Durable bond pad structure for electrical connection to extreme environment microelectronic integrated circuits

Abstract

A durable bond pad structure is described that facilitates highly durable electrical connections to semiconductor microelectronics chips (e.g., silicon carbide (SiC) chips) to enable prolonged operation over very extreme temperature ranges.

Claims

1. An apparatus comprising: a bond pad metal stack having an interface in direct physical, mechanical, and, conductive electrical contact with a surface of a conductive region of a semiconductor, wherein the conductive region of a semiconductor possesses a single doping polarity, a first insulating dielectric layer comprising a patterned via, wherein the patterned via extends through only the first insulating dielectric layer, wherein the first insulating dielectric layer is in contact with an outside lateral interface of the bond pad metal stack and also in direct contact with the surface of the conductive region of the semiconductor, and at least one electrically conductive metal layer that is not in physical contact with the bond pad metal stack, but is in electrical and physical contact through the patterned via with the conductive region of the semiconductor, wherein an electrical signal path flows through the bond pad metal stack, the conductive region of the semiconductor, and through the at least one electrically conductive metal layer that carries electrical signals to/from semiconductor devices and circuits residing elsewhere on the semiconductor.

2. The apparatus of claim 1, further comprising a second upper insulating dielectric layer that overcoats the apparatus, except that a top surface of the bond pad metal stack is devoid of the second upper insulating dielectric layer.

3. The apparatus of claim 1, wherein the first insulating dielectric layer and the second upper insulating dielectric layer comprise different materials.

4. The apparatus of claim 1, wherein the conductive region is below the first insulating dielectric layer.

5. The apparatus of claim 4, wherein the conductive region is a microscopically flat/smooth, mechanically hard, and conductive region of a silicon carbide (SiC) single-crystal semiconductor.

6. The apparatus of claim 1, wherein the conductive region is a conductive SiC layer.

7. The apparatus of claim 1, wherein the bond pad metal stack comprises an iridium interfacial stack (IrIS) comprising a TaSi.sub.2 layer, a lower platinum layer, an iridium layer, and an upper platinum layer.

8. The apparatus of claim 7, wherein a top gold layer is added above the IrIS.

9. The apparatus of claim 7, wherein the TaSi.sub.2 layer has a depth of about 400 nm and all other layers of the IrIS have a depth of about 200 nm.

10. The apparatus of claim 1, wherein the at least one electrically conductive metal layer comprises a high temperature refractory metal or metal alloy.

11. The apparatus of claim 10, wherein the at least one electrically conductive metal layer comprises titanium.

12. An apparatus comprising: a bond pad metal stack having a bottom interface in direct physical, mechanical, and, conductive electrical contact with a surface of a conductive region of a silicon carbide (SiC) single-crystal semiconductor, wherein the conductive region of the SiC single-crystal semiconductor possesses a single doping polarity, a first insulating dielectric layer comprising a patterned via, wherein the patterned via extends through only the first insulating dielectric layer, wherein the first insulating dielectric layer is in contact with an outside lateral interface of the bond pad metal stack and also in direct contact with the surface of the conductive region of the SiC single-crystal semiconductor, a second upper insulating dielectric layer that overcoats the apparatus, except that a top surface of the bond pad metal stack is devoid of the second upper insulating dielectric layer, and at least one electrically conductive metal layer that is not in physical contact with the bond pad metal stack, but is in electrical and physical contact through the patterned via with the conductive region of the SiC single-crystal semiconductor, wherein an electrical signal path flows through the bond pad metal stack, the conductive region of the SiC single-crystal semiconductor, and through the at least one metal layer that carries electrical signals to/from semiconductor devices and circuits residing elsewhere on the SiC single-crystal semiconductor.

13. The apparatus of claim 12, wherein the first insulating dielectric layer and the second upper insulating dielectric layer comprise different materials.

14. The apparatus of claim 12, wherein the bond pad metal stack comprises an iridium interfacial stack (IrIS) comprising a TaSi.sub.2 layer, a lower platinum layer, an iridium layer, and an upper platinum layer.

15. The apparatus of claim 14, wherein a top gold layer is added above the IrIS.

16. The apparatus of claim 14, wherein the TaSi.sub.2 layer has a depth of about 400 nm and all other layers of the IrIS have a depth of about 200 nm.

17. The apparatus of claim 12, wherein the at least one electrically conductive metal layer comprises a high temperature refractory metal or metal alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a top view schematic of a prior art bond pad configuration.

(2) FIG. 1B is a side view cross section schematic of a prior art bond pad configuration.

(3) FIG. 2A is a top view schematic of a bond pad configuration according to an embodiment of the innovation.

(4) FIG. 2B is a side view cross section schematic of a bond pad configuration according to an embodiment of the innovation.

(5) FIG. 3A is a top view schematic of a bond pad configuration according to an embodiment of the innovation.

(6) FIG. 3B is a side view cross section schematic of a bond pad configuration according to an embodiment of the innovation.

(7) FIG. 4 is a photograph of an embodiment of the innovation recorded using scanning electron microscopy.

(8) FIG. 5 is a graph depicting an example of experimentally measured bond pad to bond pad resistance over time.

DETAILED DESCRIPTION

(9) According to an aspect of the innovation, a bond pad structure is disclosed that may be used for integrated circuits that function for prolonged time in extreme temperatures. In one embodiment, the bond pad structure may be directly attached to the semiconductor. In one embodiment, the conductive bond pad metal stack may be directly attached to a conductive region/layer of the semiconductor. The conductive portion/layer of the semiconductor may be a layer of silicon carbide (SiC). The typical prior art bond pad structure included bond pad metal attached to a dielectric layer between the bond pad metal and the semiconductor as depicted in FIGS. 1A and 1B.

(10) It was discovered that attaching the bond pad metal to a dielectric layer between the pad metal and semiconductor provides for inferior mechanical adhesion making the relatively thicker metal bond pad film susceptible to damage from CTE mismatch stress especially imparted by operation across extreme temperature ranges. It was also discovered that superior mechanical adhesion is obtained by directly depositing the bond pad onto the mechanically strong and flat semiconductor (e.g., SiC semiconductor). The extremely flat/smooth semiconductor surface foundation improves the flatness/smoothness of the subsequently deposited bond pad metal films, which in turn can facilitate improved/easier wire bonding and bond durability and strength.

(11) According to an aspect, the structure according to the innovation reduces the impact of physical dielectric crack formationif and when it occurs due to large thermally-induced stressto regions where there are no oxygen-sensitive metals or metal interfaces. This simultaneously allows thermally-induced film stress (which can be significant for extreme temperature operating ranges) to be relieved without the cracking exposing the oxygen-sensitive metal layers and oxygen-sensitive metal-SiC interfaces to air, directly or via interfacial lateral enhanced diffusion path. Such exposure leads to device operation degradation. Cracks that penetrate from the dielectric surface to the SiC surface without contacting or passing through metals, on the other hand, do not impact device functionality. Also, the direct connection of the bonding metallization to the pristine and flat and extra-hard SiC surface (rendered by final wet etch of dielectric just prior to deposition) provides maximum adhesion and mechanical strength for the bond pad to better withstand bonding and thermal-stress forces with reduced damage/degradation. This structure also promotes durability and repeatability because the bond pad stack metal film interfaces and surfaces have a smooth microscopic morphology. As described below, the bond pad design according to the innovation has been tested and shown thousands of hours of operation at 500 C.

(12) In one embodiment, the conductive layer of the semiconductor is a microscopically flat/smooth, mechanically extremely hard, and conductive region of a silicon carbide (SiC) single-crystal semiconductor.

(13) In one embodiment, the bond pad may be laterally patterned as an isolated island of metal that is not in direct physical contact with any other patterned metal or interconnect. As depicted, for example, in FIGS. 2A and 2B, the bond pad metal stack 220 does not come in direct contact with the conductive interconnect (e.g., metal layer 210) that carries electrical signal between the bond pad and the rest of the integrated circuitry residing on the semiconductor chip. In this embodiment, the bond pad metal 220 is completely laterally surrounded by layers of dielectric (e.g., layers 230, 240). In some embodiments, insulating dielectric layers 230 and 240 may be comprised of the same dielectric insulating material, but lower dielectric layer 240 is deposited and suitably patterned in a separate processing step prior to the subsequent deposition and patterning of upper dielectric layer 230. In alternative embodiments, the upper dielectric layer 230 may be comprised of a different dielectric material than lower dielectric material 240. The upper dielectric layer 230 is configured so that it overcoats and protects conductive interconnect metal layer 210 from being exposed to atmospheric oxygen during extreme temperature operation that otherwise could oxidize and degrade metal layer 210 from performing its electrical function of conducting electrical signals. The lower dielectric layer 240 is configured with patterned etched vias 215 that enable metal layer 210 to directly contact and make electrical connection to the surface of conductive region 270. Portions of the bond pad metal 220 and the lower dielectric layer 240 and the metal layer 210 are adjacent to and in contact with at least a portion of the conductive region 270 of the semiconductor.

(14) According to this embodiment of the innovation, the electrical signal must flow through the conductive region 270 of the semiconductor, as depicted by dashed arrows 211 in FIG. 2B. In conventional bond pad structures, patterned metal carrying electrical signals elsewhere within the IC physically contact/connect the bond pad metal (as seen in FIG. 1, electrical signal flow 111 only flows through metals).

(15) In one embodiment, the dielectric laterally surrounding the bond pad metal (e.g., 230B and 240B) is itself completely laterally surrounded by a pattern of high-temperature interconnect metal 210. FIG. 2b depicts a cross-section showing an upper dielectric layer 230 and a lower dielectric layer 240. A portion of upper dielectric 230B is adjacent to bond pad metal 220 and metal layer 210. In some embodiments, such as the one depicted in FIG. 2B, the small portion of lower dielectric 240B that resides immediately adjacent to bond pad metal 220 is laterally surrounded by a ring of metal 210, and also contacts the conductive SiC layer 270 below and the SiC substrate 250.

(16) In one embodiment, the lateral distance between the laterally enclosing metal ring 210 and bond pad island metal 220 is between 1 and 10 the wafer fabrication process lateral patterning minimum dimension (lambda, known in the art). In some embodiments, the latter distance between the bond pad 220 and metal ring 210 may be reduced. Reduction of the lateral distance between bond pad island 220 and metal ring 210 will improve the quantitative electrical performance by reducing the electrical series resistance of the bond pad structure. However, if this lateral distance is made too small it disadvantageously increases the chances that a crack formed in the dielectric might undesirably reach metal 210 which in turn would undesirably lead to oxidation and failure of metal 210 under hightemperature operation.

(17) In one embodiment, the laterally enclosing metal is electrically connected to the bond pad metal through vias 215 through the lower oxide dielectric 240 overlying the outer lateral region of the conductive wide band gap semiconductor feature. As illustratively indicated by dashed arrow path 211 in e.g., FIG. 2B, the electrical signal flows down from the bond pad metal 220, into and then laterally through the conductive wide band gap semiconductor feature 270, then up into metal 210 residing inside/at oxide via 215 and then laterally through metal 210 to the rest of the integrated circuit (not shown). It is known in the art that the direction of electrical signal can also be reversed from the depiction shown in FIG. 2B (i.e., the arrow directions reversed, indicating opposite signal flow direction from the chip out of the bond pad) depending on electrical signal bias conditions and purpose.

(18) In one embodiment, the conductive semiconductor layer 270 through which electrical signal flows is formed with first doping polarity (n-type or p-type, known in the art), and resides atop underlying semiconductor (e.g., an SiC semiconductor) region whose conductivity is intentionally made insulating or opposite-type from first doping polarity type via doping techniques known in the art.

(19) According to an embodiment, the innovation may include a bond pad metal stack having multiple layers. In one embodiment, the bond pad metal stack may comprise or include an iridium interfacial stack (IrIS). In one embodiment, the layers of the bond metal stack may be arranged as follows:

(20) TABLE-US-00001 Gold Layer Upper Platinum Layer Iridium Layer Lower Platinum Layer TaSi.sub.2 Layer Conductive SiC Layer

(21) In one embodiment, the bond pad metal stack may comprise the following layers (in order of deposition, from bottom contacting the SiC to top exposed bonding surface) 4000 Angstrom thick layer of TaSi.sub.2, 2000 Angstrom thick layer of Platinum, 2000 Angstrom thick layer of Iridium, 2000 Angstrom thick layer of Platinum, ultrahigh vacuum sputter deposited in suitable sputter system to permit these layers to be deposited without breaking vacuum, capped by 1 micron thick gold deposited by vacuum E-beam evaporation. Sputter-deposited IrIS bond pad layers are disclosed in U.S. Pat. No. 9,013,002, which is incorporated herein in its entirety. Other bond pad metallization layer structures could be employed besides IrIS, so long as they meet electrical and mechanical durability requirements for extreme temperature. Such requirements are described in detail in U.S. Pat. No. 9,013,002 and include ability to form strong bond wire attachment, the ability to conduct electrical signals for long (e.g., months or years in duration) exposures to 500 C. oxidizing air environment.

(22) For example, the Ti/TaSi2/Pt metal contact stack described in Okojie, et al., Reliability assessment of Ti/TaSi2Pt ohmic contacts on SiC after 1000 h at 600 C., J Appl. Phys., 91:10, pp. 6553-6559 (2002), which is incorporated herein in its entirety. In one embodiment, the total thickness of the bond pad stack is less than 2 micrometers as thinner stacks desirably reduce top metal surface roughness and can be deposited more quickly in mass-production. Thicknesses of such stacks can suitably range between about 0.2 micrometers to about 20 micrometers, about 0.5 micrometers to about 15 micrometers, about 0.75 micrometers to about 10 micrometers, about 1 micrometer to about 5 micrometers, or about 1 micrometer to about 4 micrometers. In one embodiment, the stacks have a thickness between about 1 micrometer to about 3 micrometers.

(23) Turning now to FIGS. 2A and 2B and FIGS. 3A and 3B, embodiments of the present innovation are disclosed. The insulating dielectric layers cross-sections seen in FIGS. 2B and 3B are omitted from top-view depictions FIGS. 2A and 3A for clarity. According to the innovation, the bond metal stack (220, 320) is directly above and in contact with the conductive layer of semiconductor 270, 370 (e.g., a conductive SiC layer) and at least one insulating dielectric layer (e.g., 230, 240 or 330, 340).

(24) According to these embodiments, the bond metal stack (220, 320) does not directly contact the metal layer 210. In one embodiment, the metal layer may comprise a high temperature refractory metal or metal alloy. The metal layer may possess a high melting temperature and high crystallization temperature. Suitable metals include elemental films and compounds and alloys of tantalum, silicon, carbon, aluminum, tungsten, molybdenum, nickel, titanium, hafnium, iridium, platinum, gold, silver, germanium, gallium, or a combination of two or more thereof.

(25) As shown by the dashed arrows in FIGS. 2B and 3B, the electrical signal path, 211, 311 flows through the bond metal stack 220, 320, the conductive layer 270, 370, and then through metal layer 210, 310. As discussed above, the configuration in which the bond pad metal stack 220, 320 is attached to the semiconductor (conductive) layer 270, 370 provides for superior mechanical adhesion making the relatively thicker metal bond pad film less susceptible to damage from CTE mismatch stress. The extremely flat/smooth semiconductor surface foundation improves the flatness/smoothness of the subsequently deposited bond pad metal films, which in turn can facilitate improved/easier wire bonding and bond durability and strength.

(26) The upper dielectric layer 230, 330 overcoats and protects/passivates interconnect metal 210, 310 from exposure to environmental impurities and atmosphere that could undesirably degrade device/circuit electrical properties. Such dielectric protection, also called passivation or a passivation layer, is well-known and practiced in the art of conventional integrated circuit manufacture, and is shown as element 130 in FIG. 1B. For extreme temperature integrated circuit operation, this layer performs the additional function of protecting interconnect from exposure to atmospheric oxygen that would otherwise cause oxidation at the extreme operating temperature and harm electrical conduction of interconnect metal (e.g., 210, 310). It is known in the art that most modern integrated circuits contain more than one layer of conductive interconnect. Typically, only one such metal interconnect layer is used to connect the bond pad to the rest of the integrated circuit.

(27) This is in comparison to the bond pad structure of the prior art as depicted in FIGS. 1A and 1B that is used and sufficient for typical integrated circuits that operate over conventional electronics temperature below 200 C. As with FIGS. 2A and 3A, the dielectric layers in FIG. 1A is omitted for clarity. As depicted in FIG. 1B, the bond metal stack 120 is adjacent to and in contact with the metal layer 110. The electrical signal path 111 flows from the bond metal stack 120 through the metal layer 110. As noted above, this configuration may suffer from failure related to extreme temperature induced buckling and de-lamination of the metal 110 connected to bond pad metal 120 at/along the dielectric/metal interface. In particular, the adhesion of the metal film to the dielectric is insufficient to withstand the much larger stress buildup that arises due to CTE mismatch combined with the much larger temperature range of operation, and the relatively large lateral area of bond pads compatible with package connection techniques (wire bonding or flip-chip attachment) known in the art. The larger CTE mismatch stress buildup of higher temperature operation may sometimes also lead to crack formation/propagation of the upper protective dielectric passivation films. When cracks in upper dielectric 130 happen to form/propagate above interconnect metal 110, they provide a path for atmospheric oxygen to reach, oxidize, and fail the underlying metal interconnect 110 during high temperature operation.

(28) According to an aspect of the innovation, a durable bond pad structure may be made for microelectronic integrated circuits in extreme environments, including extreme temperatures of 400 C. and above. In one embodiment, the structure includes a bond pad metal stack structure that includes multiple layers. In one embodiment, the bond pad metal stack may be formed in a deposition process that takes place near the end of the wafer fabrication process after deposition of all dielectric layers at temperatures greater than 500 C. has occurred. In such case the patterned removal of all dielectrics in regions where the bond pad metal is to be deposited should preferably be accomplished immediately prior to bond pad metal deposition. While such patterned dielectric removal can be accomplished by a combination of dry etching and/or wet etching known in the art, it is preferable that the removal of the bottom-most dielectric be accomplished by wet buffered oxide etchant known in the art to leave a clean and flat SiC surface as the foundation for the bond pad stack metal deposition. In one embodiment, the method may include loading the SiC wafer into the vacuum deposition as soon as possible following the final patterned dielectric wet etch.

(29) In one embodiment, the lateral circular diameter of the patterned bond pad metal stack (e.g. the IrIS stack) is close to 160 micrometers, but this size can be adjusted to between 40 micrometers and 300 micrometers depending upon the specifications of the wire bonding or flip-chip process being used to make electrical connection to the bond pad.

(30) In some embodiments, the bond pad diameter should be selected large enough for subsequent attachment of robust gold-alloy wires via ball-type wire bonding known in the art. In another embodiment, the lateral bond pad shape may consist of a rounded-corner square-like shape with suitably rounded corners or a rectangular shape with suitably rounded corners. Lateral bond pad shapes with sharp corners are undesirable as sharp corners are known to undesirably enhance/concentrate stress.

(31) In one embodiment, the laterally enclosing metal (e.g., 210, 310) is a lateral extension one of the metal interconnect layer patterns used in the construction of interconnects between transistors within the transistor integrated circuit, with selected material, thickness, and deposition method selected to meet the integrated circuit fabrication requirements. In one embodiment, the laterally enclosing metal is selected to be hollow ring shape that simultaneously desirably minimizes the electrical series resistance of the invention.

(32) In one embodiment, metal interconnect (e.g., 210, 310) may be comprised of TaSi.sub.2 that may be deposited by close-proximity sputtering process (target-to-substrate distance of less than 3 cm) and patterned by reactive ion etching and may further include photolithographic patterning to form an enclosing metal pattern (e.g., 210, 310). In one embodiment, the lateral distance between the laterally enclosing metal 210, 310, and bond pad island metal (e.g., 220, 320) is approximately 10 micrometers, which is approximately 1.6 lambda dimension of 6 micrometers of this particular process.

(33) In one embodiment, the SiC semiconductor may comprise 4HSiC with a patterned and thermally activated ion implant of n-type dopant (e.g., nitrogen and/or phosphorous dopants) to increase the electrical conductivity of conductive region of semiconductor 370. (See, e.g., FIG. 3b.) In one embodiment, the dielectric may comprise 1 micrometer thick SiO.sub.2 deposited at 720 C. by low-pressure chemical vapor deposition (LPCVD) using tetraethyl orthosilicate (TEOS) precursor.

(34) In one embodiment, patterned vias through the SiO.sub.2 layer are patterned by dry and wet etching prior to metal TaSi.sub.2 deposition to facilitate desired electrical connection between the interconnect metal to the conductive 4HSiC layer. In one embodiment, a different contacting metal layer can be inserted at the bottom of the via between the TaSi2 and the conductive 4HSiC layer. The details of a suitable via etch followed by deposition of different contacting metal plus TaSi.sub.2 metal into the via are disclosed in U.S. patent application Ser. No. 15/438,130, hereby incorporated in its entirety. Using these processes and the bond pad structure represented in FIG. 2, bond pads were successfully fabricated and shown to electrically function for thousands of hours at 500 C. with little degradation/change in electrical conduction as shown, for example in FIG. 5).

(35) What has been described above includes examples of the innovation. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject innovation, but one of ordinary skill in the art may recognize that many further combinations and permutations of the innovation are possible. Accordingly, the innovation is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term includes is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term comprising as comprising is interpreted when employed as a transitional word in a claim.