DE2554691C2 - Process for producing electrical conductors on an insulating substrate and thin-film circuit produced therefrom - Google Patents

Description

The invention relates to a method according to the preamble of claim 1 and to a Thin-film circuit according to the preamble of claim 11. A method and a thin-film circuit of this type are known from Di-OS 21 08 730.

Integrated thin-film and hybrid circuits are used, for example, in filters and in memories for switching and transmission systems are used. The electrical applied to an insulating substrate The conductors of an integrated thin-film circuit consist of a multilayered titanium-palladium-gold structure together. The gold layer takes over the main role in the power line and also serves at the same time : 'Is good bond layer when connecting (bonding) with the Connection terminals of the thin-film circuit. Such a Ti-Pd-Au structure has a satisfactory one Behavior, but requires relatively large amounts of gold. As a rule, the gold layer is etvv.i soOOnm thick and must be created over a fairly large area of the interconnect pattern, resulting in corresponding leads to high material costs in the production of such thin-chip circuits.

To reduce the gold content in thin-film circuits, it is known from DE-OS 21 08 730 the dielectric substrate has a multilayer titanium-copper-nickel'GoId structure to raise. With such a structure, the power conduction becomes substantial taken over by the copper layer, while the gold layer only serves as a bonding layer and is therefore in their thickness compared to a Ti-Pd-Au structure

can be reduced. The Timnschicht provides one good adhesion of the copper layer to the substrate and the nickel layer serves as a dilusion barrier between the Copper and gold layers. With the well-known Ti-Cu-Ni-Au structure however, the adhesion of the copper layer to the tilane layer is only complete when used Satisfactory special processes for layer production, but inadequate for other layer production processes, although it would be expected that the adhesion between the two non-precious metals would be copper and titanium should be good in general.

In contrast, the object of the invention is to in a method of the type mentioned at the outset, satisfactory adhesion between the titanium layer and the Achieve copper layer regardless of the type of application of these two layers.

This object is achieved by the features of claim 1.

The method according to the invention is for improvement the adhesion between the titanium and the copper layer a palladium layer arranged, its effect It is surprising in so far as precious metal is not made of non-precious metal generally do not form a strong bond. The reason for this surprising improvement in adhesion by means of the palladium layer could not yet be clarified.

Advantageous developments and refinements of the method according to the invention result from the Dependent claims 2 to 10. A thin-film circuit. their connecting lines using the inventive Process are produced is specified in the dependent claim II.

The invention is described in detail below with reference to the drawing.

Show it:

Fig. 1 is a perspective view of part of a ceramic substrate with thin metal layers deposited thereon, the individual layers being enlarged are shown.

2 is a perspective view of the substrate according to FIG Fig. 1 after working out the conductor and contact points from the thin layers.

3 shows a flow chart of the one / a method steps for two alternative method sequences.

Fig. 4 is a perspective view of a thin film sound jng. which is produced in accordance with the embodiment of the present method.

5 shows an enlarged section along the cutting line 3-3 in Fig. 4.

In the embodiment "according to FIG. 1, a z. B. substrate 10 consisting of ceramic material initially a titanium layer 12 with a thickness of about 170 down to 250 nm deposited, preferably vapor-deposited, to increase the adhesion of the subsequently deposited metal layer / u. Then it is applied to the TiLmschicht a preferably 300 nm thick palladium layer 14 is vapor-deposited, aiii soft in turn one with gold plated copper layer 16 is plated

You don't need the gold layer for a good one oxidized bonding surface. However, since copper is light diffused through üold and the bonding over * surface is destroyed, a nickel layer serving as a diffusion barrier is created between the copper and gold layers arranged. Accordingly, the composite conductive thin film 17 exists on the substrate 10 of titanium and palladium, which are successively vapor-deposited on the substrate, followed by copper, Nickel and gold sequentially plated onto the palladium. The thin layer 17 becomes In a subsequent production step, a pattern 22 made up of individual conductors 18 and contact points 20 the substrate 10 formed (Fig. 2).

The following are based on the flow chart of FIG. 3 describes two embodiments of the method.

The sequence of process steps shown in FIG. 3 preferably begins after the deposition of the

ίο Resistor and capacitor elements, usually are made up of tantalum or tantalum nitride, on the insulating, mostly consisting of aluminum oxide Substrate. The creation of the pattern 22 began with the deposition of a titanium layer on virtually the entire surface of the substrate. For this purpose, electron beam evaporation was used, which, however is not mandatory, since other well-known methods such. B. atomization, can be used. The thickness of the titanium layer is preferably between 150 and 300 nm. to serve as an adequate adhesive layer can and to avoid bonding problems /.ti that usually occur when the thickness is less than about 150 nm. A titanium layer thickness of about 250 nm obviously forms the optimum. A palladium layer about 50 nm thick was then placed on top of the titanium layer preferably knocked down by the same method. The palladium layer serves to improve the adhesion between the titanium layer and the copper layer to be described. A suitable thickness for the palladium layer lies between 20 and 300nm.

Next was a thin layer of copper. im considered Example case also by electron beam evaporation, deposited on the palladium layer. This vapor-deposited copper layer is used mainly for producing a highly electrically conductive layer for the subsequent electroplating steps. Suitable layer thicknesses for the vapor-deposited copper layer are preferably between about 300 and 700 nm at about 500 nm.

Before carrying out the photoresist masking required for the above-mentioned electroplating, it is advantageous to cover the vapor-deposited copper layer with a thin chromium layer in order to increase the adhesion of the photoresist in a known manner. Following the photolithographic masking process ¹ 'urde. as can be seen from FIG. 3. On the selected, not covered by the photoresist parts of the vapor-deposited copper layer further copper, deposited galvanically. For this purpose, for example, the substrate was made in an electrolytic cell / anode, the current density being 20 mA cm ² and the bath used containing about 68 grams liters of CuSO ₄ and 180 liters of H ₂ SO _4. This bath was for complete occupancy The total thickness of the vapor-deposited and electrodeposited copper was around 3500 nm. The suitable thickness range in relation to the thickness range of the subsequent nickel and gold layers will be discussed in more detail below. It should be noted that prolonged exposure to the Copper surface causes oxidation of the copper surface in relation to air and thus leads to poor adhesion.Therefore, the copper electroplating should follow the step of copper vapor deposition as quickly as possible, and the subsequent electroplating of nickel should also be carried out as long as the copper is still wet Next, nickel was on the fre The enclosed areas of the copper surface are galvanically deposited.

The bath specially used for this purpose basically consists of nickel sulfamate and boric acid. The nickel layer was about 1000 nm thick. In order to obtain an adequate diffusion barrier between the electroplated copper layer and the subsequently deposited gold layer while achieving a reasonable square film resistance, the thickness of the nickel layer should be between 800 and 2000 µm. A layer thickness below about 800 nm leads to a porous layer that does not prevent the diffusion of copper into the gold layer at the high temperatures during the subsequent further processing. The thickness of the nickel layer is therefore an important parameter. The current density used was again about 20 mA cm ² , which resulted in a sufficiently dense layer. To obtain a sufficiently dense nickel layer (of the order of 9 g cm '), the minimum current density for the nickel layer deposition should be around 10 mA cm'

The production of the uppermost gold layer can, as in Fig. 3 is shown, basically take place according to two alternative method steps. With the first alternative the photoresist layer previously produced on the substrate was used as a mask for the galvanic deposition of the Gold layer used on the entire exposed area of the underlying nickel layer. The second Alternatively, the photoresist layer was stripped off and a second photoresist layer was applied, exposed and designed to only those areas of the nickel layer to uncover those as bonding sites for integrated training platelets or as fastening points for components outside the substrate. In In both cases, a gold plating bath was used as the plating bath with 20gr I potassium gold zyamd. 50gr I ammonium citrate and 50 grams per liter of ammonium sulfate at a current density of approximately 2 mA cm '. the The thickness of the gold layer was about 2000 nm. To ensure a good bonding surface, the thickness should be the gold layer in the range between 1500 and 2500 nm lie.

The next step involves molding of the connection line pattern by etching the vapor-deposited copper and titanium layers that are covered by the Later electroplated layers were not covered. For this purpose, the one used for electroplating was initially used Photoresist removed In both alternative process steps for producing the gold layer, the evaporated copper using an ammonium persulphate solution and removing the titanium layer by means of hydrofluoric acid in the removal of the evaporated copper care must be taken that the nickel is etched and the galvanically deposited undercutting Copper layer is avoided. Usually this is Etching time to remove 500 nm of copper with ammonium persulfate about 60 seconds. As a precaution, the etchings were removed from the etchant as soon as all visible copper signs had disappeared, whereupon the stains were immediately rinsed off to a avoid further etching.

The thicknesses of the copper, nickel and gold layers for a correct square surface resistance R _z can be calculated from the following equation:

_{R =} I

Here, f is the thickness and ρ is the specific resistance of the specified metals. As a stable final value, a square resistance of about 0.005 ohms or less is desirable for most applications. For a nickel layer thickness of 800 to 2000 nm and a gold layer thickness of 1500 to 2500 nm, a copper layer thickness of 2500 to 4000 nm is the optimum for a proper square area against each other.

After the connection manager has been created normal further processing takes place in a known manner, for example shaping the resistance pattern. hot pressing ^ bonding, soldering, etc.

A perspective view of a simple thin film circuit immediately after the manufacture of the The connecting line pattern prior to further processing is illustrated in FIG. 4, as can be seen from this. define the inside of a Ker.tmiksuhstrate 10 of the location 42 provided bonding locations 41 the area in which the integrated circuit chip (not shown) can be arranged. Bonding points 43 in the vicinity of the edges allow the sonication with components outside the substrate. In the example shown, the thin-film circuit comprises a first resistor element 52 and a second Resistance element 44 in series with a capacitor element 45. Resistance elements 44, 52 usually made of tantalum nitride and the capacitor element 45 made of a multilayered tantalum-tantalum oxide conductor structure are constructed in a known manner. The line pre-connections between the bonding slips and the scfialtkreiselemente are as described above Process made. F. Fig. 5 shows an enlarged section along section line 3-3 groove 46 denotes the vapor-deposited titanium layer and 47 denotes the palladium layer, while the vapor-deposited Thin layer and a galvanized reinforcement layer consisting of copper layer denoted by 50 As can be seen from FIG. 5. the gold layer 51 was formed only in those areas where a later bonding treatment is planned: i.e. the second alternative method was used During the subsequent heat treatment, alloys of copper are indeed formed to a small extent.

Nickel and gold mainly at the boundaries of the nickel layer, but it still remains the basic one Maintain Cu-Ni-Au multilayer structure.

To show that the Ti-Cu-Ni-Au layer sequence defies the mentioned change in the composition is compatible with all thin-layer processing sequences. the Ti-Cu-Ni-Au conductor system was heated to different temperatures for different lengths of time according to the conditions in the usual circuit processing. For example, 5 "structures with a nickel layer thickness of about 1000 nm were heated to 250 ° C. for 5 hours, which corresponds to the usual heat treatment for stabilizing Ta ₂ N resistors. An Auger analysis of the resulting structure showed an insignificant diffusion of copper or gold through the nickel layer and a change of the square sheet resistance of only 4%. a 1,000-hour-long heat treatment at 150 ⁰ C resulted in no detectable diffusion through the nickel layer and to a square sheet resistance change of only 1.5%. the upper limit of a heat treatment of the conductor system is located at Heating at 350 ^° C. for 4 hours, since in this case considerable exchange diffusion took place and the change in square surface resistance was approximately 17%.

The Ti-Cu-Ni-Au system was also exposed to ambient atmospheres made up of air plus dry and moist HCl or moist NO ₃ and SO _2. In dry

HCI, the examined system showed the same good corrosion resistance as the well-known Ti-Pd-Au system. ! in humid environments (HCl, SO ₂ or NO,), ie. When subjected to a more rigorous corrosion resistance test, the system examined showed Kortlakl resistance levels approximately similar to those of dry HCM exposure.

In the case of thin-film circuits, after all high-temperature treatment has been carried out, bonding is usually carried out by means of hot pressing. To test the compatibility of the system under consideration with regard to such a treatment, it was therefore necessary to expose the metal layers to high temperatures over long periods of time. The Ti-Cu-Ni-Au system, provided with nickel layers of varying thickness between 200 and 1000 μm, was subjected to various heat treatments at 150, 250, 300 and 350 ° C., followed by bonding and testing of the tensile strength. Maximum tensile strengths were found generally to a system with 1,000 to nickel layer thickness, with sufficient Festigkcilswerte after heat treatments at l50 ° Cbiszu 1,000 hours at 250 ° C for up to 10 hours at 300 ⁰ C up to four hours and at 350 ⁰ C to 2 hours showed. It is recommended that the heat treatment be carried out for at least 30 minutes.

In general, the tensile strength of the conductor system under consideration was essentially the same as that of the known, similarly treated Ti-Pd-Au system. Therefore, the experiments carried out show that a Wiirmpreß bonding after a 4 hour Wafmbehandlung the Ti-Cu-Ni-Au Struklur is possible at 300 ⁰ C, which is the commonly applied heat treatment to the used in aufplatlierten crossovers insulating cure. Furthermore, I am a Wafampfß-Bonderi after heat treatments of up to

ιό two hours at 35O ⁰ C possible, which represents the heat treatment provided for resistance stabilization prior to the comparison with laser beams. The thickness of the nickel layer was around 1000 nm.

Another surprising property of the bctrachlcten Ladder system is its compatibility with soldering processes. The soldering of the well-known Ti-Pd-Au system with common Sn-Pb solder leads to the formation of brittle intermetallic Sn-Au connections. At the observed System, the gold layer is sufficiently thin that there are no brittle intermetallic compounds develop. Furthermore, the gold protects the nickel surface from oxidation and therefore allows light weight Wet the nickel surface with the solder. The speed of dissolving nickel in solder is sufficient slowly to allow enough time for soldering and desoldering available for repair purposes.

For this purpose 3 sheets of drawings

Claims (11)

Patent claims: 1. A method for producing electrical conductors on an insulating substrate, in which - a metal layer of titanium is produced in a desired pattern on the substrate, - a metal layer made of copper is produced above the titanium layer, - Additional copper is galvanically deposited on the copper layer in selected areas will, - a metal layer of nickel is galvanically deposited on the copper layer, - At least in sections, a metal layer of gold is deposited galvanically on the nickel layer will, and, - those areas of the titanium layer and the copper layer that are not covered by the galva- are covered with precipitated layers, characterized in that on the titanium layer before the production of the copper layer a Palladium layer is produced. « 2. The method according to claim 1, characterized in that the titanium slide 'in a thickness of 150 up to 300 nm. the copper layer in a thickness of 2500 to 4000 nm. the nickel layer in a thickness of 800 to 2000 nm and the gold layer can be produced in a thickness between 1500 and 2000 nm. 3. Verfa ·: s according to Claim I. characterized in that the palladium layer practically the entire surface of the titanium layer a substantially d <to manufac ¹ "- is ·· palladium existing metal layer in a Dicice from 20 to 300 deposited nm!. 4. The method according to claim 3, characterized in that mm production of the copper layer first of all practically on the entire surface of the palladium layer, a portion of the copper layer consisting essentially of copper is evaporated in a thickness of 300 to 700 nm and then! \ E-Bend the selected areas of the vapor-deposited copper layer corresponding to the desired connection pattern are electroplated with additional copper, and / was of such a thickness that the total thickness of the resulting copper layer / is between 2500 and 4000 nm. 5 The method according to claim 3 or 4, characterized in that the galvanically applied il of the copper shield is produced using an electroplating bath containing CuSO ₄ and H ₂ SO ₄ , that the nickel layer is produced using an a.ilvanischen bath that Nickelsull.imat contains and that the gold layer is made from an electroplating bath that contains gold / yanide 6. A method according to any one of claims I to 5, characterized in that the resulting multilayer structure is heated door a period of about 30 minutes to LO hours at a temperature between 200 ⁰ C and 400 "C ^5. 7. The method according to claim 6, characterized in that the layer assembly is heated ^{to about 300 ° C. for a period of about 30 minutes to 4 hours.} 8. Procedure according to; Claim 6, characterized in that that the layers were built for a Diuiervön heated to about 350 ° C for about 30 minutes to 2 hours. 9. The method according to claim 4, characterized in that those parts of the vapor-deposited Copper layer not covered by at least one of the electrodeposited layers are removed by immersion in a caustic solution of ammonium persulfate and ciaß those Parts of the vapor-deposited titanium layer, not soft from at least one of the electrodeposited Layers covered can be removed by immersion in an etching solution of hydrofluoric acid. 10. The method according to any one of claims I to 9, characterized in that the gold layer only on those portions of the nickel layer is made soft for the connection with a electrical circuit are provided. 11. Thin-film circuit, its electrical components and using interconnection lines on the main surface of an insulating substrate of the method according to claim 1 are produced, with a titanium layer produced on the substrate, a copper layer produced above the titanium layer. a nickel layer produced on the copper layer. and a gold layer produced at least in sections on the nickel layer,
characterized in that a palladium layer is arranged between the titanium layer and the copper layer.