Method for anisotropic etching of silicon

Details Method for anisotropic etching of silicon Method for anisotropic etching of silicon

1999-07-16

2001-09-04

Gulakowski, Randy (Department: 1746)

Etching a substrate: processes

Etching and coating occur in the same processing chamber

C438S694000, C438S733000

Reexamination Certificate

active

06284148

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for anisotropic etching of silicon.
BACKGROUND INFORMATION
It is known to etch defined features, for example trenches, combs, tongues, flexural beams, or the like, anisotropically with low to medium selectivity into silicon substrates that are preferably utilized with the semiconductor technique. The individual features to be etched in are usually defined by way of etching masks applied onto the silicon substrate, via so-called masking layers, for example a photoresist layer. In the anisotropic etching technique it is necessary to arrive at exactly laterally defined recess in the silicon. These recesses, penetrating in the depth direction, must possess lateral boundaries which are as accurately perpendicular as possible. The edges of the masking layers which cover those silicon substrate regions that are not to be etched must not be underetched, so as to maximize the lateral accuracy of the feature transfer from the mask into the silicon. This results in the need to have etching proceed only on the nature floor, and not on the previously created sidewalls of the features. It is proposed in German Patent 42 41 045 to perform the etching of profiles into silicon substrates using a method which alternatingly provides plasma polymer deposition and plasma etching steps. In this context, deposition and etching steps are performed in a chemical context based exclusively on fluorine compounds; during the inherently isotropic etching steps, forward advancement of the sidewall polymer film applied during the previous deposition steps already effectively passivates the freshly exposed portions of the silicon sidewall, so that the inherently isotropic etching step becomes locally highly anisotropic. This technique of local anisotropy by way of forward advancement of a sidewall film allows relatively wide etching steps at very high speed without etching into the sidewall, which consequently exhibits only minor roughness. In general, the performance of this plasma etching, when it is done with microwave excitation (propagation ion etching or PIE), does not create any appreciable wall roughess. This process does, however, create serious problems in a so-called inductively coupled system with high-frequency plasma excitation (ICP=inductively coupled plasma). With this, a pronounced recess becomes etched into the silicon directly beneath the edge of the photoresist mask. This recess is an effect of the inductive excitation, which is associated with magnetic and electric fields in the region of the substrate, for example a silicon wafer, and appears with greater or lesser severity on ICP systems of various designs. Inductively coupled plasma systems are playing an increasingly important role because of their robustness and versatility, and are inherently well suited for the process described above. A factor playing a major role in the formation of the etching recesses is the fact that the transition region between photoresist mask and silicon represents a discontinuity in conjunction with electric fields of the plasma source, and is exposed to a greater ion bombardment than the lower portion of the sidewall. In addition, the mechanism of the advancing sidewall film is not yet completely effective at the mask edge, and passivation of the sidewall there is thus weaker. This results in the so-called underetching, so that the etched-in silicon features no longer exhibit the necessary accuracy and critical dimensions.
SUMMARY OF THE INVENTION
The method according to the present invention for anisotropic etching in silicon avoids the problems of underetching in previously conventional ICP etching processes by the fact that the quantity of polymer deposited in the course of the polymer deposition steps which are accomplished alternatingly with the etching steps is initially too great, and then gradually decreases. The shortage of sidewall polymer at the beginning of the process is thus remedied by an excess of polymer, so that the sidewall remains sufficiently passivated. The greater ion bombardment at the discontinuity point now also no longer results in any etching of the critical region. It is thus an essential feature and a particular advantage of the process that it operates first with an excess of polymer, and that this excess then decreases in the course of the alternating polymer deposition and etching steps. The disadvantages of an excess deposition of polymer that have occurred hitherto include the occurrence of so-called positive-slope profiles, i.e. the sidewall of the etched trench is no longer perpendicular, but rather tapers with increasing etching depth, until a pointed tip occurs. A correction made, for example, only once to this profile by way of suitable process parameters results once again in the recess problem at the transitional discontinuity point. This also is eliminated by the gradual reduction in deposited polymer in the course of the polymer deposition steps. The so-called initially over-rich process with a positive profile begins with no recess formation, so that the process transitions gradually to one which generates perpendicular profiles with less polymer deposition, and the greater part of the etching operation is performed therewith.
Advantageous embodiments and developments of the method.
In a particularly advantageous embodiment of the method according to the present invention, adjustment of the polymer deposition steps is performed continuously from one process cycle to the next. It is advantageous to begin with a so-called “rich” (i.e. high-polymer) process parameter set, which would result in a positive profile but definitely prevents any recess formation beneath the mask edge. With each of the following cycles the polymer concentration in the process is then slightly scaled back, so that this continuous adjustment of various process parameters minimizes both discontinuities in the profile transitions and the risk of forming a recess. Advantageously, adjustment of the parameter set is accomplished over the first 20 &mgr;m of etching depth if, for example, etching is to occur to a depth of 100 &mgr;m into silicon.
In a further advantageous development of the method according to the invention, present the quantity of deposited polymer decreases in discrete steps. The process begins, for example, with a long deposition time for each step, and etching is performed for a while with that parameter set; the deposition time is then reduced, etching continues for a certain period with the new parameter set, and so on until a parameter set generating a vertical profile has been arrived at.
Advantageously, it is possible to control the decrease in polymer quantity by varying the duration of the etching steps or the polymer deposition steps, since no internal plasma properties change when the duration of the individual steps is changed. Varying the duration of the etching step or the polymer deposition step is simple, and the effects of modification are readily apparent.
A further preferred possibility is to control the polymer quantity by varying the substrate temperature, or to change the pressure during deposition. It is important to note in this context that a sudden transition from a parameter set generating a positive profile to a much less positive one also creates the risk of recess formation: when a transition is made to a perpendicular profile, if it occurs suddenly, the positive profile is very similar in shape to the profile of the resist mask edge (or of the SiO
2
mask edge, if a hard-material mask is being used), and the transition to a less-positive profile shape once again constitutes a definite and significant discontinuity in the sidewall, which once again can cause problems with recess formation in ICP systems. It is therefore advantageous not to perform this adjustment in very large steps, but to transition to a vertical parameter set in several steps. It is advantageous in particular if etching is first performed for a certain distance with the positive parameter set before transi

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