Method and apparatus for anisotropic etching of substrates

Details Method and apparatus for anisotropic etching of substrates Method and apparatus for anisotropic etching of substrates

1998-08-06

2001-04-10

Utech, Benjamin L. (Department: 1765)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

C438S711000, C438S729000, C438S731000

Reexamination Certificate

active

06214161

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and apparatus for anisotropic etching of substrates using a plasma.
BACKGROUND INFORMATION
Etching of substrates using a plasma is known. It is used principally with substrates made of silicon for semiconductor technology, in particular in the manufacture of chips from silicon wafers, in which patterns defined by etching masks, for example valleys, are etched into the substrate. The etching masks are masking layers, for example photoresist layers, applied onto the surface of the substrates. The plasma is ignited by exciting reactive gases or gas mixtures using high-frequency electromagnetic radiation. This is done, for example, by using an inductively coupled plasma source (ICP source) with high-frequency excitation. A typical ICP source has an excitation coil, placed around the plasma volume and having one or more turns, through which a high-frequency current, for example at a frequency of 13.56 MHZ, flows in order to excite the plasma. One end of the coil (the “hot” end) is thus connected to a high-frequency source.
An etching method of the species for deep silicon etching using an ICP source is described in German Patent Application No. 42 41 045.
The alternating field or magnetic field of the excitation coil is, however, inhomogeneous. This inhomogeneity causes an increase in the quantity of ions and reactive particles at the edge of the plasma as compared with the middle. Because of this excitation geometry, different etching rates are observed in the center and at the edge of the substrate. For example, the etching rate in the middle of a wafer with a diameter of 150 mm can be up to 20% less than in the edge region of the substrate.
In addition, the electrical leakage fields proceeding from the “hot” end of the coil that is at high voltage lead to a correspondingly severe deformation of the inductive plasma. At the same time, the plasma is displaced out of the center of the excitation coil toward the “hot” end. Ablation of the etching mask is accordingly also highly nonuniform, and is displaced from the center of the substrate toward the edge region. Ablation is less in the region of the substrate that is adjacent to the end of the excitation coil in the plasma source that is at high voltage than in the regions of the substrate remote from the “hot” coil end. These inhomogeneities result in inaccuracies in the etched structures.
SUMMARY OF THE INVENTION
The method and apparatus according to the present invention are advantageous is that the inhomogeneities described are compensated for to a considerable extent. By installing and optimizing an aperture according to the present invention, the inhomogeneity of the etching rate can be at least halved. At the same time, the irregular ablation of the etching mask is almost completely suppressed. In fact, a slight overcompensation may even be observed, i.e. ablation of the etching mask decreases slightly toward the substrate edge. The mask profile is now centered. This effect is advantageous because it allows the photoresist to be applied at thinner layer thicknesses. Associated with this are greater patterning accuracy, and a reduction in achievable pattern widths.
The method and apparatus according to the present invention are based on the recognition that the plasma inhomogeneities described above are kept away from the substrate by the method and the apparatus according to the invention. The aperture according to the invention offers an enlargement of the effective surface for electron-ion recombination, while the acceptance angle with respect to the substrate decreases. Expanding the plasma below the aperture toward the substrate results in a dilution of the reactive particles, i.e. of both the ions and the free radicals, which is effective particularly in the edge region of the substrate.
The substrate is thus shielded from the inhomogeneity of the high-frequency coil field of the excitation coil, and from the influence on the plasma of the electrical fields proceeding from the coil end that is at high voltage. Compensation is based on an electron-ion recombination on the walls of the aperture, thus decreasing the ion current density in the edge region of the substrate; and on a dilution effect which is also effective in the edge region of the substrate.
The present invention thus provides an improvement in the uniformity of an ICP source with respect to the substrate.
Another embodiment of the aperture according to the present invention includes an orifice plate having an approximately cylindrical tube that is set on top of it and is introduced into the plasma. The cylindrical tube offers an increase in the effective area for electron-ion recombination, by way of an elongated path length for the electrons and ions in the volume enclosed by this recombination surface.
By coating the substrate electrode around the edge of the substrate with an absorber made of a material which consumes the respective reactive particles, it is additionally possible to simulate a substrate load which extends over the edge region of the substrate and consumes reactive particles. A further cause of nonuniform ablation of the etching mask can thereby be compensated for. Fewer reactive particles are absorbed by the substrate itself in the edge region of the substrate than in the middle. At the same time, the inhomogeneity of the plasma means that production of reactive particles is more intense at the edge. The concentration of reactive particles is thus increased in the edge region of the substrate. The collector shield absorbs this excess. Depending on the dimensions of the collector shield, absolutely homogeneous etching over the substrate surface can be achieved, or the etching rate distribution over the substrate surface can in fact be inverted (etching rate decreases toward the edge).


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