Method for Producing a Multi-Step Structure in a Substrate

The invention concerns a method for producing a multi-step structure in a substrate.

In many technical fields it is necessary to produce a step structure in a substrate. Single-step structures are generally produced by removing substrate with the aid of, for example, mechanical, physical or chemical processes. For larger numbers and higher accuracy requirements, photoprocesses are frequently employed for defining the location of the step on the substrate. In such processes, a photoactive substance is deposited on the structure, exposed through a mask, and developed, so that areas for the removal of substrate are passivated or substrate is removed through a mask overlying the substrate during removal.

For producing a multi-step structure, a sequential approach is normally adopted. Irritially, a single-step structure is formed, in which further steps are produced. This necessitates however that the first step in the substrate has a uniform depth so that irregularities existing therein are not repeated in the next steps. Apart from this, it is difficult with this approach to position the second photomask and inaccuracies may occur when the second step is formed in the substrate. Thus, this approach is only suitable for steps of a particular size for which the accuracy requirements are not too exacting.

Such step structures are frequently used for computer components, such as the wiring planes of semiconductor chips or the thin-film wiring on multilayer ceramic substrates.

As components of this type are being increasingly miniaturized and have to meet ever higher accuracy requirements, the previously described approach has reached its limits so that a new concept had to be found for producing such substrates.

From IBM Technical Disclosure Bulletin, Vol. 33, No. 2, July, 1990, p. 447 "Dual-Image Resist for Single-Exposure Self-Aligned Processing" a photoresist system is known which consists of a wet developable first and a dry developable second photoresist which are radiated at different wavelengths. Such a system may be used to fabricate step-shaped structures from a substrate by exposure to a first wavelength and development, followed by a second exposure, development and processing after the first opening has been formed. This approach is limited however since photoresists responsive to different wavelengths are expensive and the number of steps thus produced cannot be increased at random, not to mention the fact that the precision obtainable depends on how sensitive the second photoresist is to the first wavelength.

Therefore, it is the object of the invention to provide an inexpensive method by which finest multi-step structures can be produced in a substrate with great accuracy.

This object is accomplished by the method according to the invention comprising the steps of:

a) producing a substrate (1);

b) depositing a first photoresist layer (2) on the substrate (1);

c) forming a first opening (4) in the first photoresist layer (2) by exposure through a first mask (3), development and postbaking of the first photoresist layer (2);

d) depositing a second photoresist layer (5) on the first photoresist layer (2) and on the substrate (1) in the area of the first opening (4);

e) forming a second opening (7) in the second photoresist layer (5) by exposure through a second mask (6), development and postbaking, with said second opening (7) overlying and being larger than the first opening (4);

f) transferring the first opening (4) to the substrate (1);

g) removing the first photoresist layer (2) to the extent to which it is not covered by the overlying second photoresist layer (5) and simultaneously removing the top-most photoresist layer (5);

h) transferring the second opening (7) to the substrate (1) and simultaneously deepening the first opening (4) in the substrate (1);

i) removing the first photoresist layer (2).

By this method the topography problem that inevitably arises during the production of finest vertical steps in a substrate is shifted to thin layers to be applied to the smooth substrate surface, which are then sequentially processed.

An advantageous further development of the invention consists in steps f) and g) as well as h) and i) being carried out simultaneously. A method thus modified requires fewer steps.

It is also within the scope of the invention that after completion of step

e) the following steps are carried out at least once:

d') depositing a further photoresist layer (8) on the existing photoresist layers (2, 5) and on the substrate (1) in the area of opening (4);

e') forming a further opening (10) in the further photoresist layer (8) by exposure through a mask (9), development and postbaking, with said further opening (10) overlying and being larger than the opening (7) in the photoresist layer (5); and that after completion of step h), the following steps are carried out at least once:

g') removing photoresist layer (2) to the extent to which it is not covered by the overlying photoresist layer (7) and simultaneously removing the top-most photoresist layer (5);

h') transferring the further opening (10) to the substrate (1) and simultaneously deepening the existing openings (4, 7) in the substrate (1).

In this way, an arbitrary number of steps can be formed in the substrate.

If the invention is used, for example, to produce conductors for computer components it may be expedient for the first opening (4) to extend through the substrate (1).

The invention is described in detail below with reference to drawings. Although the invention may be used for any technical applications requiring a multi-step structure to be produced in a substrate, it will be described here by way of an example concerning the production of computer components.

In the drawings:

FIG. 1A is a perspective view of a multi-step structure producible by the method according to the invention;

FIG. 1B is a sectional view along line 12--12 of the structure of FIG. 1A;

FIG. 2A shows step a of a first specific preferred embodiment of the process of the invention.

FIG. 2B shows step b of a first specific preferred embodiment of the process.

FIG. 2C shows step c of a first specific preferred embodiment of the process.

FIG. 2D shows step d of a first specific preferred embodiment of the process.

FIG. 2E shows step e of a first specific preferred embodiment of the process.

FIG. 2F shows step f of a first specific preferred embodiment of the process.

FIG. 2G shows step g of a first specific preferred embodiment of the process.

FIG. 2H shows step h of a first specific preferred embodiment of the process.

FIG. 2I shows step i of a first specific preferred embodiment of the process.

FIG. 3A shows step a of a second specific preferred embodiment of the process of the invention.

FIG. 3B shows step b of a second specific preferred embodiment of the process.

FIG. 3C shows step c of a second specific preferred embodiment of the process.

FIG. 3D shows step d of a second specific preferred embodiment of the process.

FIG. 3E shows step e of a second specific preferred embodiment of the process.

FIG. 3F shows step f of a second specific preferred embodiment of the process.

FIG. 3G shows step g of a second specific preferred embodiment of the process.

FIG. 3H shows step h of a second specific preferred embodiment of the process.

FIG. 3I shows step i of a second specific preferred embodiment of the process.

FIGS. 1A and 1B show a step structure formed in a substrate 1 by means of the inventive method. Substrate 1 contains a first opening 4 which leads to a second opening 7, with the latter opening 7 being wider than opening 4. Adjacent to opening 7, there is a trench 11 which is formed in substrate 1 simultaneously with opening 7. Openings 4 and 7 are shown to extend through substrate 1 down to substantially the same depth, although the method according to the invention is also suitable for forming non-continuous openings without debris and grasslike structures, with the depth and shape of each opening in the substrate being arbitrarily selectable.

For producing computer components, such multi-step structures are formed inter alia in wiring planes of semiconductor chips and in the thin-film wiring of multilayer ceramic substrates. According to FIGS. 1A and 1B, for example, a dielectric substrate 1 is used for such applications. In this substrate, a trench 11 is formed which, if filled with a conductive substance, constitutes a conductor. This conductor must be connected through a via, i.e. openings 4 and 7 filled with conductive substance, to an underlying conductor.

FIGS. 2A-2I and 3A-3I show the method steps required according to the invention for producing such a substrate. FIG. 2A-2I shows a method for, producing a two-step structure and FIG. 3A-3I a method for producing a structure with more than two steps by way of a three-step structure. The substrate 1 of step a) may consist of organic or inorganic material and must merely be removable by a mechanical, physical or chemical process which does not attack the photoresist layers. Preferably the substrate is of a continuous material as shown for substrate 1 in all the figures. Owing to the relatively high selectivity they permit during removal compared to known photoresists, organic substrate materials are an obvious choice for this purpose. Such materials include a number of polymerized and non-polymerized materials, of which polyimide, for example, is particularly suitable. If the original surface is unsuited for the proposed approach, the substrate 1 may be deposited thereon in the form of a uniform layer.

This is the case, for example, during the production of multilayer ceramic substrates, where a layer of, say, polyimide, which may be planarized by micromilling in the solid state, is deposited as substrate 1 on the sintered glass ceramic substrate.

When substrate 1 has a planar surface, a first photoresist layer 2 is deposited thereon (step b). The choice of photoresist depends on substrate 1 and its removal. The photoresist must adhere well to substrate 1 and not be attacked, at least at the beginning of the removal process. Positive or negative working liquid or dry resists may be used for such applications.

For multilayer ceramic substrates, a conventional positive working photoresist is employed, for example. It may be advantageous in such cases to use a silylated photoresist or to silylate a positive resist after development in particular for an organic substrate material.

In step c), the photoresist layer 2 is exposed through a first mask 3 with at least one opening and then developed and postbaked.

On this first photoresist layer 2, a second photoresist layer 5 is also deposited (step d) in the area of the first opening 4 thus formed. This second photoresist layer 5 may consist of the same or another material than the first photoresist layer 2. The individual photoresist layers may have different thicknesses.

A second opening 7 larger than and overlying the first opening 4 is also formed in the second photoresist layer 5 by exposure through a second mask 6, development and postbaking (step e). In this manner, a double layer is obtained, whose first opening 4 and second opening 7 respectively correspond to the via and the conductor to be formed later on.

For forming more than two steps in the substrate, steps d) and e) are repeated (steps d' and e' in FIG. 3D and 3E respectively), thus producing a random number of photoresist layers with openings overlying each other. The double or multiple layer produced in this way is sequentially processed in the subsequent steps.

Initially, the first opening 4 is transferred to the substrate 1 (step f), using a mechanical, chemical or physical process suitable for removing substrate 1. For this purpose, a chemical or physical etch process may be employed. Thus, for example, dry etch steps which are extremely selective and uniform with respect to the photoresist layers, may be used to remove a polyimide substrate 1 without debris and grasslike structures at high etch rates. An oxygen plasma, for example, yields a uniformity exceeding 5%, so that an etch stop layer is not required.

Then the first photoresist layer, to the extent to which it is not covered by the overlying second photoresist layer 5, is simultaneously removed along with the latter layer (step g), so that the pattern of photoresist layer 5 is transferred to photoresist layer 2. For this purpose, any available removal process is generally suitable. However, for the above-described material constellation it is expedient to change the etch gas and to use an etch gas containing a fluorine compound, such as CF.sub.4. Such an etch gas removes even silylated photoresist layers with a high degree of uniformity.

Subsequently, in step h), the second opening 7 is transferred to the substrate, and the first opening 4 is formed down to the desired depth in substrate 1. Depending upon the application of the step structure, it may be expedient for the first opening 4 to extend through substrate 1, as, for instance, for conductors with a via connecting them to underlying conductors. The second opening may also be structured as a conductor and be transferred as such to the substrate. For step h), the same transfer process as for step f) is generally used.

When more than two photoresist layers have been deposited and structured, they are subsequently transferred to the substrate as additional steps by repeating steps g) and h) and then deepened (steps g' and h' in FIG. 3A-3I).

In a further step i) , after the double or multiple layer has been processed, the remaining photoresist layer 2 is removed by a process as in step g).

Steps f) and g) as well as steps h) and i) may be carried out simultaneously by combining the removal processes required for them, for example, by admixing up to about 3% of a fluorine compound, such as CF.sub.4, to an oxygen plasma or by removing the photoresist layer in a pure oxygen plasma.

As in the described anisotropic etch process the different etch steps on the photoresist layers and in the substrate are carried out by merely varying the etch gases, the removal tool does not have to be exchanged. Thus, the various etch steps required for implementing the method may be carried out in a plasma etch reactor of a more recent type operating at a very low pressure (below 10 microbar), such as a DECR (Distributed Electron Cyclotron Resonance) tool. If necessary, an ion beam etch step for removing a chromium layer may be carried out in the DECR tool on a pad below the substrate. The DECR tool yields a uniformity of about 1.5% and better. The anisotropic etching process for producing stepped structures inherently produces openings with lateral side-walls extending about perpendicularly from the substrate surface to an etched bottom-wall or through another surface of the substrate (as shown in FIGS. 2 & 3), and having wall surfaces inherently exhibiting the specific microstructure typical of the etching process used for the material of the substrate I. By shifting the topography problem previously encountered during the production of finest step structures to thin layers which are deposited on a smooth surface and then selectively processed, it is possible to produce even minimum step structures for which laser ablation is unsuited. Thus, the method according to the invention permits integration densities which so far have been unobtainable for thin-film packaging products.