BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of anisotropic plasma etching of substrates preferably defined with an etching mask in which the etch rate and selectivity is increased. The method can be well implemented for manufacturing microelectromechanical system (MEMS), as well as microelectronic devices.
2. Background of the Related Art
Anisotropic plasma etching, particularly for single crystal silicon, can work independent of crystal orientation of the substrate or doping level. This method also applies to doped or undoped polysilicon. Preferred fields of applications are MEMS technology, where structures have a high aspect ratio, i.e., a high structural height to width ratio. Other examples include surface wave technology, where narrow grooves and vertical walls are etched to produce actuators, surface wave filters, delay lines, etc. Additional microelectronics applications include storage cells, insulation, collector contacts, etc.
The Reactive Ion Etching (RIE) processes which are commonly used for anisotropic silicon etch employ relatively high energy ions (≧100 eV) and reactive halogens, such as fluorine, chlorine or bromine, which are used directly in the plasma or are released from corresponding compounds, like CF4, CF3Br, C2F6, CCI4, CHCI3. The resulting ion bombardment of the etching ground, i.e., the area to be etched, initiates the reaction of the radicals with the silicon to be depleted. The etching of the sidewalls is minimal due to the directionality of the ions.
Problems occur when, to increase the speed of silicon removal (i.e., the etch rate), one tries to enhance the plasma density by increasing the power coupled to the plasma discharge. This can be accomplished by either increasing the power of the source for the plasma discharge or by increasing the value of the polarization voltage applied to the substrate. However, as the power is increased more hot ions are produced and the direction of ion movement becomes more random. The results is that more ions and radicals are depleted by the walls of the trenches, with the inevitable loss of anisotropy of the etch. To overcome this problem, one must reduce the etch rate, resulting in the loss of the throughput.
An additional problem encountered is mask degradation. As etch rate is reduced, etch time, and therefore mask exposure time, are increased, leading to more rapid mask degradation, i.e., reduced selectivity.
U.S. Pat. No. 5,501,893 discloses an etching method that includes alternating etching and polymerizing steps where the purpose of the polymerizing step is to provide a polymer layer on the surfaces that were exposed in the previous etching step to form a temporary etch stop. Thus the side walls are protected from etching during the etching steps. However, the gas mixtures introduced during the etching and polymerizing steps are different such that different gas mixtures are cycled during the respective etching and polymerizing steps. In the etching step the gas mixture includes SF2 and Ar and in the polymerizing step the gas mixture includes CHF3 and Ar.
The problem with cycling different gases is that the time ratio of the etch deposition cycle depends on the speed of the gas mixtures and varies from point to point, affecting the uniformity. Also, the time for species to arrive at the bottom of the etched trench varies drastically for trenches having different sized openings. Also, this method typically requires more complex hardware and controls to introduce the two different gas mixtures in cycles.
It is desirable to make the plasma as cold as possible with coexisting polymer-producing unsaturated monomers and fluorine, bromine or iodine radicals. The energy level sufficient for ionization is different for each gas. In certain cases, the activation energy for polymer-producing gases (C4F8, CHF3) is two times higher than for radical producing gases (SF6). The object of the invention is to establish a method for enhancing the treatment of the silicon surface being etched, by using the differences in the energies of activation of the reactive gases to arrive at optimal conditions for both etching and passivation, and alternating those conditions at a high rate to produce a high aspect ratio, and high selectivity etch process.
SUMMARY OF THE INVENTION
The object of the present invention is accomplished by providing a method of anisotropic plasma etching of substrates (typically silicon) comprising the following steps:
a) placing the substrate with the surface to be selectively etched on an electrode connected to an electromagnet power source;
b) introducing mixed gases consisting of an etching gas (SF6) and a passivation gas (CHF3, C4F8, etc.) into the processing chamber;
c) exciting the mixed gases with lower power (100-800 W) electromagnetic radiation sufficient to produce a plasma containing ions and radicals for etching;
d) concurrent with step (c), applying high polarizing voltage (50-500 eV) to the substrate via its electromagnet power source to produce a highly anisotropic etch;
e) exciting the mixed gases with high power (1000-3000 W) electromagnetic radiation to produce in the plasma unsaturated monomers for protective polymer coating formation;
f) concurrent with step (e), applying low polarizing voltage (0-25 eV) to the substrate to form a conformal polymer coating on the exposed side walls of the surfaces being etched; and
alternating steps c) and d) with steps e) and f) to achieve an anisotropic etch with a high etch rate and selectivity than is currently being achieved using other methodologies.
The method used in this invention enables the substrate to be etched without using helium gas as a cooling medium. This is because lower power is used to excite the etching gas resulting in less heat being generated during the process.
A further advantage of this invention is that a constant flow of mixed gas is injected into the process chamber during processing, resulting in a process that is more stable and repeatable.
The following is a description of the process of etching the substrate with reference to FIGS. 2(a) to 2(c). The substrate 14, including a silicon substrate 40 and an etching mask 40 that exposes the regions of the silicon substrate 40 that are intended to be anisotropically etched, is placed on the substrate holder/electrode 12 and subjected to the first etching step. In this step, a mixture of gases containing etchant and passivating gases (e.g., SF6, C4F8, and CHF3) with a certain flow rate, preferably in the range of 100 to 400 sccm and pressure in the range of 0.1 to 10 Pa, is introduced into the chamber 10. The plasma is stimulated by applying a relatively low RF power, preferably in the range of 100 to 800 W, from the power source 30. At the same time, a polarization potential is provided by the substrate generator 18 to produce an electrical field of a relatively high value in the range of 50 to 500 eV, and preferably 80-300 eV. The low RF power applied for plasma stimulation provides the directional etch with an extremely high rate. Specifically, the plasma is “cold” due to the low energy applied so that the directionality of the ions can be controlled. Also, the high potential of the substrate 14 provides strong acceleration of the ions toward the etched surface. Both factors provide excellent directionality of the ions, resulting in a high anisotropic etch. The etched portion 44 of the silicon substrate 40 is shown in FIG. 2(a).
After a certain period of time, e.g., 10 to 100 sec, the power source 30 switches to a relatively high power, preferably in the range of 1000 to 3000 W, to create high energy excitation of the plasma and the polarization potential developed by the substrate generator is reduced to a relatively low, or even zero, value (i.e. 0-25 eV). High energy excitation of the plasma creates a condition that results in the formation of a polymer layer 46. Further, isotropic movement of species causes the thickness of the polymer layer 46 on the bottom of the trench to be the same as the thickness on the sidewalls, as shown in FIG. 2(b). During this time, the low polarization on the substrate 14 prevents any etching. After a certain time, e.g., 0.5 to 3 sec, the protective polymer layer 46 of a predetermined thickness is formed on the trench walls, as shown in FIG. 2(b). This protective polymer layer 46 prevents erosion of the trench walls that were formed in the previous etching steps during the subsequent etching step, performed in the manner discussed above.
Since numerous changes can be made without departing from the spirit of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. For example, the invention is not limited to the list of etchant and passivation gases discussed above. Additional silicon etching gases include CF4, NF3, NF3HF, HBr, CCL4, CF2Cl2, CFCl3, Br2, Cl2, 12, HCl, CIF3 and BCl3. Also, additional passivation gases include CH4, CH2F2, H2, C2H4, C3H8, CH3Br, C2F6, C2F4, and C3F6.