading to premature etch stop. It would therefore be advantageous to develop an etching process whereby the formation of polymeric residue can be controlled to avoid the etch stop phenomenon during the etching process of for example, low-k materials.
Attempts to employ a cleaner etching chemistry (less deposition of polymeric resi
Chaio Li-Chih
Liu Jen-Cheng
Su Yi-Nien
Taiwan Semiconductor Manufacturing Co. Ltd.
tung & Associates
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