Controlled Ion Implant Damage Profile for Etching

This invention relates to methods for etching patterns on substrates, and more particularly to methods for etching substrates by implanting ions in patterns on the substrates, and then chemically etching the resulting damaged zones. The Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00789, awarded by the United States Department of Energy to AT&T Technologies, Inc.

The formation of a variety of devices such as, for example, waveguides and semiconductors, typically includes one or more steps of etching the surface of a substrate which may be made of inorganic materials, such as, for example, SiO.sub.2, garnet and LiNbO.sub.3. Conventional etching techniques, such as masking a substrate with a patterned photoresist and then contacting the resulting patterned substrate with a chemical etchant to selectively dissolve material from the area defined by the pattern, present problems in controlling vertical and lateral penetration of the etchant into the substrate.

Some materials, such as for example, LiNbO.sub.3, are sufficiently resistant to chemical attack that they present the problem of finding a suitable mask or resist for the substrate which can withstand the chemicals which are used for etching.

In one method of overcoming this problem, the pattern to be etched is bombarded with ions in order to damage the patterned area. The resulting damaged area is etched with chemicals at a significantly greater rate than the remainder of the material. This technique of implanting ions within a material to be etched is disclosed, for example, in Johnson et al U.S. Pat. No. 3,808,068; Tefft et al U.S. Pat. No. 4,325,182; Aklufi U.S. Pat. No. 4,450,041; and, Jain et al U.S. Pat. No. 4,652,334. However, these processes, which use ions having a single incident energy, are capable of producing a sufficient level of damage only within a short distance from the surface.

In another method of providing patterns in chemically resistive material ions are impinged onto the patterned surface to erode material from the substrate; however, the mask used to define the pattern is subjected to the same erosive forces as the substrate material, and the erosion of the mask can diminish the lateral resolution of the desired pattern.

It is accordingly one object of this invention to provide an improved method of etching a material.

It is another object to provide an improved method for etching a material which is highly resistive to etching solutions.

In accordance with this invention, there is provided a process for etching a pattern on a substrate comprising irradiating the surface of the substrate with ions having a plurality of different ion energies to form the desired pattern, and then contacting the resulting ion-implanted area with an etchant.

The ion-implantation step of this process produces a tailored depth profile of displacement damage which controls the subsequent etching step, and determines both the maximum depth that will be etched in the substrate and the sidewall profile of the etched features. This process can be carried out with an ion fluence which is significantly lower than the ion fluence in prior art ion-milling or reactive-ion-etching techniques, thereby minimizing the problems of erosion of masks and loss of lateral resolution. For example, typical ion-milling etch rates of 50 nm per minute with an ion current density of 1 mA/cm.sup.2 require a total ion fluence of 3.times.10 .sup.18 ions/cm.sup.2 to remove 500 nm of material as compared to as little as 5.times.10.sup.15 ions/cm.sup.2 with the present process. This lower ion fluence reduces or eliminates significant erosion of mask material.

The maximum depth of etching is determined by the range of the most energetic ions rather than by the time of exposure to an etching solution, which greatly reduces the care needed to achieve precise control of etching depth during the etching process. The use of multiple ion energies permits etching to greater depths with a lower ion fluence than is possible with a single ion energy.

This process is especially useful for etching LiNbO.sub.3 because of the low chemical reactivity of this compound, and the invention will be illustrated in more detail by describing steps in which LiNbO.sub.3 is the material to be etched. It will be recognized by workers in the art that other materials can be treated in a similar manner by implanting ions in a plurality of ion energies.

The ions which are to be implanted within a body of LiNbO.sub.3 may be ions which are conventionally used in reactive-ion-etching techniques, such as for example, helium ions.

The maximum ion energy level to be used is determined by the depth to be etched, greater depths requiring higher ion energies. A typically optically useful etching depth of 500 nm for LiNbO.sub.3 corresponds to an expected penetration of 150 keV ions.

The number of kinetic energies of the ions impinging on the substrate surface must be at least two and preferably at least three. An increase in the number of ion energies increases the uniformity of the profile of implant damage.

In the preferred method of carrying out the invention, the ion fluence is approximately the same at each energy level in order to produce substantially uniform damage throughout the depth to be etched. As used herein, the term "ion fluence" refers to the total number of ions impinging on a unit area.

Typically, ion energies for etching LiNbO.sub.3 to a depth of about 500 nm are about 30, 60, 90, 120 and 150 keV in five steps. The ion fluence in each step may range from about 1.times.10.sup.15 ions/cm.sup.2, or less to 1.times.10.sup.16 ions/cm.sup.2 or more.

Having thus described the invention, the following examples are offered to illustrate it in more detail.

A body of LiNbO.sub.3 was implanted with helium ions to an ion fluence of 2.times.10.sup.16 ions/cm.sup.2 at each of three energy levels of 50, 100 and 150 keV. The material was subsequently etched at room temperature by a 24% aqueous solution of HF. An etched depth of 500 nm, which corresponds to the expected range of the 150 keV ions, was reached in less than 9 minutes.

A body of LiNbO.sub.3 was implanted with helium ions at a fluence of 2.7.times.10.sup.15 ions/cm.sup.2 at a 30 keV and a fluence of 1.times.10.sup.15 ions/cm.sup.2 at each of the four energies of 60, 90, 120 and 150 keV. The resulting implanted surfaces were etched to a depth of 500 nm in 120 minutes, using 49% HF in water at room temperature.

The foregoing Examples are offered to illustrate the invention and are not intended to limit the scope of the invention. For example, it is obvious that changes may be made in the material to be etched, the ions to be implanted, the ion energies, the number of different ion energies which may be used, and the ion fluence for each ion energy.