WO2003061356A1

WO2003061356A1 - Embossing die for fabricating high density interconnects and method for its fabrication

Info

Publication number: WO2003061356A1
Application number: PCT/CH2003/000027
Authority: WO
Inventors: Philippe Steiert; Gerhard Staufert; Alex Dommann; Andreas KÜNDIG; Peter J. Uggowitzer
Original assignee: Elmicron Ag
Priority date: 2002-01-17
Filing date: 2003-01-17
Publication date: 2003-07-24
Also published as: AU2003201247A1

Abstract

The embossing die (61) according to the invention comprises an embossing surface and ridge shaped protrusions (61.1) protruding from this surface. The ridge shaped protrusions (61.1) serve for forming groove shaped indentations into a dielectric substrate during an embossing process. These groove shaped indentations serve for being filled with conducting material during a subsequent plating step, which conducting material may serve as conductor path. In addition to the ridge shaped protrusions, also peg shaped or cone shaped protrusions (61.2) for pre-forming through connections etc. may be present. At least a part of the embossing die (61) comprising the embossing surface and the protrusions are made from one piece of material, namely from an amorphous metal.

Description

EMBOSSING DIE FOR FABRICATING HIGH DENSITY INTERCONNECTS AND METHOD FOR ITS FABRICATION

Inventors: Philippe Steiert, Gerhard Staufert, Alex Dommann, Andreas Kϋndig, Peter J. Uggowitzer

FIELD OF THE INVENTION

The invention is in the field manufacturing High Density Interconnects (HDIs). It more specifically relates to an embossing die for mechanically deforming a dielectric substrate in order to provide it with channel shaped depressions and/or with peg shaped or cone shaped protrusions for pre-forming through connections. Such depressions may be filled with conducting material in order to form conductor paths of the HDI.

BACKGROUND OF THE INVENTION

Traditionally, conductor path structures of HDIs as well as of Printed Circuit Boards (PCBs) are fabricated by photochemical methods. The increasing miniaturization, the wish to fabricate more economically and environmental aspects make alternatives to this fabrication method desirable. The international patent application publication WO 01/50825 discloses a method for fabricating PCBs or HDIs which is based on a new concept. In order to form current paths, channel like depressions are manufactured in a dielectric substrate by means of an embossing tool. Then, the substrate is plated until the depressions are filled. Finally, conducting material is removed in a manner that the areas of the substrate that should not have any conductive surface are free from any conducting coating. The depressions filled with conducting material then form the conductor paths. During the embossing step, also openings for forming through connections or other structures may be formed.

The fabrication of embossing tools for such a procedure is not trivial, especially if the product to be fabricated is a HDI with very fine structures. The sizes of HDI structures due to the increased miniaturization may be as small as a few tens of micrometers or even a few micrometer. Usually, embossing tools of this kind are fabricated by some sort of LIGA technique using parallel X-ray beams in combination with a special resist layer and chemical etching. Recently, a low cost alternative to this procedure based on the use of Ultraviolet Radiation instead of X- rays has been developed.

In the Swiss Patent Application 870/01, a method for producing embossing dies is described, which relies on mechanically deforming a block of material into a master tool and then on electroforming the embossing tool. This method is more flexible and economical than the previously known methods. The produced embossing tools, however, tend to have lifetime which is limited to an extent that the production cost of the tool is not a negligible factor in the total cost of the resulting product.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide an embossing die and a method of fabricating an embossing die which overcomes disadvantages of existing embossing dies and of existing fabrication methods and which especially allows to fabricate HDIs even more economically. Further, the embossing die should be such that it can be manufactured with an embossing surface which is even and smooth down to a very small, sub-micrometer or even nanometer scale so that structures for increasingly miniaturized HDIs can be manufactured.

This object is achieved by an embossing die and a method according to the definition of the claims and according to the description given hereafter.

The embossing die according to the invention comprises an embossing surface and ridge shaped protrusions and/or peg shaped or cone shaped protrusions protruding from this surface. The ridge shaped protrusions serve for forming groove shaped indentations into a dielectric substrate during an embossing process. These groove shaped indentations serve for being filled with conducting material during a subsequent plating step, which conducting material may serve as conductor path. The peg shaped or cone shaped protrusions may be present in addition to the ridge shaped protrusions and are for pre-forming through connections etc.. At least a part of the embossing die comprising the embossing surface and the protrusions are made from one piece of material, namely from an amorphous metal.

A first achievement of this invention is that it has surprisingly been found that amorphous metal material is highly suited for forming embossing dies for forming indentations into a substrate during the fabrication process of High Density Interconnects, despite the requirement that the embossing die structures are extremely miniaturized, i.e. have dimensions in the micrometer region. Embossing dies for producing HDIs have protrusions with dimensions which are different from the dimension of protrusions of existing embossing tools, typically by orders of magnitude. This brings about different requirements for a large variety of material properties including the surface geometry (i.e. evenness, smoothness, corrugation) and physical surface properties (hardness, elasticity, sticking coefficient etc.).

It has been found that amorphous metals allow for an excellent smoothness. One of the reasons for this could be the absence of any crystalline structures and thus of any grain boundaries etc. which could cause some roughness. If the embossing die is fabricated by a replication process from a master or a sub-master, the amorphous state causes it to replicate very accurately and true to the master (or sub-master). In contrast, in the crystalline state energy, minimization, which always governs shaping processes, sometimes causes deviation from perfect replication, the orientation of the grains being boundary conditions. The absence of any grain brings about excellent casting properties. If a master tool has an even and smooth surface, also the cast amorphous product has this property. Structures of the master tool are replicated even down to a nanometer scale. As a consequence, a very smooth surface which may be fabricated results in a low sticking coefficient. This makes possible to form indentations into the substrate which are very close together without substrate material sticking between the indentations corresponding to the protrusions.

A further range of requirements for the tools relates to HDIs being a mass products, and factors such as embossing tool longevity are cost relevant. It has been found that amorphous materials do not exhibit any fatigue. This might be due to the fact that fatigue is often an effect involving grain boundaries, which are absent in amoφhous metals. The lifetime of amoφhous metal embossing tools is found to be longer than for 'ordinary' embossing tools by at least a factor of 10 and often by a factor 100 and more. Also, since there are no grains and thus no grain boundaries, there is no internal friction.

A further advantage of the invention is related to the small (in diameter) dimensions of the embossing tool protrusions in preferred embodiments. The embossing tool, during the embossing process, is pressed against a viscous or plastically deformable substrate material. In this process, a capillary effect causes the viscous material to closely follow the course of the surface even if the pressure applied on the tool is comparably low.

There is no phase transition when the material is cooled, thus there is no change in volume upon cooling from the liquid to the solid state. Instead of a phase transition temperature, these materials have a glass transition temperature. Also this effect has been found to be advantageous under certain conditions.

Still further, The materials have a broad elastic region. In the elastic region, they are almost completely hysteresis free.

A special class of amorphous metals are the bulk-solidifying amoφhous alloys. One known example of such metals is the VITRALLOY 105. Bulk-solidifying amoφhous alloys are materials comprising Zr and are, among other things, characterized by the fact that they solidify in an amoφhous structure even at relatively low cooling rates. Due to this, with these alloys, casting of some structures has been done successfully. According to this invention, the properties of a-metals are to be used for the manufacturing of embossing dies for HDIs. However, for HDIs with their very fine, small size structures, this is not easy for the following reasons:

- Bulk-solidifying amoφhous alloys are highly reactive in the liquid state. There are almost no master tool (or mould) materials which are resistant.

- Of course, if an embossing die comprising a surface with an evenness and smoothness sufficient for HDI embossing puφoses is to be formed, the surface of the master tool has to equally smooth and even. Crystalline metal surface may only be prepared to be smooth on a micrometer scale at most on a tenth of micrometer scale. Thus, metallic moulds are not sufficient.

Also existing methods for forming semiconductor moulds are not sufficient.

According to a special embodiment of the invention, these challenges are met by manufacturing the embossing die with the following method:

In a first step, a crystalline semiconductor material block is provided, e.g. a Si wafer. Then, this block is appropriately structured, e.g. using the Deep Reactive Ion Etching (DRIE) technique. The parameters for a DRIE process resulting in the appropriate surfaces are readily available by equipment producers, e.g. the firm of STS. The resulting surface is precise in form and is sufficiently smooth but is not resistant to Zr. Then, the surface is undergone a material property changing treatment. .This treatment is such that material of microscopic elevations is more affected than material in microscopic pores etc and that a surface layer has different, resistant properties. If the structured block is an Si wafer, the surface treatment may e.g. be an oxidizing the surface resulting in an Silicon oxide layer of a thickness between 0.1 μm and 10 μm, preferably between 0.4 μm and 1.5 μm. The oxidizing process is based on a fluid (i.e. a gas or a liquid) acting upon the surface in a way that the microscopic elevations are more strongly exposed to the oxidizing fluid than e.g. microscopic indentations, this resulting in a highly efficient leveling process. The resulting silicon oxide layer surface is smooth on a nanometer scale. Also, the Silicon oxide exhibits some resistance to Zr corrosion, i.e. it is resistant at least as long as about one second or a few seconds. The product resulting after the surface property changing treatment may thus serve as master tool.

In a next step, bulk-solidifying amoφhous alloy is cast onto the structured surface of the master tool. As alloy material, any bulk-solidifying amoφhous alloy or any alloy with similar relevant properties may be used. Examples comprise VITRALLOY 105 and other alloys with Zr (e.g. between 25 and 80 %), optionally Beryllium (preferred additive) or Scandium (an additive which is preferred for countries in which the use of Beryllium is not legal) and additionally transition metals such as Ni, Cu, or Ti, or other metals such as aluminum, etc. Pertaining to bulk-solidifying alloys, there composition and their production it is referred to the US patents 5,288,344 and 5,368,659, the contents of which are incoφorated herein by reference.

For casting, an element of the alloy placed adjacent to the structured master tool surface and is heated up to a temperature around the glass transition temperature such that it softens or 'melts'. At the same time, in some manner a pressure is applied (e.g. by just placing the element on top of the surface, the pressure being applied by the gravitational force). After softening, the alloy material very rapidly wets the surface and fills the indentations. It is then subjected to a very rapid cooling process. The resulting product meets the above requirements for an embossing die.

The cooling process may be caused by a cool block of material with a high thermal conductivity and a high heat capacitance being pressed against the alloy material. It may as an alternative be caused by the presence of a fluid which partially surrounds the material and which has a comparably high heat conductivity, e.g. He gas or liquid nitrogen.

According to a special embodiment, the amoφhous alloy block during its heating is placed between two elements which are pressed against each other. One of the two elements is the master tool, while the other one is e.g. a flat element such as an unstructured surface oxidized Si wafer or a flat glass plate etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, examples of embossing tools and of fabrication processes of embossing tools according to the invention are explained with reference to drawings. The examples illustrate aspects of the invention but should not be inteφreted as limiting of the scope of the invention. In the drawings

Fig. 1 represents a schematic cross section of two embossing tools and a dielectric substrate,

Fig. 2 shows a schematic cross section of another embossing tool, Fig. 3 represents a schematic cross section of yet another embossing tool,

Figs. 4 through 10 schematically show production steps of a manufacturing process of an exemplary embossing die according to the invention,

Fig. 11 represents a schematic perspective view of an embossing tool according to the invention.

Figs. 12 and 13 schematically represent variations of the manufacturing method according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embossing tools 1, 101 of Figure 1 are made of an amoφhous metal. In principle, any amorphous metal which is stable at room temperature or at a temperature slightly above room temperature and which has a certain hardness may be used. The preferred amoφhous metal material are the mentioned bulk-solidifying amoφhous alloys.

The embossing tools comprise an essentially flat surface 1.1, 101.1 with ridge like or ridge shaped protrusions 1.2, 101.2 for forming groove shaped indentations in a dielectric substrate 2. The height h and width w of the ridge like protrusions in section are usually between 1 μm and 100 μm, preferably below 50 μm or even below 30 μm. The embossing process may, depending on the material of the substrate, be done at an elevated temperature. Concerning the embossing process, it is referred to the publication WO 01/50825 being incoφorated herein by reference. The groove shaped indentations may in a subsequent plating step, preceded by a coating step, be filled by conducting material for serving as HDI conductor paths. Also this plating step is described in the publication WO 01/50825, which is incoφorated herein by reference.

The embossing tools may further also comprise pin like protrusions 1.3, 101.3 for forming through connections during the embossing step.

The protrusions 1.2, 101.2, 1.3, 101.3, of course, do not have to have the rectangular shape shown in Figure 1. As is symbolized by Figure 2, the cross section of the protrusions may vary and be adapted to the dimension of the protrusions, the dielectric substrate etc. The embossing tool 201 of Figure 2 comprises ridge shaped protrusions with a rectangular cross section 201.2 as well as tapered ridge like or peg like protrusions 201.4, and 201.5, respectively. Also other shapes of protrusions are possible and may be useful, for example protrusions with a small inclination of for example between 0.3° and 3^ό.

The embossing tool 301 of Figure 3 is not entirely made of an amoφhous metal. Instead, only a proportion 311 of the embossing tool is made of amoφhous metal. The proportion 311 of the tool that is made of amoφhous metal comprises the surface with the protrusions. The non-amoφhous proportion 312 is made of any suitable material, e.g. Ni or Cu which e.g. may be plated onto the amoφhous proportion. In such a plating process, the material may be provided with additives which cause the back side of the embossing tool to be particularly smooth and even. This embodiment is especially well suited for embossing tools with a relatively high total tool thickness t of above 300 μm, where the e.g. only a surface layer of around 30 μm to 100 μm thickness has to be made of amoφhous metal. More generally, the total embossing tool thickness is usually chosen to be between 30 μm and 10 mm.

It is understood that the shape of the embossing tool for fabricating HDIs may be varied in many ways in order to suit the particular requirements of the HDI to be fabricated.

Next, possible manufacturing methods of the embossing die according to the invention are quickly discussed referring to the very schematic figures.

Figure 4 shows a schematic representation of a block 21 for forming a master tool, i.e. of a four inch Si wafer. Figures 5 and 6 symbolize steps of a DRIE structuring process of the wafer 21 using photoresist layer 23 which is partially illuminated by Vis.-, UV or X-ray radiation 25. DRIE is a method for providing Si wafers with structures with vertical walls or with walls showing a wanted small inclination, and being smooth on a 10 nm scale. Since the DRIE process as such is known, the process step leading from Figure 4 to Figure 6 is not described in more detail here. The structured casting surface 21.1 of the SI wafer comprises indentations 21.2 corresponding to the indentations to be formed in the HDI substrate. After removal of the photoresist layer 23. In a next step, the casting surface 21.1 of the wafer is oxidized, resulting in a silicon oxide layer 21.3 of a thickness between 0.1 μm and 10 μm. The surface of this silicon oxide layer 21.3 is flat on a nanometer scale. The resulting product 31 in the following serves as a master tool (Figure 7). In a next step, the master tool 31 is placed in a high vacuum chamber with a pressure of e.g. 10^"5 mbar or less. On top of the master tool, a piece 33 of a bulk-solidifying amoφhous metal alloy is placed. In the embodiment described here, the piece is disk shaped. Then, the piece is heated, e.g. by induction. Figure 8 symbolizes heating coils 35 for this process. When a certain temperature around the glass transition temperature, e.g. slightly below the glass transition temperature, is reached, the alloy material becomes soft and gets into the indentations 21.2 of the master tool. This is a very rapid process, due to the excellent mutual wetting properties of the alloy and of Silicon oxide. Immediately thereafter, for example, within less than a second after the softening of the alloy, a large copper block 41, which may optionally comprise cooling means, is placed on top of the amoφhous alloy piece 33, instantly cooling and thus hardening it (Figure 9).

Finally, the embossing tool has to be separated from the master tool. This can for example be done by a material selective etching process in which the entire master tool is dissolved. Due to the longevity of the embossing tool according to the invention, re-using the master tool is not absolutely required for economical reasons. A further option is the mechanical removal of the master tool. Figure 10 shows the finished die 51 after it is released from the master tool.

An example of an embossing die 61 with ridge like protrusions 61.1 and peg like protrusions 61.2 is shown very schematically in Figure 11.

The piece 33 of bulk-solidifying amoφhous metal material may, in the softened state, be compressed in order to be flattened and to cover a larger area. This may for exmple be done by a further oxidized silicon wafer 71 or a glass plate with a relatively even surface, which is pressed onto the piece 33 as shown in Figure 12. The further wafer or glass plate etc. may or may not be mounted on the copper block 41. Since it does not have as high a thermal conductance, the alloy does not immediately harden upon being in contact with it but may be flattened before it hardens still relatively rapidly, i.e. within one second.

The cooling process may be caused by means different from the ones shown above. Of course, the solid block of material does not have to be a copper block but may be any other material with a sufficient heat conductance and heat capacitance. As an alternative, the cooling may be caused by the formed piece 33 being immersed in a fluid. E.g. the vacuum chamber may after the heating process be instantly filled with Helium as symbolized by Helium supply means 81 in Figure 13, or by an other appropriate gas/fluid (e.g. liquid nitrogen),.

Depending on the thickness of the amoφhous metallic material, the embossing die may be completed by a non-amoφhous proportion on the backside (the frontside being the side with the embossing surface), e.g. a Ni or a Cu proportion.

Embossing die elements fabricated according to the above method may be cut to size very accurately. A plurality of such elements can then be added, with an accuracy corresponding to the accuracy of the positioning of the individual structures, i.e. with a micrometer accuracy, to form a larger embossing die.

Numerous other embodiments can be envisaged without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. Embossing die for being used in an HDI manufacturing process comprising

an embossing surface,

said embossing surface comprising ridge like protrusions for forming grooves in a dielectric substrate,

said protrusions and at least a proportion of embossing die being made from one piece of material,

wherein said proportion of the embossing die comprises at least the embossing surface

and wherein said one piece of material is essentially made of an amoφhous metal.

2. Embossing die as claimed in claim 1 further comprising pin like protrusions for forming through connections.

3. Embossing die as claimed in claim 1, wherein the ridge like protrusions in cross section have a width w of between 1 μm and 100 μm and a height of between 1 μm and 100 μm.

4. Embossing die as claimed in claim 1, wherein the amoφhous metal is a bulk- solidifying amoφhous alloy, e.g. comprising Zr.

5. A method for manufacturing an embossing die for embossing dielectric substrates in an HDI manufacturing process, comprising the steps of

providing a master tool with a cast surface comprising indentations corresponding to the indentations to be formed in the dielectric substrates,

covering the cast surface by material of a metallic alloy in a manner that the metallic material has an amoφhous structure, and

removing the master tool from the metallic material.

6. A method as claimed in claim 5, wherein the master tool is manufactured by structuring, preferably by deep reactive ion etching, a cast surface of a semiconductor substrate block, and by treating the cast surface in a manner that corrugations and microscopic elevations and indentations are leveled out and that a protective layer is created.

7. Apparatus comprising means for carrying out the method as claimed in claim 5 or 6.

8. Use of an amoφhous metallic alloy for producing an embossing tool for manufacturing conductor paths in High Density Interconnects (HDIs).

9. Use according to claim 8, wherein said amoφhous metallic alloy is a bulk- solidifying amoφhous metallic alloy.