Process for making studless thin film magnetic head

We claim:

1. A process for pattern-etching an alumina layer, including the steps of: depositing an alumina layer; depositing over the alumina layer an adhesion metallic layer; depositing over the adhesion metallic layer at least one additional metallic layer including a top metallic layer; forming a photoresist mask pattern over the top metallic layer; pattern-etching the top metallic layer through the photoresist mask; pattern-etching through the top metallic layer, in reverse order of deposition, the other metallic layer or layers; pattern-etching the alumina layer with an alumina etchant comprising HF through the metallic layers; and removing the metallic layers.

2. The process of claim 1 including a step of removing the photoresist mask following the step of pattern-etching one of the metallic layers.

3. The process of claim 1 wherein the step of depositing the adhesion metallic layer comprises depositing a material selected from the group consisting of Cr, Ti, W, Ta, Mo, Nb, V, Zr, Hf, and alloys comprising one or more elements thereof.

4. The process of claim 3 wherein the adhesion metallic layer comprises a metal from the group consisting of Cr and Ti.

5. The process of claim 1 wherein the step of depositing the adhesion metallic layer comprises depositing a material having good electrical conductivity and strong adhesion to the alumina layer and selected from the group consisting of Ni-Fe, W, Mo, and alloys comprising one or more of these metals.

6. The process of claim 3 wherein the step of depositing at least one additional metallic layer comprises the step of depositing a first additional metallic layer comprising a metal selected from the group consisting of Au, Ni-Fe, Cu, Ag, Pd, Pt, Rh, Ir, W, Mo, and alloys comprising one or more elements thereof.

7. The process of claim 6 wherein the first additional metallic layer comprises a metal from the group consisting of Au and Ni-Fe.

8. The process of claim 5 wherein the step of depositing at least one additional metallic layer comprises the step of depositing over the adhesion metallic layer an additional metallic layer comprising a metal selected from the group consisting of Au, Cu, Ag, Pd, Pt, Rh, Ir and alloys comprising one or more thereof.

9. The process of claim 8 wherein the step of depositing an additional metallic layer comprises plating an Au layer.

10. The process of claim 9 wherein the additional metallic layer comprises a high purity soft Au layer with thickness of at least 0.1.mu.m.

11. The process of claim 10 wherein the thickness of the additional metallic layer is in the range 0.2-0.5.mu.m.

12. The process of claim 6 including a further step of depositing over the first additional metallic layer a second additional metallic layer comprising a metal selected from the group consisting of Au, Cu, Ag, Pd, Pt, Rh, Ir and alloys comprising one or more thereof.

13. The process of claim 12 wherein the step of depositing a second additional metallic layer comprises plating an Au layer.

14. The process of claim 13 wherein the second additional metallic layer comprises a high purity soft Au layer with thickness of at least 0.1.mu.m.

15. The process of claim 14 wherein the thickness of the second additional metallic layer is in the range 0.2-0.5.mu.m.

16. The process of claim 12 wherein the adhesion metallic layer comprises a metal from the group consisting of Cr or Ti, the first additional metallic layer comprises vacuum deposited Au, and the second additional metallic layer comprises plated Au.

17. The process of claim 6, 8, or 12 wherein the alumina etchant comprises a solution of HF in water.

18. The process of claim 17 wherein the concentration of HF in water in the alumina etchant is in the range of 10% to 25% by weight.

19. The process of claim 18 wherein the concentration of HF in water in the alumina etchant is approximately 16% by weight.

20. The process of claim 17 comprising the step of removing the metallic layers, in reverse order of their deposition, by successive immersion in or spraying with etchants.

21. The process of claim 17 comprising the step of removing the metallic layers, in reverse order of their deposition, by sputter-etching or ion-milling or reactive ion etching (RIE).

22. The process of claim 17 comprising the step of removing the metallic layers by lifting-off, effected by etching the adhesion metallic layer from underneath the at least one additional metallic layer.

23. The process of claim 17 wherein the step of etching the alumina with an alumina etchant comprising HF is controlled by interrupting the etching action after a pre-set time duration.

24. The process of claim 17 comprising the step of depositing an etch-stopper in the alumina layer, the etch-stopper comprising a metal layer, cured photoresist, or a substrate.

25. A process of etching alumina comprising the step of exposing the alumina to an etchant comprising a mixture of HF and water, wherein the concentration of HF is in the range of 10% to 25% by weight.

26. The process of claim 25 wherein the concentration of HF is approximately 16% by weight.

27. A process for forming a studless TFH device including the process of claim 17 wherein the alumina layer is an overcoat layer and further including the step of depositing and patterning bonding pads prior to the step of depositing the alumina layer, and wherein the step of etching the alumina layer comprises forming vias to the bonding pads.

28. The process of claim 27 wherein each of the bonding pads serves as an etch-stopper for the etching of the alumina overcoat layer.

29. The process of claim 28 wherein the step of depositing and patterning the bonding pads comprises the step of simultaneously depositing and patterning both the coil and the bonding pads, the coil and the bonding pads comprising the same metal.

30. The process of claim 29 wherein the step of depositing and patterning the bonding pads includes the steps of: depositing a metallic seed-layer selected from the group consisting of Ni-Fe, W, Mo, Ni-Co, Ni-Co-Fe, Cd, In, and alloys comprising one or more of these metals; forming a photoresist mask incorporating patterns for the coil and the bonding pads; plating through the photoresist mask a metal bonding pad layer selected from the group consisting of Au, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more elements thereof; removing the photoresist mask; and etching the metallic seed-layer from areas between the plated pattern of the metal bonding pad layer.

31. The process of claim 29 wherein the step of depositing and patterning the bonding pads includes the steps of: depositing a metallic seed-layer selected from the group consisting of Ni-Fe, W, Mo, Ni-Co, Ni-Co-Fe, Cd, In, and alloys comprising one or more of these metals; depositing by plating or by a vacuum technique a metal bonding pad layer selected from the group consisting of Au, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more elements thereof; forming an etch mask incorporating patterns for the coil and the pads over the bonding pad layer; pattern-etching the bonding pad layer and the metallic seed-layer through the etch mask by ion-milling or by reactive ion etching (RIE); and removing the remains of the etch mask.

32. The process of claim 30 wherein the metallic seed-layer comprises Ni-Fe or W and the bonding pad layer comprises Au.

33. The process of claim 31 wherein the metallic seed-layer comprises Ni-Fe or W and the bonding pad layer comprises Au.

34. The process of claim 30 wherein the step of depositing a metallic seed-layer comprises the steps of depositing a first adhesion metallic seed-layer selected from the group consisting of Cr, Ti, W, Mo, Nb, V, Zr, Hf, and alloys comprising one or more of these elements, and depositing a second metallic seed-layer selected from the group consisting of Au, Ni-Fe, W, Mo, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more of these metals following the step of depositing the first adhesion metallic seed-layer.

35. The process of claim 31 wherein the step of depositing a metallic seed-layer comprises the steps of depositing a first adhesion metallic seed-layer selected from the group consisting of Cr, Ti, W, Mo, Nb, V, Zr, Hf, and alloys comprising one or more of these elements, and depositing a second metallic seed-layer selected from the group consisting of Au, Ni-Fe, W, Mo, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more of these metals following the step of depositing the first adhesion metallic seed-layer.

36. The process of claim 34 wherein the first adhesion metallic seed-layer comprises Cr or Ti and the second metallic seed-layer comprises Au.

37. The process of claim 35 wherein the first adhesion metallic seed-layer comprises Cr or Ti and the second metallic seed-layer comprises Au.

38. The process of claim 30 comprising the step of depositing and patterning a coil having more than one different level, the different levels of the coil being deposited simultaneously with different bonding pads.

39. The process of claim 31 comprising the step of depositing and patterning a coil having more than one different level, the different levels of the coil being deposited simultaneously with different bonding pads.

40. The process of claim 34 comprising the step of depositing and patterning a coil having more than one different level, the different levels of the coil being deposited simultaneously with different bonding pads.

41. The process of claim 38 wherein one or more pads are deposited and patterned repeatedly with each of the coil levels.

42. The process of claim 28 comprising the step of depositing and patterning a coil, wherein the step of depositing and patterning the coil is separate from the step of depositing and patterning the bonding pads.

43. The process of claim 42 wherein the coil and the bonding pads comprise metals, the metal comprised in the coil being different from the metal comprised in the bonding pads.

44. The process of claim 29 wherein the step of depositing and patterning the coil and the bonding pads comprises the steps of depositing a continuous Al layer by vacuum deposition; forming a mask having patterns for the coil and the bonding pads; dry pattern-etching by ion-milling or by RIE the Al layer; and removing the remains of the mask.

45. The process of claim 43 wherein the coil comprises Al or Cu and the bonding pads comprise Au

46. The process of claim 28 further including the steps of: depositing and patterning an intermediate metallic layer over the bonding pads following the step of depositing and patterning the bonding pads and prior to the step of depositing the alumina overcoat layer; and wherein the step of removing the metallic layers comprises the step of etching the intermediate metallic layer to expose the bonding pads.

47. The process of claim 46 wherein the step of depositing and patterning an intermediate metallic layer comprises depositing a metal selected from the group of Cr, Ti, and Ni-Fe.

48. A process for etching scribe-line grooves through an alumina overcoat and an alumina undercoat in a TFH device, including the process of claim 17 wherein a substrate serves as an etch-stopper.

49. The process of claim 48 wherein the step of etching the scribe-line grooves includes the step of forming vias to the bonding pads.

50. A process for etching a crater for a recessed structure in an alumina layer in a TFH device including the process of claim 17, wherein the depth of the crater is controlled by interrupting the etching action after a predetermined period of time.

51. The process of claim 50 wherein the crater depth is controlled by placing an etch-stopper layer at a selected depth in the alumina layer.

52. The process of claim 51 wherein the etch-stopper layer is selected from the group comprising a metal, a cured photoresist, and a substrate.

53. A process for constructing lightning arresters in a TFH device by etching vias through an alumina undercoat for connecting a magnetic core with a substrate, including the process of claim 17 wherein the vias are etched through the alumina undercoat to expose the substrate and including the further step of electrically connecting the magnetic core to the substrate through the vias.

54. The process of claim 53 comprising the step of depositing a metallic layer over the substrate to serve as an etch-stopper and to provide an electrical connection with the lightning arrester.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium. It is applicable to either inductive or magnetoresistive (MR) elements.

In particular, the invention relates to a method of etching deep vias through the thick alumina overcoat in order to expose underlying bonding pads, thereby completely eliminating fabrication of the studs (or bonding posts), a method of etching the undercoat to define scribe-line grooves of streets and alleys across the wafer for sawing (or dicing) and machining of the sliders, and a method of pattern-etching the alumina undercoat, prior to the wafer fabrication, to create precise craters for recessed structures or open vias to the substrate for lightning arresters.

2. Background of the Prior Art

Magnetic TFH transducers are known in the prior art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719 for inductive devices and U.S. Pat. Nos. 4,190,871 and 4,315,291 for magnetoresistive (MR) devices.

In the operation of a typical inductive TFH device a moving magnetic storage medium is placed near the exposed pole-tips of the TFH transducer. During the read operation, the changing magnetic flux of the moving storage medium induces a changing magnetic flux upon the pole-tips and the gap between them. The magnetic flux is carried through the pole-tips and yoke core around spiralling conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between pole-tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium.

Magnetoresistive (MR) TFH devices can only operate in the read mode. The electrical resistance of an MR element varies with its magnetization orientation Magnetic flux from the moving magnetic storage medium induces changes in this orientation. As a result, the resistance of the MR element to a sensing electric current changes accordingly. The varying voltage signal is representative of the magnetic pattern stored on the magnetic medium.

In the manufacturing of TFH transducers for magnetic recording, a large number of devices are fabricated simultaneously in deposited and patterned layers on a ceramic wafer. When completed, the wafer is cut (or diced) and machined into individual slider transducers. The main elements of a TFH inductive transducer, roughly in the order in which they are deposited, are the (alumina) undercoat, the bottom magnetic pole, the flux gap material to provide spacing between the bottom and top magnetic pole-tips, one or more levels of electrical conductive coil winding interposed within insulation layers, the top magnetic pole, elevated studs (or posts) for connecting the coil to bonding pads (above the overcoat), a thick (alumina) overcoat, and the bonding pads.

Usually the studs are made of about 25-50.mu.m thick copper, plated through a thick photoresist mask right after the completion of the top magnetic pole and prior to the deposition of thick alumina overcoat. For reasons of uniformity and reproducibility, the studs are designed to extend to a level about 10.mu.m above the highest spot of the TFH device (the back yoke of the top pole). For each coil level the stud thickness must be increased by at least 6-9.mu.m. For similar reasons, the alumina overcoat thickness must exceed the stud thickness by about 10.mu.m, requiring an overcoat about 35-60.mu.m thick. Following the overcoat deposition, the studs are covered by the alumina overcoat. The wafer is then lapped (ground) down to expose the copper studs. Next, (gold) bonding pads are fabricated over the alumina overcoat to contact the exposed studs. The pads are later used for bonding wires to external circuitry to the read/write channel. Fabrication of studs and bonding pads is described in U.S. Pat. No. 4,219,853, for example. It is similar in both inductive and MR devices. For TFH devices combining an inductive write element and an MR read element, one on top of the other, the stud and alumina overcoat operations have to be repeated. This may bring the total thickness of the studs to about 40-70.mu.m, and the total alumina overcoat thickness to about 50-80.mu.m. As the alumina deposition process is relatively slow, about 1-2.mu.m/hour, it can easily become the bottleneck (1-3 days) in the process flow. Furthermore, alumina sputtering equipment is very costly. As a result, the alumina overcoat deposition is by far the costliest and the slowest step in TFH wafer fabrication.

The integrity and chipping resistance of the alumina overcoat are directly related to its thickness and stress. The thicker the alumina layer, the more stress it incorporates. The alumina overcoat's stress also adversely affects the noise performance of the TFH device, since it is in direct contact with the top magnetic pole. Moreover, tall features of the device, such as the studs, impair the integrity of the deposited alumina in the local surrounding area. Sawing and machining operations subject the alumina overcoat to local stresses which often result in excessive chipping and rejects.

Very thick studs require a relatively long plating time and the thickness uniformity is quite difficult to control. To achieve the required thickness, stud plating dictates the use of a dry film photoresist which is difficult to strip and process. The usual fabrication of studs and bonding pads demands numerous steps of photomasking, deposition of seed-layers, plating, stripping, and etching. The elaborate structure of the studs and bonding pads, including coil leads, permalloy, and all the seed-layers, may involve some 10-15 individual metallic layers deposited in succession on top of each other. The probability of bonding failures due to inadequate adhesion between any two of these metallic layers is directly proportional to the number of these layers. Thus reliability of the bond strength is adversely affected by the large number of the metallic layers in the structure. The poor integrity of the alumina overcoat in the locales surrounding the studs further degrades the bond strength and often impairs the fabrication of bonding pads due to "loops" of collapsed alumina around the studs. The latter appear following the lapping of the alumina overcoat to expose the studs, and are due to the poor integrity of the alumina surrounding the studs.

Copper residues and contamination, originating from copper seed-layers used for the coil and stud platings, may cause shorts and impair the corrosion resistance of the TFH device. Copper coil seed-layer residues are often found to be responsible for intensified corrosion susceptibility of exposed pole-tips in the air bearing surface (ABS). This poor corrosion resistance is due to galvanic action between the two dissimilar metals (Cu and Ni-Fe) contacting at the corners (or wings) of the pole-tips. Similarly, the copper seed-layer of the studs is often found to degrade coil-to-core resistance due to full or partial shorts. The copper coil itself is sometimes subject to capricious variations of its electrical resistance due to corrosion and oxidation of the copper turns during elevated temperature curing cycles of the photoresist insulation layers.

SUMMARY OF THE INVENTION

The present invention provides an economical method for fast and precise patterning of thick sputter-deposited alumina layers by etching through appropriate metallic masks. It further provides improvements in product design and performance which avoid the above-described shortcomings of the prior art. In particular, the precise patterning of thick alumina layers by etching enables the fabrication of studless TFH devices, scribe-line grooves for dicing, recessed craters for TFH devices, and lightning arresters.

Elimination of the studs simplifies the wafer fabrication in a very substantial way. It curtails about 20-30% of the total wafer processing steps and reduces the alumina overcoat thickness by about 30-50%. It thus produces the substantial benefits of reducing processing costs, decreasing device noise, and improving throughput, consistency, bonding strength, and corrosion resistance. Alumina chipping is usually a major contributor to yield loss. Etching scribe-line grooves facilitates significant reduction of alumina chipping during slider machining and later in tests and service. It thus significantly improves total device yields. Pattern-etching precise craters in the alumina undercoat facilitates the fabrication of TFH devices with recessed structures or features. These can improve device planarity and alleviate problems associated with tall features of multi-level coil designs. They can also improve the symmetry of pole-tips configuration. Similarly, etching vias through the alumina undercoat layer enables the construction of lightning arresters by electrically connecting the magnetic core to the substrate, which will be grounded in the process of packaging the slider. The lightning arresters provide grounding to prevent arcing between the magnetic core and rotating disk.

In accordance with this invention, the patterning of thick sputter-deposited alumina layers is accomplished by, first, depositing compatible metallic layers on the alumina, coating the metallic layers with photoresist, patterning the photoresist by photolithography, etching the metallic layers (in sequence) through the photoresist mask, stripping the photoresist, etching the thick alumina layer through the metallic mask, and removing (in sequence) the metallic masking layers. A preferred etchant comprising dilute HF in water (1:2 by volume ratio) has been found to provide fast (about 1.mu.m/min), extraordinarily uniform (better than .sigma.=1% across a wafer), and accurate etching of thick alumina layers. The etching can be conveniently performed by spray or immersion. A compatible metallic mask must be used with the dilute HF etchant since photoresist masks lift-off from the alumina surface after a very short time. A preferred metallic mask comprises a very thin (100-300.ANG.) Cr or Ti adhesion layer covered by a gold (Au) layer, about 0.3-0.5.mu.m thick. To avoid porosity in the gold layer and for economic reasons, at least one half of the gold thickness can be plated from an high purity (99.9%) "soft" gold bath.

In accordance with the invention a studless TFH device with gold coil and bonding pads, patterned and plated simultaneously, is provided. Following the alumina overcoat deposition, vias are etched through it to expose the bonding pads. This eliminates copper altogether from the TFH device, resulting in less maintenance and significantly improved reliability, corrosion resistance, and consistency of the coil resistance. Similarly, the thick alumina is pattern-etched to form scribe-line grooves which minimize alumina chipping during sawing and machining. Other improvements in a TFH device resulting from deep alumina pattern-etching include the formation of recessed craters to improve planarity and pole-tips symmetry and the formation of vias to the substrate for lightning arresters connecting the magnetic core to the substrate.

An object of this invention is to provide a method of pattern-etching through an alumina layer.

Another object is to provide a studless TFH device.

A further object of this invention is to provide a simple, fast, and low cost method to eliminate many processing steps and increase throughput in the process of fabricating a TFH device.

An additional object is to significantly decrease alumina overcoat thickness to further reduce processing cost and improve reliability and throughput.

Another object is to decrease alumina overcoat stress and related chipping and device noise.

An additional object of this invention is to improve bonding strength and reliability by eliminating many metallic layers in the stud structure.

An additional object is to eliminate dry photoresist processing.

An additional object is to provide gold coil and bonding pads, fabricated simultaneously, to further delete some processing steps and to entirely eliminate copper material in the TFH device

Another object of the invention is to improve the corrosion resistance of a TFH device

A further object of the invention is to provide scribe-line grooves for dicing and slider machining to reduce alumina chipping and improve yields

Another object of the invention is to provide vias through the alumina undercoat to the substrate to enable the construction of lightning arresters.

A still further object of this invention is to provide precise craters in the alumina undercoat to improve device planarity and pole-tips symmetry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a typical prior-art stud and bonding pad structure.

FIGS. 2(a) and 2(b) show cross-sections of alternative bonding pad structures embodying the present invention.

FIGS. 3(a) to 3(h) are cross-sectional views, at various stages, during the patterning of thick alumina, such as used for the studless pads of the present invention.

FIG. 4 is a cross-sectional view of a scribe-line groove after pattern-etching.

FIG. 5 is a cross-sectional view of a recessed crater after pattern-etching .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A schematic (not to scale) cross-section of a typical prior-art stud and bonding pad structure is shown in FIG. 1. An alumina undercoat 12 is first deposited over a ceramic substrate 10. The wafer is then subjected to the deposition and patterning of many layers. A typical succession of the metallic layers in the stud area may include a Cr (or Ti) seed-layer 14, a Cu seed-layer 16, a first level plated Cu coil 18, a Cr (or Ti) seed-layer 20, a Cu seed-layer 22, a second level plated coil 24, a Ni-Fe seed-layer 26 for the top pole, a first plated Ni-Fe layer 28 of the top pole, a second plated Ni-Fe layer 30, a Cu seed-layer 32 for the stud, a plated Cu stud 34, alumina overcoat 35, a Ni-Fe seed-layer 36 for the bonding pad, and a gold bonding pad 38 A wire 39 is bonded to gold bonding pad 38, typically by thermocompression or ultrasonic compression techniques. In some cases the stud structure may include other layers, such as a Ni-Fe seed-layer and plated layers of the bottom pole (between layers 12 and 14), or a Cr (or Ti) seed-layer under Cu seed-layer 32 for the stud, or under Ni-Fe seed-layer 36 for the bonding pad, or under Ni-Fe seed-layer 26 for the pole plating. In other cases, several of these layers are omitted. Typically the stud and bonding pad structure comprise about 10-15 individual metallic layers. In some cases, the bonding pad structure is placed on the alumina overcoat in a partial overlapping and staggering position relative to the stud structure. Bonding is made to the off-set portion of the pad to ensure adequate bonding strength. Such elaborate structures of the studs and bonding pads are susceptible to bonding failure due to the large number of metallic layers in the structure. In addition, the poor integrity of the alumina overcoat in the region surrounding the tall stud further impairs reliability and bonding strength.

According to a preferred embodiment of this invention, the entire stud fabrication is eliminated, resulting in a simple bonding pad structure of only two metallic layers, shown in FIG. 2(a). A single seed-layer 36, such as Ni-Fe or W, is bonded to an alumina undercoat 12, which overlies a substrate 10. Seed layer 36 must be compatible for gold plating and must have good to moderate electrical conductivity and strong adhesion to alumina. Other metals and alloys suitable for this purpose include Mo, Ni-Co, Ni-Co-Fe, Cd, and In. The thickness of seed-layer 36 in FIG. 2(a) is in the range 600-2,000.ANG., and preferably in the range 1,000-1,500.ANG.. This structure has only two metallic layers in the pad. An Au bonding pad 38 is preferably plated simultaneously with the coil turns and leads (not shown) through an appropriate photoresist mask. Both the coil and the bonding pads will then consist of the same (Au) metal. If the coil consists of more than a single level, different pads may be deposited and patterned simultaneously with different coil levels. Alternatively, one or more pads may be deposited and patterned repeatedly with more than one coil level. Such pads, however, will have more metallic layers in their structure. Following the coil/pad plating and stripping of the photoresist mask, seed-layer 36 must be removed from between the Au-plated areas. Using a different metal or alloy for seed-layer 36 from that used for pad 38 enables its removal by selective chemical etching, as described in U.S. Pat. No. 5,059,278 to Cohen et al., incorporated herein by reference. Wire 39 is bonded to the (Au) bonding pad 38 through the etched vias in the alumina overcoat 40.

In another embodiment, the pad structure comprises a total of three or more metallic layers, as shown in FIG. 2(b). The structure combines at least two seed-layers. The first seed-layer comprises an adhesion layer 14 of Cr or Ti or another refractory metal such as W, Ta, Mo, Nb, V, Zr, and Hf, or an alloy comprising one or more of these metals. The function of this layer is to enhance adhesion between alumina and the following gold layer (gold alone has poor affinity to oxygen and, therefore, poor adhesion to alumina). The thickness of the adhesion seed-layer 14 can be in the range 50-500.ANG., and preferably in the range 100-300.ANG.. The layer is usually deposited by a vacuum technique such as sputtering or evaporation. An Au seed-layer 37 is similarly deposited by sputtering or evaporation. It provides a low electrical resistance and compatible metallic surface for the plating of a gold pad 38. Other metals or alloys compatible with gold plating and having good electrical conductivity may also be used for the second metallic seed-layer 37. They include Ni-Fe, W, Mo, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more of these metals. In contrast, most refractory metals possess high electrical resistivity and are incompatible for gold plating. The thickness of the Au seed-layer 37 is in the range 500-3,000.ANG., and preferably in the range 700-1,000.ANG.. Plated gold pad 38 has a thickness in the range 1.0-5.0.mu.m, and preferably in the range 2.0-4.0.mu.m. Again, the coil turns and leads and bonding pads may be plated simultaneously through a suitable photoresist mask. Alternatively, the coil may comprise gold or another highly conductive metal which is deposited and patterned separately from the bonding pad 38.

Other combinations may also be used. In particular, bonding pad 38 of FIGS. 2(a) and 2(b) may comprise Au, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more elements thereof. Usually the same layer is also used for the coil winding turns. Alternatively, the coil turns may comprise a different metal or alloy than the pad. For example, the coil turns may comprise Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more elements thereof, while the bonding pad may comprise gold. However, different metals or alloys in the coil and pads require further processing steps, which add to manufacturing complexity and cost. It may be noted that although Al has both excellent conductivity and adhesion to alumina, it is incompatible for plating (of the coil). It is thus unsuitable as a seed-layer for any plating. However, Al as a single layer may be used simultaneously for both the coil and the bonding pad, if deposited by a vacuum technique (such as sputtering or evaporation) and patterned by etching. Appropriate etching techniques for Al patterning include ion milling and reactive ion etching (RIE). Wet etching of Al lacks the fine resolution required for the coil definition. When Al is used in this mode, seed-layer 36 of FIG. 2(a) and seed-layers 14 and 37 of FIG. 2(b) are eliminated, resulting in an even simpler structure. The noble metals Au, Cu, Ag, Pd, Pt, Rh, Ir, and alloys comprising one or more metals thereof may also be deposited as a continuous layer by plating or by a vacuum technique, and then pattern-etched simultaneously for both the coil turns and pads by ion milling or by RIE. The dry etching technique requires an appropriate etch mask combining both the coil and the pads patterns. It is formed over the top continuous metal layer. The remains of the etch mask are removed following the dry etching. Top layer noble metals require an intermediate adhesion layer similar to seed layer 36 in FIG. 2(a) and seed layer 14 in FIG. 2(b). Alternatively, the coil turns may comprise a different metal than the bonding pads and fabricated by different techniques. For example, the coil turns may comprise sputter or evaporation deposited Al and may be patterned by ion milling or RIE, while the bonding pads may consist of plated gold.

Patterning of thick alumina layers requires an etching mask which is substantially free of pores and not etched or adversely affected by the alumina etchant. When using dilute HF etchant for the alumina layer, etching masks based on photoresist alone are inadequate since they lift-off after only a short time in contact with the etchant The dilute HF rapidly attacks the bond between the alumina and the photoresist. Metal masks must be used for the prolonged etching. The metal mask must have excellent adhesion to the alumina layer and it should be substantially free of pores. It must also be immune against attack by the etchant.

FIGS. 3(a) through 3(h) illustrate the major steps involved during the patterning of thick alumina, according to a preferred embodiment of this invention. FIG. 3(a) shows the structure after a photoresist layer 48 has been coated over the metallic mask layers and patterned by photolithography. Prior to coating the photoresist layer, successive seed-layers 42 of Cr or Ti and 44 of Au or Ni-Fe were deposited by a vacuum technique over a thick alumina layer 40 Seed-layer 42 must provide excellent adhesion to the alumina surface. It comprises a refractory metal from the group Cr, Ti, W, Ta, Mo, Nb, V, Zr, Hf, and alloys comprising one or more of these metals The thickness of adhesion seed-layer 42 can be in the range 50-500.ANG., and preferably in the range 100-300.ANG.. Since most refractory metals (such as Cr, Ti, Ta, Nb, V, Zr, and Hf) are incompatible for gold plating and have poor electrical conductivity, seed-layer 44, with good conductivity and compatibility for gold plating, is deposited over adhesion seed-layer 42 Gold is excellent for this purpose and is the preferred choice for seed-layer 44 However, it requires an expensive gold sputtering target Alternatively, seed-layer 44 may comprise a metal from the group Ni-Fe, Ag, Pd, Pt, Rh, Ir, W, Mo, and alloys comprising one or more metals thereof A single seed-layer 44 comprising Ni-Fe, W, Mo, or an alloy comprising one or more of these metals, possesses good adhesion and electrical conductivity and is compatible for gold plating It can thus simplify the metallic mask by eliminating layer 42. The advantage of using Ni-Fe for seed-layer 44 is that it is already being utilized in the TFH fabrication process. The thickness of seed-layer 44 can be in the range 500-5,000.ANG., and preferably in the range 700-1,500.ANG.. Usually vacuum deposition produces layers 42 and 44 which are quite porous and permeable and, therefore, inadequate for use alone as the metallic mask for thick alumina etching.

After deposition of layers 42 and 44, the wafer is placed in a high purity (99.9%) "soft" gold plating bath, and a layer 46 of gold is plated over seed-layer 44. To ensure that plated gold layer 46 be free of pores, its thickness should exceed about 0.1-0.2.mu.m. Usually the preferably in the range 0.2-0.5.mu.m. Although it would be simpler to utilize Au seed-layer 44 alone, without the plated Au layer 46, sputter-deposited layers contain some pores even at thicknesses of more than 1.0.mu.m. Furthermore, thick (.gtoreq.2.mu.m) sputter-deposited Au layers significantly increase the manufacturing cost due to the very expensive gold targets Photoresist mask layer 48 is coated over the metallic mask layers and patterned by a regular photolithography technique.

Next, the pattern is transferred to the metallic layers to form the etching mask. FIG. 3(b) shows the structure after pattern-etching gold layers 46 and 44 through the photoresist mask. A convenient etchant for the gold layers is based on a dilute solution of KI and I.sub.2 in water:

______________________________________ Potassium Iodide KI 100-200 g/l Iodine I.sub.2 25-50 g/l ______________________________________

This etchant is compatible with the positive photoresist mask. It takes about 1-2 minutes to etch Au layers 46 and 44 using this etchant at room temperature. It can be applied by immersion with moderate agitation, or by spraying. Lower concentrations and temperature decrease the etch rate, while higher concentrations and temperature increase it.

Following the pattern-etching of Au layers 46 and 44, the pattern has to be further transferred by etching to the Cr or Ti adhesion layer 42. First the photoresist layer 48 is removed using conventional stripping method, such as by dipping in or spraying with a c