Stencil mask for high- and ultrahigh-energy implantation

Details Stencil mask for high- and ultrahigh-energy implantation Stencil mask for high- and ultrahigh-energy implantation

2002-04-30

2004-06-29

Stein, Stephen (Department: 1775)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S296000, C430S396000, C428S131000, C428S195100, C428S446000

Reexamination Certificate

active

06756162

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a stencil mask for high- and ultrahigh-energy implantation that has implantation openings in a substrate through which the implantation energy can be projected onto a wafer that will be implanted. The invention also relates to a method set up for fabricating a stencil mask of this type.
New kinds of vertical high-voltage silicon components (those that withstand voltages greater than 300 V) require vertical, pillar-type and finely patterned doping regions in the epitaxial drift zone. These so-called compensation components reduce the on resistance by up to one order of magnitude. To fabricate such vertical finely patterned doping regions, from today's standpoint high-energy implantation (up to 25 MeV boron) using silicon stencil masks appears to be the only method actually suitable in order both to expand the manufacturing capacity and to reduce the costs to a significant extent.
From a technical standpoint, fabricating suitable stencil masks for high-energy implantation is problematic since compensation components are based on the principle of balanced doping between the vertical compensation pillars and the basic doping of the epitaxial layer. Deviations in this balance by a few percent already lead to a drastic reduction in the blocking capability. Stencil masks are usually fabricated by phototechnological patterning of SOI wafers and subsequent trench etching. In this case, the trench depth for a 600 V component is approximately 35 &mgr;m. Dry-chemical etching machines which are commercially available nowadays achieve a reproducibility of the sidewall inclination of the etched trenches of about 0.5° to 1.0° on an 8 inch wafer. The conditions and the problems of such a relatively thick stencil mask M are illustrated in the form of a diagrammatic cross section in the accompanying FIG.
7
.
FIG. 7
illustrates how the implantation opening diameter 2r, which is to be regarded as the critical dimension CD, changes from its target value 2r
target
to an actual value 2r
ACTUAL
if the angle &agr;
trench
which specifies the sidewall inclination increases by the value &Dgr;&agr;
trench
. The effective CD dimension specifies a so-called “projected range” Rp, which is equivalent to a given implantation energy E.
Table 1 illustrated in the accompanying
FIG. 8
shows the dependence of the dose fluctuation of the implantation energy relative to the target dose in the event of deviation of the trench sidewall angle &agr;
trench
in the stencil mask M in accordance with FIG.
7
.
The cumulative fluctuation (error propagation) lies in the range from ±40 to 60% of the target dose. Cumulative fluctuations of ±10 to 15% are acceptable for the device (compensation component) to be fabricated. This value already takes account of fluctuations in the resist dimension and in the doping of the epitaxial layer.
The crucial disadvantage of the stencil mask patterned in accordance with
FIG. 7
is that the effective CD dimensions associated with the respective implantation energies are only controlled by the upper opening dimension of the trench and the trench angle &agr;
trench
. In particular for high implantation energies and correspondingly thick stencil masks, even slight deviations from the ideal trench angle give rise to a major effect on the critical dimension CD.
To date, such compensation components have been exclusively fabricated using the so-called construction technique:
First an n-doped epitaxial layer having a thickness of several micrometers is deposited on the substrate. Using a resist mask, a p-type doping is subsequently introduced by a low-energy implantation. In this connection, particular attention must be paid to the accuracy of the resist dimension of the resist mask since this is the parameter that determines the number of implanted ions, and consequently, the balance between the p-type and the n-type doping. The whole process, including epitaxial deposition, phototechnology and implantation, is repeated until the pillar height corresponding to the required withstand voltage has been constructed. The final sub-process includes a diffusion step that causes the implantation regions to diffuse together vertically.
Stencil masks are currently used primarily for ion projection lithography. In this technique, only very low-energy ions are used. The problem of fluctuation of trench angles in the case of very deep trenches does not arise in this case, since the silicon mask only has a thickness of 3.0 &mgr;m. The dimensionally accurate upper part of the trench extends merely to a depth of 150 nm.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a stencil mask for high- and ultrahigh-energy implantation and a method for fabricating the stencil mask which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type.
Following what has been said above, it is an object of the invention to provide a stencil mask that is suitable for a high- and ultrahigh-energy implantation and that significantly reduces the fluctuations shown in Table 1 (FIG.
8
). Thus, it is an object of the invention to enable the development of a high-energy implantation technology for such compensation components in an expedient manner.
In order to achieve the above stated object, the invention proposes departing from the concept of the simple stencil mask in which the implantation openings are defined and formed for all energies by using a CD dimension (Critical Dimension) and a single trench etching process, and instead defining a dedicated dimensionally accurate mask for each implantation energy. In the case of this mask, the critical dimension of the implantation openings is defined in a manner dependent on the respective implantation energy. In a particular embodiment, a stencil mask is composed of a plurality of individual dimensionally accurate masks, with the result that this combined stencil mask is suitable for different implantation energies. The critical dimension of the implantation openings present in a plurality of steps or stages are in each case coordinated with the required implantation energy.
Instead of this, however, it is also possible to provide a dedicated dimensionally accurate mask for each implantation energy and to use the mask in each case for an implantation with a specific energy. In the event of a changeover of the energy, the mask is changed, too. By way of example, for a compensation component constructed from five layers, it is then necessary to fabricate five separate masks with openings whose critical dimension CD is in each case coordinated with the energy used for the implantation.
Preferably, the stencil mask is constructed on an SOI (Silicon on Insulator) base material or is composed of such an SOI base material.
The following method is particularly advantageously appropriate for fabricating a stencil mask for the high- and ultrahigh-energy implantation:
I. Providing an SOI base material with an SOI layer thickness adapted to the respective ion penetration depth, for example 5 &mgr;m for 3 MeV boron, 35 &mgr;m for 20 MeV boron.
II. Etching retrograde openings from the front side of the SOI wafer as far as the SOI oxide layer. This means that for a stencil mask, for example, for a 20 MeV implantation, the first 1.2 &mgr;m (+ safety margins) of this opening (trench) must be exact. The rest of the trench need only satisfy very undemanding requirements.
III. The mask is made transparent from the rear side by wet-chemical etching, for example.
IV. The stencil mask is used as an implantation mask, for example, by bonding or precisely positioning the mask in front of the wafer that will be implanted in such a way that the front side of the mask points in the direction of the wafer that will be implanted.
With the foregoing and other objects in view there is provided, in accordance with the invention, a stencil mask for high- and ultrahigh-energy implantation of semiconductor wafers. The stencil mask includes

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