WO1991011025A1

WO1991011025A1 - A method for manufacturing of mineature impedance matched interconnection patterns

Info

Publication number: WO1991011025A1
Application number: PCT/NO1991/000005
Authority: WO
Inventors: Hans Bernhard GRØNLI
Original assignee: A.S Micro Electronics
Priority date: 1990-01-16
Filing date: 1991-01-16
Publication date: 1991-07-25
Also published as: JPH06503680A; EP0510049A1; NO900229D0; AU7068791A

Abstract

The Combi-Film-Hybrid (CFH) technology combines multilayer thick-film and thin-film on the same substrate to build high-density interconnections with integrated resistors for multi-chip modules. The technology is based on two or more multi-layer thick-film conductor layers with printable thick-film dielectric layers between the conductors including via-holes and with one or more thin-film signal interconnection layers on top of the thick-film structure using thick-film or glass between the last printed thick-film conductor and the thin-film, and with polyimide dielectric between the thin-film layers. All dielectric layers include via-holes. Each thin-film layer consists of a resistive film, a diffusion barrier film and a conductive film photopatterned to include thin-film resistors. Transmission lines for critical signals are built of thin-film and by using the underlaying thick-film layer as reference plane and hereby making possible design of high-density interconnections with characteristics impedance matched to the integrated circuits being used, to the package itself and outside interconnection.

Description

A METHOD FOR MANUFACTURING OF MINEATURE IMPEDANCE MATCHED

INTERCONNECTION PATTERNS

This invention describes a method for manufacturing mineature interconnection systems with well controlled characteristic impedances making possible good impedance matching to built- in circuit elements and with integrated resistor terminations.

The technological trend within integrated circuits is strong¬ ly influenced by increasing packing density and higher signal speed making possible building of increasingly more and more complex systems. An important limitation in system integra¬ tion is linked to the methods that are available for making interconnections between the integrated circuits. Short sig¬ nal paths combined with high-density impedance matched inter¬ connections with low cross-talk is most important for perfor- mance. The interconnection method to be used must be well controlled in a production environment with high production yield, high quality and very reliable in use.

Printed circuit board technique is the most important method to build complex interconnection systems. Traditionally, thick-film or thin-film technology are being used to improve packing density further by building multi-chip modules. Thick-film and thin-film hybrid integrated circuits are usually looked upon as techniques for different, but overlap- ping application areas.

Thin-film technology has the advantage of good line-width resolution, very stable conductors and resistors even at extreme environmental conditions and good bondability for gold wire bonding. An important limitation has been that the technology first of all is a one-conductor-layer method, although this is now in the process of being changed by using polyimide dielectrics. Another limitation is only one low-ohmic resistive layer making high-ohmiσ resistors inconvenient to design as part of the film pattern.

Thick-film has the advantage of relatively lower processing cost than thin-film, in particular in medium and higher quantities. Important technical advantages are easy proces¬ sing of multi-layers, resistance values over a large varia¬ tion range and great flexibility to use a variety of assembly methods for added active and passive components. Important practical limitations are linked to pattern resolution and edge definitions for both conductors and via-holes in di¬ electrics when using thick-film.

Transmission lines in hybrides are traditionally built by using the relatively thick substrate as a dielectric and with reference plane on the other side of the substrate. This gives wide lines and distances between lines when 50 ohm characteristic impedance is required, thus yielding geomet¬ ries which are not suited for complex patterns and bonding areas on integrated circuit chips. More optimized intercon- nection systems should have line-widths in the range 25-50 microns and distances between lines in the order of 50-100 microns and a dielectric layer between the 50 ohm transmis¬ sion line and reference plane also in the order of 20-50 microns when using dielectrics with relative dielectric constant in the order of 5-10.

Standard thick-film technology is giving dielectric layers in the thickness range 20-50 microns with good thickness control, but thick-film conductor lines-widths in the range 25-50 microns will require a very special manufacturing metho .

Transmission lines photopatterned and etched in thin conduc¬ tor films are relatively straight-forward to manufacture with line-widths in the 25-50 microns range, and multilayer thin- film technology using polyimide as signal dielectric have made high-density interconnection systems with 50 ohm charac¬ teristic impedance convenient to make on silicon as well as on ceramic substrates. Multilayer thin-film interconnection systems with polyimide dielectrics is a relatively demanding technology to develop and control in production.

The invention relates to Cornbi-Film-Hybrid (CFH)-technology which is a method for making mineature multi-layer high- density interconnection systems on thick-film quality ceramic substrates, i.e. combining thick-film technology and thin- film technology in a new way. The manufacturing process starts with a thick-film ceramic substrate. On this substrate a multilayer thick-film interconnections pattern is being built by using standard methods, e.g. including two or more conductor layers with thick-film dielectric layers with vias between the conductor layers or thick-film patterns on both sides of substrates including filled via-holes in the substrate and thick-film resistors on one of the sides. A thick-film dielectric layer with via-holes is built on top of the last made thick-film conductor layer for on-side thick- film interconnection systems or the side without thick-film resistors on two-sided patterned substrates. On top of this dielectric layer one thin-film interconnection system is being built by using vacuum deposion methods, electroplating, photopatterning and etching for circuits of lower complexi¬ ties and two-layer thin-film metal interconnection system by using polyimide dielectric with via-holes between the two thin-film conductor layers for more complex interconnection systems. Thin-film resistors are etched out in the top thin- film layer. Two alternative thin-film material systems are being used with a sandwich-structure of tantalum-nitride resistive film, molybdenum diffusion barrier film and gold conductor as one of the material systems and chromium-nickel resistive film with gold conductor film on top as the second alternative. When two thin-film interconnection layers are needed a thin layer of plated copper is beeing used on top of the first made thin-film interconnection system to obtain good adhesion between the conductor system and the overlaying polyimide. Thin- ilm resistors made of tantalum nitride are thermally stabilized at 300 degrees after etching. The thin- film resistors can be laser trimmed to minute tolerances. Thick-film interconnection patterns and thick-film resistors can be added on the other side of the substrate as part of the total process sequence by using substrates with via-holes through the substrates by filling the via-holes with metal to make electrical contacts between the two sides of the substrate. All thick-film patterns including filling the via- holes are completed before starting the thin-film processing when patterns on both sides of the substrate are being used.

The CFH-technology can be tailored to all major known assembly methods for active and passive components on the substrate, including die attach by eutectic bonding, solder bonding and glueing and making electrical connections between add-on components and the substrate conductor system by gold wire bonding or aluminium wire bonding. Tape automated bonding and beam-lead bonding can also be used as connection methods, as well as soldering of surface-mount components and "flip-chip". Standard laser trimming can be used for the integrated thin- film resistors.

The characterizing features of the invention will also appear from the attached claims and the description below with reference to the drawings.

Figure 1 is a cross-section 3-dimensional view of a CFH- substrate with two thick-film conductor layers and one thin- film conductor layer and integrated resistor showing the prefered embodiment of the invention.

Figure 2 is showing thin-film transmission lines on top of a dielectric layer over a voltage reference plane made in thick-film (figure 2 b) compared with convensional technology (figure 2 a).

Figure 3 is a cross-section view of a CFH-substrate with two thick-film conductor layers and two thin-film conductor layers with thin-film resistors in the last made thin-film layer.

Figure 4 is a cross-section view of a CFH-substrate with one thick-film conductor layer and thick-film resistors on one side of the substrate and with two thick-film conductor layers and two thin-film conductor layer and integrated resistors on the other side and with fully metallized through-holes as electrical connections between the two sides of the substrate.

Figure 5 a - 5 n show the result of a series of process steps used to construct the CFH-substrate interconnection shown in figure 4. In passing, it should be mentioned that figures 1, 3 and 4 are showing cross-sections through three different principle structures that can be made by combining thick-film and thin- film by this new method. Figure 4 is thus a more complex structure and the structures in figures 1 and 3 can be derived if some of the process steps to make the structure of figure 4 are removed.

Alumina substrate is used as starting material for processing of CFH-substrates. This will usually be 96% thick-film quality alumina. The number of conductor layers that are first built up on this substrate by thick-film screenprinting methods will depend on circuit complexity and electrical and mechanical requirements. Normally this will usually be two or more layers with double printed dielectric layer with via- holes between the conductor layers as shown in figure 1 where structure 1 is the substrate, 2 a, 2 b, 2 c, 2 d are printed conductor lines of the first printed layer, 5 a, 5 b, 5 c are printed conductor lines of a second printed conductor pattern, and 3 is a printed dielectric film with via-holes. 4 represents compensation printed conductors in these via- holes. The printed conductor layers can among others be used as signal lines, power lines and ground. In circuit with thin-film transmission lines the upper thick-film conductor layer will also be used as a reference plane for these lines. Over the last made thick-film conductor layer a new dielectric layer 6 is being made also containing via-holes 10. This dielectric layer can also be made by standard thick- film printing methods by making this layer in one or more prints by using glass-based printing materials with well controlled relative dielectric constant, giving good thick¬ ness uniformity and reproducability and planar smooth surfac with a minimum of cracks and pin-holes. Another requiremen for the dielectric layer 6 is to obtain well controlled via- holes 10 with controlled slopes down to the conductor layer 5 b below as indicated in figure 1. This is important for obtaining good control of the thin-film conductor pattern 9 in the via-holes 10. In order to obtain well controlled low- ohmic contacts 11 between the thick-film pattern 5 b and the thin-film interconnection pattern 9 gold based thick-film conductors are chosen for the contact areas 10 using gold, gold-platinum or gold palladium.

After completion of the thick-film processing a multi-layer thin-film structure is made by sputtering or evaporation, followed by electroplated full thickness of the structure. The multi-layer film consists of an underlayer of tantalum nitride resistive layer labelled 7 (7 a, 7 b, 7 c) in figure 1, a molybdenum diffusion barrier film 8 (8 a, 8 b,

8 c) and gold conductor film 9 (9 a, 9 b, 9 c). Conductor interconnection lines are defined in this multilayer film by standard photolitographic and etching methods shown as 9 a,

9 b and 9 c. If thin film resistors are to be used this is done by using two photolitographic and etching processes, defining and etching the conductors in the first process and the resistors in the following process. In figure 1, two resistors are shown, labelled 12 a and 12 b, respectively.

Figure 2 is showing how thin-film transmission lines are processed using the CFH-technology (figure 2 b) compared with conventional technology (figure 2 a). Transmission lines are made as part of the thin-film interconnection system by designing conductor lines 19 on top of the dielectric layer 18 over a reference plane of thick-film conductor, material 17. The characteristic impedance of such lines can be adjusted by varying the width of the line 19 in relation to the thickness and dielectric constant of the dielectric layer 18. The use of a dielectric layer of a thickness typi- cal for printing processes (i.e. in the order of 20-50 microns) is matching well to line widths easy to make by standard thin-film processes for 50 ohm lines. An order of magnitude reduction in line width and distance between lines is obtained by the use of the CFH-technology compared to conventional techniques using the substrate as signal dielectric. Figure 2 a shows the dielectric substrate 13 with the reference plane 14 and the transmission line 15 used according to conventional methods.

Multi-layer thin film interconnection systems are part of the CFH-technology integrating this with multi-layer thick-film. A structure containing two thick-film conductor layers and two thin-film conductor layers with thin-film resistors as part of the last made thin-film interconnection pattern is shown in figure 3. On this figure reference numeral 20 is the ceramic substrate,- 21 a and 21 b are parts of the first made thick-film conductor layer, 22 is the first made thick-film dielectric layer with via-holes filled with thick-film conductor 23, 24 a, 24 b, 24 c, 24 d, 24 e are parts of the second made thick-film conductor layer using gold or gold- based pastes in the via-areas of the dielectric layer above. Reference numerals 25 a, 25 b, 25 c are parts of the signal dielectric layer 6 of figure 1 with via-holes in the same way as explained with reference to figure 1. On top of the dielectric layer 25 a-c is the first made thin-film inter¬ connection system labelled 26 a, 26 b, 26 c, 26 d providing electrical contacts in the via-holes of said dielectric laye 25 a-c to the thick-film conductors 24 a-e below. This thin- film conductor system is, as shown in detail in figure 3 c, made of a multilayer of tantalum-nitride 35, molybdenum 36, gold 37 and with an adhesion layer of copper or nickel 38 o top in all areas covered with polyimide 39. The polyimid layer is shown as areas 31 a, 31 b, 31 c, 31' d of th 91/11025

figure 3. The tantalum nitride and molybdenum can be replaced with nickel chromium as an alternative solution. In the via- hole areas over the first made thin-film conductor layer as shown in detail in figure 3 a the copper or nickel overlayer has been removed. The second thin-film multi-layer is also vacuum deposited and plated to final thickness and etched by standard photolitographic masking methods in two photolito¬ graphic steps, one step for the conductor lines and one for thin-film resistors. The conductor lines 32 a, 32 b, 32 c, 32 e, 32 f are made using the same multilayer material system as in the first thin-film layer but not using copper or nickel top layer; using tantalum nitride resistive layer shown as layer 40 of figure 3 d, molybdenum 41 and gold 42; or as an alternative to use nickel chromium in layer 40 and gold 42. The thin-film sandwich-structure in the via-areas with electrical contact between thin-film layer one and two is shown in figure 3 a with a layer 43 of tantalum nitride, a layer 44 of molybdenum, a layer 45 of gold forming the first thin-film structure, and layer 46 of tantalum nitride, a layer 47 of molybdenum and a layer 48 of gold of the second thin-film structure. One thin-film resistor 33 is shown with its contact area in figure 3 b with tantalum nitride in layer 49, molybdenum in layer 50 and gold in layer 51.

Figure 4 illustrates the same structure as that of figure 3 with added via-holes in the ceramic substrate and an additional thick-film conductor layer and thick-film resistors on the other side of the substrate. This is to show the full flexibility of the CFH-technology making possible integration of multi-layer thick-film and thin-film on the same substrate and enabling patterns on both sides of the substrates including resistors. The metallized via-holes 52 a and 52 b in the substrate make electrical connections between the multi-layer structure on the side defined in figure 3 and 10

conductors on the other side of the substrate 53 a, 53 b, 53 c, 53 d. Thick-film resistors 54 a, 54 b are shown on the same side of the substrate. Very complex high-density interconnection systems can be designed and manufactured by using this method, thus enabling the soldering of components on the side having the interconnec¬ tions 53 a, 53 b, 53 c, 53 d, and wire-bonding or TAB-bonding of VLSI-chips on the other multi-layer side. More thick-film layers can be added on both sides of the substrate than actually shown in figure 4, simply by using standard thick- film methods.

In the disclosure below the novel CFH-technology for manufac¬ turing of mineature interconnection systems is to be described in some more detail by reference to the making of the embodiment shown in figure 4. The structure is shown for each main process step in figures 5 a - 5 n. The less complex structures shown in figures 1, figure 2 and figure 3 can be derived from the same process flow by reducing the number of process steps and using a substrate without via-holes.

The process starts with a ceramic substrate 55 with via-holes 56 a, 56 b when patterns on both sides of the substrate are needed as shown in figure 5 a. This substrate can be of alumina, aluminiumnitride or ^~ other material suitable fo thick-film printing and firing. Thick-film quality 96 alumina will in most cases be a good low-cost solution.

In the following the expression "front-side" of the substrat is referred to as the substrate side with the multi-laye thin-film and thick-film structure and "back-side" th opposite side with the single-layer thick-film pattern. 91/11

11

The first process step is to print and fire the thick-film interconnection pattern on the "back-side" of the substrate. Such a simplified pattern is shown in figure 5 b with conductor lines 57 a, 57 a, 57 c, 57 d, some of which partly filling of the via-holes 58 a, 58 b.

This is followed by a printing and firing step of a first thick-film conductor layer on the "front-side" of the substrate as shown in figure 5 c, where conductors are labelled 59 a, 59 b and with through-hole filling 60 a, 60 b from this side also.

On top of the first conductor layer and "front-side" there is printed and fired a first thick-film dielectric layer 61 a, 61 b with via-holes 62 to the conductors 59 a as shown in figure 5 d. To reduce the possibility for pin-holes in this dielectric layer two prints of the same pattern will usually be made. The via-holes are thereafter filled with thick-film conductor paste 63 and fired, as shown in figure 5 e.

The next process step is printing and firing of the second thick-film conductor layer labelled 64 a, 64 b, 64 c, 64 d, 64 e in figure 5 f. Conductor line 64 a will now have electrical contact to the conductor layer 59 a by the metallized via-hole 63.

More thick-film dielectric layers interconnection layers can be added at this point in the process sequence to build up more and more complex multi-layer structures.

When the last made thick-film conductor layer has been added the next process step will be to make the important signal dielectric layer. 1/11025

12

In the following description -a printable thick-film di¬ electric will be described. Figure 5 g illustrates how the structure is further constructed by printing the dielectri film 65 a, 65 b, 65 c with via-holes 66 a, 66 b. After printing the desired thickness the substrates are fired wit a maximum temperature reflowing the glass-based paste t obtain a smooth uniform dielectric film with good control o the size and slopes of the via-holes.

Thick-film resistors 67 a, 67 b are printed and fired on th back-side of the substrate as shown in figure 5 h, and thi must be done before thin-film deposition.

The thin-film structure 68 is deposited by evaporation or sputtering and electroplated over the complete "front-side of the substrate, as shown in figure 5 i, first by depositin the tantalum nitride layer 69, then the molybdenum layer 70 and finally a flash layer of gold. A gold film 71 is the plated to reach its full thickness. For multi-layer thin-fil interconnection systems as described here a copper film 72 i also added using electroplating. This copper layer is t obtain good adhesion between the conductor film and th subsequent layer of polyimide above. In an alternative of th thin-film 68, nickel-chromium is used in layer 69 followed b layers of gold and copper.

The thin-film interconnection pattern is defined by standar ^' photolitographic masking and etching methods resulting in structure as shown in figure 5 j with conductor lines 73 a, 73 b, 73 c, 73 d having electrical contacts 74 a, 74 b to th thick-film conductors below in the via-hole areas of th signal dielectric. For substrates with one-layer thin-fil conductors thin-film resistors will be etched out in th tantalum nitride or nickel-chromium layer as the next step. For substrates with two thin-film conductor layers the next step will now be to use photosensitive polyimide by spinning, prebaking, exposing, developing and curing the polyimide to get a structure as shown in figure 5 k. In practice this film pattern 75 a, 75 b, 75 c, 75 d, 75 e with via-holes 76 a, 76 b, 76 c, 76 d is relatively thinner than indicated on the drawing figures. After curing, the copper layer on top of the thin-film pattern 68 is etched away in the via-areas 76 a-d.

The second thin-film multi-layer structure 77 is thereafter deposited by evaporation or sputtering followed by electroplating as shown in figure 5 1 by using the material system tantalum-nitride resistive layer 78, molybdenum diffusion barrier layer 79 and gold conductor layer 80 with most of the thickness plated.

The conductor part of the interconnection pattern is thereafter defined and etched out by standard photolito¬ graphic masking and etching methods as shown in figure 5 etching through the gold and molybdenum films down to the resistive layer for substrates with resistors, conductors being shown as 81 a, 81 b, 81 c, 81 d, 81 e, 81 f in figure 5 m and with the resistive layer unetched in areas 82 a, 82 b, 82 c, 82 d, 82 e. The thin-film resistive layer is also etched away in the areas 82 a-e for substrates not having thin-film resistors.

For substrates having thin-film resistors a further photo¬ litographic masking and etching step will follow to remove the resistive layer in all areas without resistors being defined, thus yielding a defined conductor interconnection pattern 83 a, 83 b, 83 c, 83 d, 83 e, 83 f as shown in figure 5 n and integrated thin-film resistors, such as resistor 84. The tantalum nitride resistors will thereafter be stabilized 14

by an oxidation process at a temperature of about 300 degree Celcius. As an alternative the tantalum nitride and molyb denum can be replaced by nickel-chromium.

The multi-layer substrate is hereby fully processed resultin in the structure given in figure 4.

An important aspect of this described technology is tha standard gold based and also silver based thick-film materials can be used in the thick-film conductor layers, an using gold based conductors in the via-areas in the signal dielectric layer between thick-film interconnections an thin-film interconnections.

A special "low-cost" version of the CFH-technology i obtained by using copper thick-film conductors with compa tible dielectrics .and required thick-film resistors; and t replace most of the thickness of thin-film gold by copper The structure described in figure 3 will then have copper in the following areas: 21 a, 21 b; 24 a, 24 b, 24 c, 24 d 24 e; 26 a, 26 b, 26 c, 26 d; 32 a, 32 b, 32 c, 32 d, 32 e 32 f. Areas 26 a, 26 b, 26 c, 26 d will have a surface coa of gold in the via-areas shown in detail in figure 3 between layer 45 and 46; and areas 32 a, 32 b, 32 c, 32 d, 32 e, 32 f will have a surface layer of gold as surfa protection and to assure bondability in chip assemb methods.

Claims

PATENT CLAIMS

1.

A method for manufacturi g of mineaturized impedance matched interconnection systems built on dielectric substrates and using thin-film interconnection system, comprising:

a) thick-film connector lines being arranged in one layer,

b) thick-film dielectric layer being arranged with via- holes on top of the layer in step a), and

c) steps a) and b) being repeated to build up two or more thick-film conductor layers including via-holes in the dielectric layers therebetween,

characterized in

d) that standard thick-film processes with the same firing temperature are used for all the layers, and

e) that a thin-film multi-layer system is added after completion of steps a), b) and c), said system comprising at least two of the following three components

1) Tantalum-nitride or nickel-chromium,

2) Molybdenum and

3) Gold and/or copper, and where the thin-film multi¬ layer extends down through the via holes to the uppermost thick-film conductor layer and provides the electrical interconnection between the thin-film multi-layer system and the said thick-film conductor layer, and f) that thin-film transmission lines are formed in said thin-film multi-layer system through etching therein.

2.

A method according to claim 1, further characterized in etching resistors in the thin-film multi-layer system.

3.

A method according to claim 1, characterized in that gold is replaced by copper fully or partly both in thin-film conductors and thick-film conductors.

4.

A method according to one or more of the preceding claims, characterized in building a further dielectric layer with via-holes over the first made thin-film interconnection layer of steps e) and f).

5.

A method according to claim 4, characterized in building a second thin-film interconnection system over said further dielectric layer.

6. A method according to claims 4 and 5, characterized in building up a multi-layer thin-film interconnection system by repeating the building of said further dielectric layer and said second thin-film multi-layer interconnection system.

7.

A method according to one or more of the preceding claims, characterized in the use of a substrate with via-holes and adding one or more thick-film conductor layers on the side of the substrate opposite to the pattern resulting from the features of anyone of the claims 1-6 with some or all via- holes metallized through the thick film processes.