Aluminum-copper connector having a heterostructure, and method for producing the heterostructure

Abstract

A heterostructure comprising at least one first surface containing only copper and at least one second surface, opposite the first surface, containing only aluminium or an aluminium alloy with solid solutions present in the alloy, wherein a. an anchoring layer is arranged between the first and second surfaces, wherein b. each slice plane running perpendicular to the anchoring layer has at least one aluminium or aluminium-alloy island surrounded by copper, and c. at most the aluminium alloy solid solutions which are present in the alloy occur in the anchoring layer. Also, an aluminium-copper connector and a heterostructure production method.

Claims

1. A heterostructure comprising at least one first surface containing only copper and at least one second surface, opposite the first surface, containing only aluminum or an aluminum alloy with solid solutions present in the alloy, wherein a. an anchoring layer is arranged between the first and second surfaces, wherein b. each slice plane running perpendicular to the anchoring layer has at least one aluminum or aluminum-alloy island surrounded by copper, c. any aluminum alloy solid solutions present in the alloy are in the anchoring layer, d. the islands of aluminum or aluminum alloy enclosed by copper have nested and sideways protruding cantilevered structure, e. if the heterostructure is made of copper and pure aluminum, there are no mixed crystals at all, and if the heterostructure is made of copper and aluminum alloy, any mixed crystals present in the heterostructure were pre-existing in the alloy prior to forming the heterostructure and the forming of the heterostructure has not formed any new solid solution crystals, and f. the thickness of the anchoring layer is between 0.5 and 100 micrometers.

2. The heterostructure according to claim 1, wherein an X-ray diffractogram of the anchoring layer shows only the crystallites of copper and aluminum or aluminum alloy which also occurred in the bulk materials.

3. The heterostructure according to claim 1, wherein the thickness of the anchoring layer is between 10 and 50 micrometers.

Description

(1) Figures are used to further illustrate the invention. There is shown in:

(2) FIG. 1 images of sculptured aluminum in two magnifications (prior art);

(3) FIG. 2 a photograph of a sectional surface perpendicular to the anchoring layer;

(4) FIG. 3 a photograph of a sectional surface perpendicular to the anchoring layer;

(5) FIG. 4 a photograph of a sectional surface perpendicular to the anchoring layer;

(6) FIG. 5 an X-ray diffractogram of the anchoring layer;

(7) FIG. 6 photographs of copper deposits on aluminum strips after various pretreatments of the strips;

(8) FIG. 7 photographs of the copper deposits of FIG. 6 after mechanical stretching of the strips.

(9) In FIGS. 2 to 4, various sections through heterostructures are photographed with an electron microscope. The heterostructures consist here for example of rectangular strips of the technical alloy AlMg3 (>94% Al content) and circular copper thick films deposited on the aluminum. The cross-sectional images each show the surroundings of the copper-bearing surface and represent light copper (upper part of the image) and dark AlMg3 (lower part of the image). The anchoring layer can be recognized by the fact that it has both bright and dark parts of the image and thereby follows the course of the copper-plated partial surface of the aluminum strip. It is particularly noticeable that in each of the sections of copper completely enclosed islands of aluminum (here: AlMg3) can be seenhighlighted in the pictures by dashed borders.

(10) This initially gives the impression that aluminum fragments, such as grains, have somehow been mixed into the copper. However, all aluminum visible in FIGS. 2 to 4 isat least before generating the cut surfaceis definitely connected to the aluminum strip at the bottom of the picture, in particular electrically connected. The extremely nested and sideways cantilevered structure of the sculptured aluminum makes it virtually impossible to find a cut surface perpendicular to the anchoring layer in which at least one aluminum island surrounded by copper cannot be seen. This property of the heterostructure is thus very well suited as one of its characteristics.

(11) Another characteristic of the heterostructure can be found in an X-ray diffractogram of the anchoring layer, which is shown in FIG. 5. All occurring peaks of the X-ray scattering can be unambiguously assigned to the usual crystallites, existing previously in pure copper and pure aluminum or, here, the alloy AlMg3. This is also the case after energizing and after treatment in an aging cabinet under cyclic temperature fluctuations. At no time do new mixed crystals form.

(12) The two aforementioned properties of the heterostructure have the consequence that copper and aluminum are mechanically robust and permanently connected by a key-lock principle (interlocking) and also remain so because corrosion, aging and the formation of brittle intermetallic phases are avoided.

(13) The following experiment demonstrates how good the adhesion is in comparison to copper-plated aluminum according to the prior art:

(14) One strip of AlMg3 is first pre-treated and then covered with a galvanically deposited copper layer. The samples can be seen in FIG. 6, the aluminum strips being a) polished, b) sandblasted on the basis of U.S. Pat. No. 1,457,149 and c) etched in accordance with the invention. FIG. 6d) shows a coating of copper on aluminum according to the teaching of U.S. Pat. No. 2,495,941.

(15) The copper-plated aluminum strips are then mechanically stretched beyond the elastic range. FIG. 7 shows the experimental results.

(16) With the lengthened polished and sandblasted aluminum strips, the copper layers simply break off as a whole. In FIGS. 7a) and b) they are placed again on the strips for the photographs. In the lower photograph areas, the areas are recognizable, where they were previously contacted with the strip.

(17) The strip with the heterostructure according to the invention in FIG. 7c) retains a perfectly adherent copper layer even during stretching; this was stretched together with the aluminum. There are no signs of damage to the coating integrity.

(18) In FIG. 7d), the copper layer also stretches with the strip, however the layer ruptures. It can be concluded that the force application of the expanding aluminum to the copper layer was not uniform everywhere, i.e. there were areas of better and worse adhesion under the copper layer. This is also supported by the visible delamination of parts of the copper layer. In the cracks of the copper layer the aluminum substrate is visible, i.e. there was a partial detachment instead.

(19) The heterostructure according to the invention avoids the delamination and the degradation of the electrical and thermal conductivity under mechanical, electrical and thermal stress.

(20) Preferably, therefore, an aluminum-copper connector is created by producing a body of aluminum or an aluminum alloy having at least one copper-plated partial surface having a heterostructure according to the invention. In this case, the anchoring layer should follow the course of the copper-plated partial surface at a predetermined depth below the copper-plated partial surface.

(21) In an advantageous embodiment for electrical conduction the AlCu connector is formed as an aluminum cablewith freely selected cross-section, possibly surrounded by insulationwith at least one copper-plated cable end. If insulation completely covers all non-coppered aluminum surfaces, the cable behaves virtually like a full copper cable and can be so used as well.

(22) A further advantageous embodiment of an AlCu connector is the equipment of a commercially available aluminum heat sink, preferably a heat sink filled with water or other cooling liquid, with at least one copper-plated partial surface. Pure copper is too heavy and too expensive as a heat sink, but the rapid removal of heat from the place of origin into the heat sink is thus promoted.

(23) Finally, a two-step process for the generation of the heterostructure will be presented.

(24) For the electrochemical etching of the sculptured aluminum surfaces with steps and undercuts a salt water solution is used as etching electrolyte, the common salt (NaCl) with a concentration from the interval of 200 mmol/l to 800 mmol/l and sodium sulfate (Na2SO4) with a concentration of 5 mmol/l to 100 mmol/l. For silicon-containing aluminum alloys such as AA4018, sodium fluoride (NaF) with a concentration in the interval from 5 mmol/l to 100 mmol/l can additionally be added to the etching electrolyte.

(25) As an advantage, it should be emphasized that the etching electrolyte has a chemical composition similar to seawater and contains no critical environmental toxins. It can be easily and inexpensively manufactured and disposed of again.

(26) In the electrochemical etching of pore structures in semiconductors and metals, it is basically the case that the shape of the structures achieved is determined by the passivation of surfaces against the etching attack. The passivation takes place by the addition of at least one passivation species to the vulnerable surface, which slows down the etching in the attachment or even prevented. The passivation species can be very different, for example, chlorine-containing molecules or phosphate or sulfate ions can passivate. US 2013/0264196 A1 proposes, inter alia, the addition of sodium nitrate (NaNO3) as a passivation species, using high concentrations which stabilize the pore walls. At the same time, etch current densities of 100 to 1000 mA/cm 2 are used, so that etching still takes place at the pore tips, because the passivation species does not reach the pore tips adequately by diffusion limitation. This then leads to drilling (drilling) deeper, tunnel-like pores in aluminum.

(27) The etching electrolyte of the present invention relies primarily on chlorine ion-containing molecules as the passivation species. By an inventively low etch current density in the range between 10 mA/cm.sup.2 and 100 mA/cm.sup.2 and etch bath temperature between 10 C. and 40 C., an advantageous reaction kinetics can be achieved with the etching electrolyte, i.e. that sets up a ratio between passivation and resolution of the aluminum surface favorable for structuring. In particular, there is nowhere a diffusion limitation of the passivation species, but in particular it is uniformly slowly etched everywhere.

(28) Outside the mentioned temperature range, the reaction kinetics is noticeably impaired. In addition, if the etching current density is too great or too small, either a diffusion limitation of the passivation species occurs, or the passivation cannot be interrupted, so that in both cases formation of the desired structures does not occur.

(29) For the copper deposition, a galvanic electrolyte is provided which contains an aqueous solution containing copper sulfate (CuSO4) with a concentration in the interval from 40 mmol/l to 120 mmol/l, boric acid (H3BO3) with a concentration in the interval from 10 mmol/l to 30 mmol/l and polyethylene glycol (PEG) with a concentration in the interval from 0.15 mmol/l to 0.55 mmol/l. Each of the three components has a specific function within the electrolyte. Copper sulfate serves as a source of copper ions, boric acid and polyethylene glycol are necessary to control copper deposition kinetics to completely encase the sculptured aluminum surface structures and eliminate copper voiding in the heterostructure. It is also important for copper deposition on the sculptured aluminum surface that the naturally formed aluminum oxide layer be dissolved in the copper electrolyte while at the same time not destroying the etched aluminum surface structures by chemical dissolution. The deposition current density should be set in the range between 1 mA/cm.sup.2 and 30 mA/cm.sup.2. At a higher current density, voids may form in the heterostructure, while at too low a current density, copper deposition may be too slow.

(30) The copper deposited in the region of the etched aluminum surface structures, together with the said structures, form the anchoring layer by mechanical positive locking, which is the essential feature of the heterostructure of copper and aluminum according to the invention.

(31) The process for producing a heterostructure according to the invention should in summary comprise at least the following steps: a. Providing an etching bath with an aqueous etching electrolyte containing between 200 mmol/l and 800 mmol/l sodium chloride and between 5 mmol/l and 100 mmol/l sodium sulfate; b. Providing a plating bath with an aqueous electroplating electrolyte containing between 40 mmol/l and 120 mmol/l copper sulfate and between 10 mmol/l and 30 mmol/l boric acid and between 0.15 mmol/l and 0.55 mmol/l polyethylene glycol; c. Introducing an electrically contacted object made of aluminum or an aluminum alloy and a counter electrode into the etching bath; d. Applying and keeping constant an etching current density at the interval of 10 mA/cm 2 to 100 mA/cm 2 for a predetermined etching time at a predetermined temperature; e. Introducing the etched object and a counter electrode into the plating bath; f. Applying and keeping constant a deposition current density from the interval of 1 mA/cm2 to 30 mA/cm2.

(32) As a concrete example of the method for producing a heterostructure comparable to that of FIG. 6c), the following procedure is adopted:

(33) First, a polycrystalline aluminum alloy rolled strip (e.g. AA5754) is patterned on its surface by electrochemical etching. The etching electrolyte for this purpose is water containing 500 mmol/l NaCl and 56 mmol/l Na2SO4. The aluminum structuring is carried out galvanostatically at a constant current density of about 50 mA/cm.sup.2.

(34) The etching time depends on the selected etching current density, on the composition and temperature of the etching electrolyte and on the desired structural depth in the aluminum; it is here for example 30 min. The person skilled in electrochemistry is familiar with the fact that when changing an etching parameter, he has to adapt the etching time to the new conditions, which he can accomplish easily by means of simple preliminary experiments.

(35) The galvanic copper deposition, with which the aluminum-copper heterostructure is produced, takes place in an aqueous electroplating electrolyte containing 72.1 mmol/l copper sulfate, 17.8 mmol/l boric acid and 0.33 mmol/l polyethylene glycol 3350. The deposition is carried out galvanostatically at a current density of 15 mA/cm.sup.2. The deposition time is freely selectable in view of the selected deposition current density and the desired copper layer thickness. The electrolyte temperature here is 20 C. in both baths.

(36) Another advantage of the above-described two-stage process in two separate electrolyte baths is that the electrolytic plating bath for copper deposition is not contaminated with aluminum etchants. This ensures that the reproducibility of the deposition process and the purity of the deposited copper layer are high, which also simplifies the control of the electrical resistance of the heterostructures. The division into an etching bath and a deposition bath also advantageously increases the service lives of the electrolytes. If the electroplating electrolyte is depleted of copper, it can easily be re-enriched with copper in-situe.g. by means of copper counter-electrodeor ex-situ.