PAD METALLIZATION SYSTEMS AND RELATED METHODS

Implementations of a method of forming an interconnect may include providing a silicon carbide semiconductor substrate including a plurality of aluminum pads thereon, each pad surrounded by a passivation material and forming a barrier layer over the plurality of aluminum pads and the passivation material. The method may include forming a seed layer over the barrier layer; patterning a layer of photoresist with a plurality of openings exposing the plurality of aluminum pads; forming a copper layer over each of the aluminum pads by electroplating into each opening of the plurality of openings; and forming a nickel layer over the copper layer. The method may include forming one of a gold layer or a palladium layer over the nickel layer; removing the layer of photoresist; and etching the barrier layer and the seed layer to form a plurality of copper-containing interconnects.

BACKGROUND

1. Technical Field

Aspects of this document relate generally to methods for forming electrical interconnects. More specific implementations involve electrical interconnects that include copper pads.

Semiconductor packages have been devised that allow for routing of electrical signals from a semiconductor die to components of the package used to interface with a circuit board or motherboard to which the semiconductor package is attached. Semiconductor packages also have been developed that assist with protecting the semiconductor die from humidity, thermal conditions, or shock and vibration during operation and use.

SUMMARY

Implementations of a method of forming an interconnect may include providing a silicon carbide semiconductor substrate including a plurality of aluminum silicon copper pads thereon, each pad surrounded by a passivation material and forming a barrier layer over the plurality of aluminum silicon copper pads and the passivation material. The method may include forming a seed layer over the barrier layer; patterning a layer of photoresist with a plurality of openings exposing the plurality of aluminum silicon copper pads; forming a copper layer over each of the aluminum silicon copper pads by electroplating into each opening of the plurality of openings; and forming a nickel layer over the copper layer. The method may include forming one of a gold layer or a palladium layer over the nickel layer; removing the layer of photoresist; and etching the barrier layer and the seed layer to form a plurality of copper-containing interconnects.

Implementations of a method of forming an interconnect may include one, all, or any of the following:

Forming the copper layer, the nickel layer, and the one of the gold layer or the palladium layer on each aluminum silicon copper pad of the plurality of aluminum silicon copper pads further may include forming using electrolytic plating.

The copper layer may be 10 microns thick, the nickel layer may be between 2 microns to 4 microns thick, and the gold layer may be 0.5 microns thick.

Forming the barrier layer or the seed layer further may include using sputtering.

The method may include forming a backmetal on the silicon carbide semiconductor substrate.

Each aluminum silicon copper pad of the plurality of aluminum silicon copper pads may be formed of a top layer of aluminum silicon copper that contacts the barrier layer and a bottom layer of aluminum silicon copper.

The top layer of aluminum silicon copper may be 1 micron thick.

Implementations of a method of forming an interconnect may include providing a silicon carbide semiconductor substrate including a plurality of aluminum pads thereon, each pad surrounded by a passivation material and forming a barrier layer over the plurality of aluminum pads and the passivation material. The method may include forming a seed layer over the barrier layer; patterning a layer of photoresist with a plurality of openings exposing the plurality of aluminum pads; forming a copper layer over each of the aluminum pads by electroplating into each opening of the plurality of openings; and forming a nickel layer over the copper layer. The method may include forming one of a gold layer or a palladium layer over the nickel layer; removing the layer of photoresist; and etching the barrier layer and the seed layer to form a plurality of copper-containing interconnects.

Implementations of a method of forming an interconnect may include one, all, or any of the following:

Forming the copper layer, the nickel layer, and the one of the gold layer or the palladium layer on each aluminum pad of the plurality of aluminum pads further may include forming using electrolytic plating.

The copper layer may be 10 microns thick, the nickel layer may be between 2 microns to 4 microns thick, and the gold layer may be 0.5 microns thick.

Forming the barrier layer or the seed layer further may include using sputtering.

The method may include forming a backmetal on the silicon carbide semiconductor substrate.

Each aluminum pad of the plurality of aluminum pads may be formed of a top layer of aluminum that contacts the barrier layer and a bottom layer of aluminum.

The top layer of aluminum may be 1 micron thick.

Implementations of a method of forming an interconnect may include providing a silicon carbide semiconductor substrate including at least two pads thereon; forming a first interlayer dielectric layer over the at least two pads; and planarizing the first interlayer dielectric layer. The method may include forming a second interlayer dielectric layer over the first interlayer dielectric layer; forming a plurality of vias in the second interlayer dielectric layer and in the first interlayer dielectric layer; and forming a source pad over the plurality of vias. The method may include forming a first passivation layer over the source pad; etching the first passivation layer to expose the source pad; and forming a second passivation layer over the first passivation layer, the second passivation layer having an opening therein exposing the source pad; wherein one of the at least two pads may be a gate feed.

Implementations of a method of forming an interconnect may include one, all, or any of the following:

The method may include planarizing the first passivation layer.

The method may include forming a first patterned layer before forming the first interlayer dielectric layer and removing the first patterned layer after forming the first interlayer dielectric layer.

The method may include forming a second patterned layer before forming the second interlayer dielectric layer and removing the second patterned layer after forming the second interlayer dielectric layer.

The method may include forming a third patterned layer before forming the first passivation layer and removing the third patterned layer after forming the first passivation layer.

The method may include: forming a barrier layer over the source pad and the second passivation layer; forming a seed layer over the barrier layer; patterning a layer of photoresist with an opening exposing the source pad; forming a copper layer over the source pad by electroplating into the opening; forming a nickel layer over the copper layer; forming one of a gold layer or a palladium layer over the nickel layer; removing the layer of photoresist; and etching the barrier layer and the seed layer to form a copper-containing interconnect.

DESCRIPTION

In various semiconductor devices, logic circuits and power circuits can be combined. For example, in the case of a semiconductor device used to operate an automatic window opener in an automobile, the logic portion of the device senses the control signal indicating the window is to be raised or lowered and then activates the power portion (a metal oxide field effect transistor [MOSFET]) to transfer power to the motor for the specified period of time to allow the window to physically be raised or lowered. The voltage and power used by the logic portion of the device is typically much smaller in comparison with the voltage and power needed to operate the motor, so if the electrical interconnects used for the logic portion are used for the power portion of the device, the electrical interconnects can be damaged through overheat and/or excessive current load, even during the relatively short time that the high voltage/amperage is experienced during operation. In such systems, electrical interconnects adapted for higher power operation, such as thick copper interconnects like pads/bumps/pillars greater than 10 microns in height can be used to help the interconnects avoid damage during the high voltage transient load period. Similar structures in the form of metal lines greater than 10 microns in height are also used, and the principles disclosed herein can also be applied with metal lines. As wirebonding is often used to connect the source and drain pads of the MOSFET, for example, the ability to create reliable wirebonds with a wire bondable material of the thick copper interconnects is a design consideration when such combined logic and power circuits are combined in a semiconductor device.

In some process implementations, thick copper pads with a layer of gold and/or nickel on their upper surface are used. Since the sidewalls of the thick copper pads remain exposed in this process, the likelihood that particulate contamination can cause shorting between pads is increased as there is nothing present to fill the gaps between adjacent pads. In other process implementations, to assist with reducing the likelihood that particulate contamination can cause failures with the thick copper pads, the use of polyimide layers that fill in the space between the pads and cover the upper surface of the thick copper pillars is used in applications such as, by non-limiting example, automotive and other vehicle industries. Openings in the polyimide layer to expose the upper portion of the thick copper pillars are made into which the wirebonds are formed. In order to get the polyimide to adhere to the gold on the upper surface of the thick copper pads however, a film of palladium needs to be used as the top layer over the upper surface. The polyimide can also adhere to a layer of aluminum formed at the top layer over the upper surface of the thick copper pads, but does not adhere to the gold of the thick copper pads themselves.

Various methods for forming thick copper interconnect systems are disclosed in this document that utilize various layers of materials to which a polyimide material will adhere over the top surface of a thick copper pad/bump/pillar. These methods are merely exemplary, and combinations of these methods and materials disclosed herein may be employed in various system and method implementations. While in this document the term “pads” is used primarily to describe the methods and structures herein, it should be understood that the same principles may be applied with other metallization structures including bond pads, metal lines, or other large areas of metallization like dummy structures used to ensure local and wafer-level uniformity.

One of the challenges of forming wirebonds with pure copper pads is that the presence of copper oxide on the bond surface of the pads interferes with the formation of the intermetallic and alloy compounds that form a good mechanical and electrical connection between the bond wire and the copper pads. Thus, the ability to minimize and reduce the formation of copper oxide during the processing steps assists with the creation of good wirebonds. The various methods implementations disclosed herein show various approaches to reduce copper oxide formation on the bond surface/top surface of the copper pads disclosed herein.

Referring to FIG. 1, a side cross sectional view of a semiconductor device 2 is illustrated which includes semiconductor substrate portion 4 on which is formed a plurality of pads 6. These pads 6 represent the outer layer of a stack of materials that are formed on/in the semiconductor substrate portion 4 which, for the sake of easier illustration, are not shown in FIG. 1 or the other figures, but are understood to be present as forming the active device(s) included in the semiconductor device 2. This stack of material helps forms structures such as, by non-limiting example, transistors, diodes, interconnects, traces, vias, insulating layers, and any other component of an active semiconductor device. Around each of the pads 6 is a layer of passivation material 8 which may be, by non-limiting example, silicon nitride, a polyimide, or another electrically insulating material. The particular material of the semiconductor substrate may be, by non-limiting example, silicon, silicon carbide, silicon on insulator, ruby, sapphire, gallium arsenide, gallium nitride, or any other semiconductor material type. The particular types of semiconductor devices that may employ the method implementations disclosed here may include, by non-limiting example, transistors, MOSFETs, insulated gate bipolar transistors (IGBTs), diodes, power devices, high electron mobility transistors (HEMTs), rectifiers, or any other power semiconductor device or other active semiconductor device type. While in FIG. 2 just two pads are illustrated, it is understood that this is merely for illustration convenience as the method implementations disclosed may be employed at the die-level or wafer-level portions of the fabrication process and many more pads than just two pads may be employed in various implementations.

Referring to FIG. 2, a side cross sectional view of the semiconductor device 2 of FIG. 1 is illustrated following formation of a seed layer 10 thereon for use in electroplating. The use of the seed layer 10 permits the formation of an electrical connection to the seed layer material covering each of the pads 6 during the electroplating operation. In various implementations, the seed layer 10 may be formed by sputtering. The seed layer 10 may include, by non-limiting example, a titanium and tungsten film, a copper film, any combination thereof, or another metal film type that permits electroplating of pure copper to the seed layer.

Following formation of the seed layer 10, referring to FIG. 3, a layer of photoresist 12 is applied which is then patterned. The application process for the photoresist 12 may include, by non-limiting example, spin coating, dispensing, stencil printing, squeegee application, or any other method for forming a uniform layer of material above the seed layer 10. The patterning process used may be specific to the particular type of photoresist employed. For example, where a positive photoresist is employed, the regions of the photoresist above the pads 6 are exposed with an electromagnetic radiation source through a mask to cause a corresponding reaction in the material of the photoresist that permits a developing solution to wash away the exposed regions. Where a negative photoresist is employed, the regions of the photoresist outside the pads are exposed to electromagnetic radiation causing a corresponding reaction in the material of the photoresist that prevents the developing solution from washing away the exposed regions. As illustrated in FIG. 3, the openings 14 in the photoresist 12 are larger than the size of the pads to allow the passivation material and any other under bump materials to assist in supporting the to-be-formed bump and help prevent issues like bump cracking later during operation. However, in other implementations, the size of the openings 14 may be the same as or substantially the same as the size of the pads. The height/thickness 16 of the photoresist 12 is set to be higher than or substantially the same as the height of the copper pads to be electroplated to help form the sidewalls and a flat upper/bonding surface for each of the copper pads.

Referring to FIG. 4, the semiconductor device 2 is illustrated following completion of the electroplating process that forms a copper pad/bump/pillar 18 within each of the openings 14 of the photoresist 12 which is mechanically and electrically connected with each corresponding pad 6. In this implementation, a pure copper pad has been formed, meaning that substantially all of the material of the pad is formed of just copper. In other implementations, however, a copper alloy could be electroplated, depending on the electrical or mechanical characteristics desired for the pads.

Referring to FIG. 5, the semiconductor device is illustrated following formation of one or more layers of material 24 on the top side/bonding surface 22 of each of the copper bumps 18 and after stripping/removal of the photoresist. In various method implementations, the one or more layers may be formed using electroless plating. Where the one or more layers are formed using electroless plating, the electroless plating may be carried out while the photoresist is still present around the copper pads 18 as illustrated in FIG. 4 after electroplating of the copper pads. In other implementations however, the electroless plating may take place after the photoresist has been removed and after removal of the seed layer 10 as in the structure illustrated in FIG. 6. While the one or more layers 24 are illustrated as being present on only the top of the copper pads 18 in FIG. 5 for ease of illustration, where the electroless plating takes place after removal of the photoresist and seed layer, it is understood that the material of the one or more layers may deposit on all exposed surface of each of the copper pads 18. Thus, if the one or more layers 24 are desired to be formed only on the bonding surface 22 of the copper pads 18, then the electroless plating would take place while the photoresist 12 is still in place as illustrated in FIG. 4.

While the previous discussion discloses the use of electroless plating to form the one or more layers of material 24, in other method implementations, sputtering may be used to form the one or more layers of material 24. In such implementations, the sputtering takes place following the electroplating of the copper pads 18 while the photoresist 12 is still in place around the pads as illustrated in FIG. 14. The photoresist 12 is then stripped/removed as previously described, leaving the one or more layer of material 24 on the bond surface 22 of each of the copper pads 18 as illustrated in FIG. 5.

The materials used in the one or more layers of material 24 depend on the particular method of formation. Where electroless plating is employed, the material of the one or more layers may be nickel, gold, or palladium as a single layer in some implementations. In others, the material of the one or more layers may be nickel, gold, or palladium in two or more separate layers each formed through electroless plating. In yet other implementations, the material of the one or more layers may be any combination of nickel, gold, and/or palladium formed in a single layer or in multiple layers using electroless plating. Where the use of sputtering is employed, the material of the one or more layers may be, by non-limiting example, either titanium, nickel, and gold or titanium, nickel, and palladium formed in either a single layer or as separate layers.

The process of removing the photoresist may take place using various methods, including, by non-limiting example, solvent stripping, washing, ashing, etching, or any other method for removing polymer material. As illustrated, the as-plated copper pads 18 are then exposed and rise above the surface of the seed layer 10 at a desired height/thickness 20. As the seed layer 10 electrically shorts all of the copper pads 18 and all of the pads 6 together, it needs to be removed to electrically isolate all of the pads 6 from each other once again.

Referring to FIG. 6, the semiconductor device 2 of FIG. 5 is illustrated following removal of the seed layer material 10 from around the copper pads 18, leaving the seed layer 10 present only as an under bump material of each of the copper pads. The removal of the seed layer 10 may be carried out using etching in various method implementations. FIG. 6 also illustrates that, after the seed layer 10 has been removed, the one or more layers of material 24 remain formed onto the top surface/bonding surface 22 of each of the copper pads 18. In various implementations, the one or more layers of material 24 may be formed of a material that is not removed/etched by the process used to remove/etch the seed layer.

Referring to FIG. 7, the semiconductor device 2 is illustrated following application of a layer of polyimide 26 over the copper pads 18 and the patterning of opening 28 over the top surface/bonding surface 22. Note that not every copper pad 18 may have the top surface 22 with the one or more layers of material 24 exposed through an opening in the layer of polyimide 26. This may be because not all of the pads 6 are actually used for electrical interconnects and may be dummy pads used to preserve a certain amount of metal density at the pad layer to help improve processing uniformity or to help provide mechanical support/strength to the semiconductor device. The particular patterning method and process used to form the opening 28 in the polyimide 26 depends on the material of the polyimide and whether it is patterned photolithographically using positive or negative exposure processes like those previously described in this document. Because material types used in the one or more layers of material 24 are those previously described in this document, the polyimide 26 will adhere to the one or more layers 24 to form the opening 28 and allow for wirebonding into the opening 28. FIG. 8 illustrates the semiconductor device 2 following wire bonding of a bond wire 30 onto the one or more materials 24 and to the copper pad 18.

Prior to the wirebonding process, the semiconductor device 2 may have the semiconductor substrate portion thinned through, by non-limiting example, backgrinding, lapping, separating, polishing, or any other thinning process. Because of the use of the polyimide 26, the mechanical strength of the remaining portion of the semiconductor substrate portion may be improved to support a thinner die than could otherwise be produced without the presence of the polyimide 26 in the material stack. A wide variety of method variations may be constructed using the principles disclosed in this document.

Referring to FIG. 9, a second semiconductor device 32 is illustrated in a side cross sectional view that is similar in structure to the semiconductor device 2 of FIG. 1. The semiconductor device 32 includes pads 34 which are surrounded by passivation material 36. In the various method implementations, the material of the semiconductor substrate portion 38 of the semiconductor device 32 may be any disclosed in this document. In the various method implementations, the material of the passivation material 36 may also be any disclosed in this document. Also, the semiconductor device 32 may be any semiconductor device type disclosed in this document.

Referring to FIG. 10, the semiconductor device 32 is illustrated in a side cross sectional view following formation of a seed layer 40 over the pads 34. The material of the seed layer 40 may be any disclosed in this document used for seed layers for electroplating copper. FIG. 11 illustrates the semiconductor device 32 following formation of a patterned layer of photoresist 42 over the seed layer 40 with openings 44 over the pads 34. The material of the photoresist 42 may be any disclosed in this document and may be patterned using any of the patterning method implementations disclosed herein in various method implementations. The height/thickness 46 of the photoresist 42 above the seed layer 40 is again set at a level to achieve/support formation of copper pads at a desired thickness during electroplating.

Referring to FIG. 12, the semiconductor device 32 is illustrated in a side cross sectional view following electroplating of copper pads 48. The electroplating process for forming the copper pads 48 and the material of the copper pads 48 may be any disclosed in this document. FIG. 12 also illustrates the semiconductor device 32 following the formation of one or more layers 50 over the top surface/bonding surface 52 of each of the copper pads 48. In various method implementations, the one or more layers 50 may include different materials formed and processed using corresponding methods. In a particular implementation, the one or more layers 50 is an organic coating that, when applied, prevents or substantially prevents reaction of oxygen with the newly plated copper of the copper pads 48, thus reducing the formation of copper oxide on the bonding surface 52. This organic coating is applied soon after the completion of the electroplating of the copper pads, thus helping to seal the bonding surface 52 of the copper pads 48 from reacting further with oxygen. Furthermore, the organic coating is a material to which a subsequently applied polyimide material will adhere. Also, the organic material in this implementation is one that a wirebond can be formed through during the wirebonding process and does not involve a cleaning or removal step.

In another implementation, the one or more layers 50 is an organic material that is thicker or more tightly bonded to the copper layer than the previously disclosed organic material of the previous method implementation. This organic material has the same ability to prevent further formation of copper oxide on the bonding surface 52 of the copper bumps 48, but needs to be removed/cleaned from the bonding surface 52 prior to the actual wirebonding using a cleaning step that will be discussed hereafter. In various implementations, the organic materials that may be employed in either method implementations may be organic solderability preservatives marketed by RBP Chemical Technology Inc. of Milwaukee, WI; MacDermid Alpha Electronics Solutions of Waterbury, CT; or Shikoku Chemicals Corporation of Marugame, Kagawa, Japan.

In another implementation, the one or more layers 50 is a layer of aluminum that is deposited on the bonding surface 52 that reacts with oxygen immediately to form aluminum oxide (alumina). In particular implementations, the layer of alumina is deposited using atomic layer deposition and may be between 1 to 5 atoms thick. In other implementations, however, other materials other aluminum may be used in the one or more layer 50, such as, by non-limiting example, titanium nitride, tantalum nitride, any combination thereof, or other material capable of being deposited using ALD. Such a layer is sufficiently thick to prevent diffusion of oxygen to the copper pad 48 and the formation of copper oxide but thin enough to be wire bonded through during the wirebonding process. In various method implementations where atomic layer deposition is employed, the aluminum is deposited while the photoresist 42 is present in the atomic layer deposition chamber leading to the structure illustrated in FIG. 12. In other implementations, however, the aluminum may be deposited after the photoresist 42 has been stripped from the seed layer 40 but before the seed layer 40 is etched. This may take place to avoid issues with outgassing or contamination of the process from the large thickness of photoresist present after the electroplating process has been completed. The aluminum adds the additional benefit in that a subsequently applied polyimide will adhere to the aluminum layer.

In other method implementations, a layer of aluminum between about 1 to about 4 microns is sputtered to form the one or more layers 50. In some implementations, the layer of aluminum may be about 2 microns thick. The layer of aluminum, when applied soon after plating of the copper, may assist with reducing copper oxide formation and also assist with adhesion of the polyimide subsequently formed over the copper pads.

Referring to FIG. 13, the semiconductor device 32 is illustrated in a side cross sectional view following removal of the photoresist. In the various method implementations, the photoresist may be removed using any of the methods disclosed in this document. In FIG. 13, the one or more layers 50 illustrated of the copper pads 48 may be either of the organic materials previously mentioned or the layer of aluminum deposited using atomic layer deposition previously discussed. FIG. 14 illustrates the semiconductor device 32 following removal of exposed portions of the seed layer 40 that are not part of the copper pads 48. The removal of the seed layer 40 may take place using any of the methods disclosed herein. In various methods, the particular removal method may also take into account the resistance of the material present of the one or more layers 50 to the particular removal method. In some implementations where the aluminum layer is present which was applied after removal of the photoresist, causing the aluminum to be deposited on the seed layer 40, a removal method that is capable of removing the aluminum either separately initially or in combination with the material of the seed layer 40 may be employed.

Referring to FIG. 15, the semiconductor substrate 32 is illustrated following application of a polyimide layer 54 over the copper pads 48 and patterning of that layer to form opening 56. FIG. 15 is a side cross sectional view following removal of the material of the one or more layers 50 that exposed through the opening 56. In this method implementation, the material of the one or more layers 50 is the organic material that needs to be removed/cleaned from the bonding surface 52 of the copper pads 48 prior to wirebonding. In this method implementation, because the organic material cannot be bonded through, it needs to be removed following the formation of the polyimide layer 54 while still providing adhesion to the bonding surface 52 of the copper pads 48 for the polyimide.

FIG. 16 illustrates an alternative structure that results from an alternative method implementation used where the aluminum layer or the organic coating forms the one or more layers 50 formed over the bonding surface 52. As in FIG. 15, a polyimide layer 54 has been formed over the copper pads 48 and patterned using any of the methods disclosed herein to form opening 56. However, the exposed material of the one or more layers 50 is not removed in this method implementation and so wire bonding takes place by bonding through either the organic coating or the aluminum layer formed by atomic layer deposition. In this way, the bond is not affected by copper oxide while a pure copper pad is still being bonded to while the polyimide layer 54 is still able to adhere to the bonding surface 52 of each of the copper pads 48.

Referring to FIG. 17, copper pads 58 are illustrated after removal of the photoresist as illustrated in FIG. 12 and after etching of the seed layer 60. In this method implementation, however, no layer is applied over the top/bonding surface 62 of the copper pads following formation and prior to etching of the seed layer 60, leaving the top/bonding surface 62 as copper. FIG. 18 illustrates the copper pads 58 of FIG. 17 following formation of a polyimide layer 64 and formation of opening 66 to expose the top/bonding surface 62. In some method implementations, the polyimide layer 64 sufficiently sticks/bonds with the top/bonding surface 62 of the copper pads 58 so that no additional layer is applied to the top/bonding surface 62. In such implementations, the bonding is then carried out directly onto the top/bonding surface 62.

In other method implementations, like the one illustrated in FIG. 19, following forming the opening 66 in the polyimide 64, one or more layers 68 is formed over the top/bonding surface 62 of the copper pad 58. In some implementations, electroless plating may be used to form a layer of nickel, a layer of palladium, a layer of gold, or any combination thereof to form the one or more layers 68. In other implementations, sputtering may be used to apply titanium and aluminum (either as separate layers or a single layer) to form the one or more layers 68. In other implementations, sputtering may be used to apply titanium tungsten and aluminum (either as separate layers or a single layer) to form the one or more layers 68. In yet other implementations, atomic layer deposition may be employed to form the one or more layers 68 of any of the materials disclosed herein used for atomic layer deposition. As illustrated in FIG. 19, where sputtering is used to form the one or more layers 68, the material of the one or more layers 68 may rise up around the edges of the polyimide layer 64 of the opening 66.

A wide variety of possible combinations of electroless plated/sputtered/atomic layer deposition may be employed to form various layers on the top/bonding layer 62 of the copper pads 58 in various method implementations.

The various semiconductor device implementations disclosed in the following discussion related to devices made specifically with silicon carbide or gallium nitride substrates specifically. Referring to FIG. 20, an implementation of a power metal oxide field effect transistor (MOSFET) 70 is illustrated with just the far backend interconnect layers shown on the semiconductor substrate 72 and with the remaining interconnect/device layers under metal 1 below these layers and formed in the substrate not shown for the purposes of more clear illustration. While a power MOSFET device is illustrated in FIG. 20, the principles disclosed herein could be employed with any other silicon carbide or gallium nitride semiconductor device type (diode, high electron mobility transistor, power semiconductor device, rectifier, etc.) that utilizes pads for forming subsequent interconnects with a circuit/motherboard using wirebonds, bumps, or pillars.

In FIG. 20, a source pad 74, gate pad 76, and a gate runner 78 are illustrated. The source pad 74 includes a first layer of aluminum silicon copper 80 and a second layer of aluminum silicon copper 82. Gate pad 76 includes a first layer of aluminum silicon copper 84 and a second layer of aluminum silicon copper 86 separated by gate oxide layer 88. In this implementation, gate runner 78 is made of a single layer of aluminum silicon copper, though in other implementations two layers of aluminum silicon copper could be used. The gate runner 78 is routed away from the source pad 74 and gate pad 76, which means that part of the surface area of the power MOSFET device is dedicated to the location of the gate runner.

In a particular power MOSFET device design, the thickness of the first layer of aluminum silicon copper 80 is about 3 microns and the total thickness of the two layers is about 7 microns to about 10 microns. During temperature cycling reliability testing designed to simulate operation of the power MOSFET over its entire design lifetime, recrystallization of the aluminum silicon copper and corresponding deformation of the first and second layers of aluminum silicon copper 80, 82 has been observed following failure analysis. The deformation has been observed to be so significant that cracking of the passivation layers on the die edge and dielectric layer material cracking in gate runner structures next/under the source pad has been observed. Where an electroless plated nickel palladium gold layer is plated over the first layer of aluminum silicon copper 80, 84, cracking of the nickel palladium gold layer has also been observed during known good die testing due to softening of the aluminum silicon copper layers caused by heating of the layers which permits recrystallization and corresponding deformation to begin.

To combat the interlayer dielectric cracking around the gate runner, the gate runner 78 can be routed a sufficient distance away from the source pad 74, but this increases the total surface area needed for the power MOSFET device thus increasing the die size. Also, where soldering is used to attach to the nickel palladium gold layer the nickel can become fully consumed in the joint forming process or during subsequent operation of the device which creates a weak joint between the aluminum silicon copper layer and the solder which can result in device failure. The silicon carbide MOSFETs disclosed herein may be operated in harsher application environments that demand higher power dissipation and/or continuous operation at about 185 C to about 200 C. Under these conditions, the degradation of the electroless plated nickel palladium gold in combination with the deformation of the about 5 micron thick first and second layers of aluminum silicon copper 80, 82 for the source pad 74 may cause enough failures during reliability testing indicating that field failure likelihood is unacceptably high.

To combat the deformation of the first and second aluminum silicon copper layers, reduction of the thickness of the first layer is disclosed in this document. Elimination of electroless plated layers and the use of electroplated layers is also disclosed herein. Also, in a particular method and systems implementation, the use of only aluminum in the pad layers instead of aluminum silicon copper is disclosed.

Referring back to FIG. 20, in this power MOSFET implementation 70, the thickness of the first layer of aluminum silicon copper 80 is between about 1 micron to about 2 microns instead of about 3 microns. The ability to reduce the thickness of the first layer of aluminum silicon copper correspondingly reduces the likelihood of heat-induced deformation. It also results in a reduction of the cost to form the interconnects as less aluminum silicon copper material is needed in the interconnect structure. This reduction of the thickness of the first layer of aluminum silicon copper is facilitated by the use of the electroplated layers. In the implementation of FIG. 20, the combined thickness of the first layer of aluminum silicon copper 80 and the second layer of aluminum silicon copper 80 may be between about 7 microns to about 10 microns. In some implementations, aluminum silicon only or aluminum only pads could also be used in the system and methods disclosed herein.

In FIG. 20, a layer of passivation material 90 is illustrated formed around the gate runner 78, source pad 74, and gate pad 76. In this implementation the passivation material is a photodefinable polyimide (PDPI) material which is initially applied over the pads and then exposed and developed to form openings through which the source pad 74 and gate pad 76 are exposed. While the use of a polyimide is illustrated in FIG. 20, in other implementations materials like a die coat material or a benzocyclobutene (BCB) material could be utilized as well. In some implementations, a hard passivation material could also be used, though the use of a compliant polymeric material may aid with handling stresses during temperature cycling caused during operation. As illustrated in FIG. 20, an additional layer(s) of passivation material 92 may also be formed over the gate runner 78 and adjacent to the source pad 74 and gate pad 76. In this implementation, the passivation material 92 is a hard passivation material (in this case silicon nitride).

The various device structure implementations disclosed herein may be formed using various implementations of a method of forming an interconnect. Referring to FIG. 21, the power MOSFET implementation 70 of FIG. 20 is illustrated following formation of barrier layer 94 over the passivation material 90 and first layer of aluminum silicon copper 80 of the source pad 74 and the first layer of aluminum silicon copper 84 of the gate pad 76. The barrier layer 94 may be formed of, by non-limiting example, titanium tungsten, titanium tungsten nitride, titanium nitride, any combination thereof, or another material designed to prevent diffusion of copper into the die stack materials and material of the silicon carbide substrate. FIG. 21 also illustrates seed layer 96 that has been formed over the barrier layer 94. In this implementation, the seed layer 96 is made of copper to facilitate copper electroplating. Because the seed layer is deposited over the entire surface of the substrate, electrical connections needed to electroplate copper are present on the substrate.

Referring to FIG. 22, the power MOSFET implementation 70 is illustrated following application of a layer of photoresist 98 over the seed layer 96. The thickness of the layer of photoresist may be determined by the desired thickness of the metal layer(s) that will be electroplated onto the seed layer in various method implementations. The layer of photoresist may be formed using spin coating, stencil printing, squeegeeing, or through a dry film application. FIG. 23 illustrates the layer of photoresist 98 following patterning using a lithographic exposure and develop process. The unexposed (in the case of negative photoresist) or exposed (in the case of a positive photoresist) material is removed to expose the source pad 74 and gate pad 76 with the seed layer 96 thereon. At this point, the power MOSFET implementation 70 is ready for electroplating.

Referring to FIG. 24, the power MOSFET implementation 70 is illustrated after completion of electroplating. As illustrated, three layers have been sequentially electroplated, one on top of the other. The first layer is copper layer 100. In various method implementations, the copper layer 100 can be between about 2 microns to about 25 microns thick. In a particular implementation, the thickness of the copper layer is about 10 microns thick. The thickness of the copper layer may be determined by the power dissipation requirements for the particular power MOSFET device (RDSon). The next layer is nickel layer 102. In various method implementations, the nickel layer 102 may be between about 1 micron to about 5 microns thick. In a particular implementation, the thickness of the nickel layer may be between about 2 microns to about 4 microns thick. The thickness of the nickel layer may be determined by the particular continuous operation temperature of the particular power MOSFET device and/or the mechanical/bonding requirements for the interconnect (solder reflow temperature profile, sintering process parameters, etc.). The next layer is gold layer 104 as gold is used in the implementation illustrated in FIG. 24. In other method implementation, however, a palladium layer may be electroplated instead. The thickness of the gold layer 104 in various implementations may be about 0.15 microns to about 0.5 microns thick. The thickness of the palladium layer would be between about 0.05 microns to 0.2 microns thick in various implementations. In a particular implementation, the thickness of the gold layer 104 may be 0.5 microns. Because of the higher mechanical strength of the combined copper layer 100, nickel layer 102, and gold/palladium layer 104, the thickness of the first layer of aluminum silicon copper can be reduced to about 1 micron to about 2 microns.

In various method implementations, following electroplating of the copper layer 100, an additional layer can be placed over the copper layer 100 to reduce/prevent continuing oxidation during the other electroplating operations. The additional layer may be, by non-limiting example, an organic layer that is removed as the nickel plating operation begins, a thin layer of copper oxide, or a 1-10 nanometer thick layer of a metal other than copper applied using atomic layer deposition. The use of the additional layer may help assist with prevent increasing resistance in the interconnect caused by the uncontrolled formation of copper oxide as the power MOSFET implementation 70 awaits nickel electroplating for a period of time that may vary from substrate to substrate.

Referring to FIG. 25, the power MOSFET implementation 70 is illustrated following removal of the photoresist layer 98 using an ashing process, solvent stripping process or other method adapted for the particular material of the photoresist 98. FIG. 26 illustrates the power MOSFET implementation 70 following etching of the seed layer 96 and the barrier layer 94 from the passivation layer 90. The etching process may be carried out using an etching process selective to the material of the barrier layer 94 and seed layer 96 relative to the gold/palladium layer 104. At this point the source pad 74 and gate pad 76 are again electrically isolated from each other.

Also illustrated in FIG. 25 is the power MOSFET implementation 70 after formation of a backmetal layer (backmetal) 105 on a back side of the silicon carbide substrate. The backmetal layer may be formed of one layer or multiple layers of the same or dissimilar materials which may be, by non-limiting example, nickel, titanium nickel, titanium nickel vanadium silver, titanium nickel vanadium gold, aluminum nickel palladium gold, aluminum nickel vanadium gold, aluminum nickel vanadium silver, gold, palladium, aluminum silicon copper, or any combination thereof. Where nickel is present in a sputter backmetal material a corresponding portion of vanadium from the target may be present. The use of the backmetal may allow the power MOSFET implementation 70 to be cooled from both sides (dual side cooled) which may be a key capability where the device is operated under the extremely high stresses of 2000 V and higher and/or at high continuous operating temperatures. The stress of the backmetal thickness/material, etc. may also be adjusted to adjust the warpage of the silicon carbide substrate and/or the semiconductor die that is ultimately singulated form the silicon carbide substrate.

While the use of two layers of aluminum silicon copper to form the source pad 74 has been illustrated in the foregoing implementation, a single layer of aluminum silicon copper or aluminum silicon is possible, however, it requires the size of the pad to be larger than a two layer pad to deal with the stress of the pad. Thus the use of the two layers reduces stress. Reduced stress allows for smaller pads which allows for a smaller die size and the corresponding cost savings.

In the foregoing example, the use of a silicon carbide substrate has been illustrated. In various other device and method implementations, the substrate may also be a gallium nitride substrate structured for lateral GaN devices or vertical GaN devices. In various gallium nitride substrates, the entire substrate may be composed of gallium nitride or the substrate may include a layer of gallium nitride formed thereon.

The foregoing system and method implementations have involved pads formed of aluminum silicon copper. In other system and method implementations, the pads may be formed of only aluminum. Particular implementations include the use of two layers of aluminum as illustrated in FIGS. 20-26 to assist with stress reduction in a similar way as previously described. In these implementations, the thickness of the first aluminum layer on the source pads and gate pads would also be about 1 micron to about 2 microns instead of about 3 microns, enabled by the mechanical strength of the electrolytic plated copper/nickel/gold or palladium interconnect structure previously described. Because aluminum is sufficiently ductile, it is able to act as a stress buffer between the material of the die stack/silicon carbide substrate and the copper/nickel/gold layers of the interconnect during operation of the device.

Where the pads are made of only aluminum, in a particular implementation, the first layer of the pads may be made of copper while the second layer of the pads may be only aluminum. In such implementations, the aluminum may provide sufficient stress relief in a single layer to allow the first layer of the pads to be made of electroplated copper. In such implementations, the method of formation would be modified to include providing a silicon carbide substrate that includes a plurality of aluminum pads (as the second or bottom layer of the pads) thereon. A barrier layer and seed layer (which may be made of the same material in various implementations) are then formed over the aluminum pads and the remainder of the exposed surface of the silicon carbide surface. A photoresist layer is then applied and patterned to expose the aluminum pads. A copper electroplating process is then carried out to form the second layer of the pads made of copper. Following the electroplating process, the photoresist is removed using an ashing or solvent stripping process like any disclosed in this document. The seed layer is then etched using a process consistent with selective removal of the seed layer material. At this point the pads include a first layer of copper and a second layer of aluminum and are now ready for additional processing to finish forming the interconnect structure. In various implementations, the thickness of the first copper layer of the pads may be about 2 microns to about 25 microns. In some implementations, the thickness of the first copper layer of the pads may be between about 5 microns to about 10 microns.

Depending on the dimensions desired for the interconnect, because the first layer of the pad is copper, in a particular method implementation, the seed layer may not be etched and the photoresist layer may not be removed and additional copper electroplating using the existing seed layer may be carried out to form the bottom layer of the interconnect. This copper electroplating process is followed by electroplating of the nickel and gold/palladium layers as previously described. In such implementations, the same seed layer may be used to form the electrical connection to the pads to enable the electroplating, and it is etched away after electroplating is completed and removal of the photoresist layer is completed. Thus, in this method, no new seed layer may be needed to form the interconnect structure. In various method implementations, any of the copper oxidation control/prevention methods/systems previously described may be utilized to manage oxidation of the copper layer on the pads and/or the copper layer in the interconnect.

Where the interconnect structure is desired to be not substantially coextensive with the shape of the copper layer of the pads (wider, smaller, etc.), the method includes, after the photoresist layer is removed and the seed layer is removed, forming a passivation material around the pads (like a patterned polyimide or other material like those disclosed herein). A second barrier layer and seed layer (which may be of different materials or the same materials like any disclosed in this document) is then deposited over the copper layer of the pads and the passivation material. The copper layer of the interconnect is then electroplated followed by the electroplating of the nickel layer and the gold or palladium layer using processes like those previously described. The thicknesses of the copper, nickel and gold/palladium layers may be any of those previously described. In various method implementations, any of the copper oxidation control/prevention methods/systems previously described may be utilized to manage oxidation of the copper layer on the pads and/or the copper layer in the interconnect.

The foregoing discussion involves pad structures that formed of two layers of metal formed directly over each other. However, because the use of the electroplated copper/nickel/gold or palladium layers in the interconnect, a different pad design can be employed with the silicon carbide substrate to allow the gate runner to be routed below the source pad without the issues of interlayer dielectric cracking. This design allows for a reduction in the size of the resulting semiconductor die. Also, because this pad design involves planarizing various layers, it results in a structure that has less stress when formed on a silicon carbide substrate which increases its ability to handle operational stresses and stress testing successfully.

Referring to FIG. 27, an implementation of a source pad system 106 that includes planarized intermetal dielectric layers and/or passivation layers is illustrated. As illustrated, a silicon carbide substrate 108 is employed upon/within which is built the various components of a semiconductor device (though a gallium nitride substrate like any disclosed herein could be used). For the sake of easier illustration, none of the layers of the die stack below metal 1 are illustrated in the implementation of FIG. 27. As illustrated, pads 110 are formed on the silicon carbide substrate along with gate runner 112. The pads 110 and gate runner 112 in this implementation are formed of aluminum silicon copper. In other implementations, however, the pads 110 and gate runner 112 could be formed of aluminum silicon or just aluminum. Surrounding pads 110 and gate runner 112 is a first interlayer dielectric layer 114 which has been planarized. A second interlayer dielectric layer 116 has been formed over the first interlayer dielectric layer 114. A plurality of vias 118 have been formed through the first interlayer dielectric layer 114 and second interlayer dielectric layer 116. Source pad 120 is coupled over the plurality of vias 118 and the second interlayer dielectric layer 116 and over gate runner 112. In this implementation the source pad 120 is made of aluminum silicon copper, though it may be made of just aluminum in some implementations. The ability to planarize the first interlayer dielectric layer 114 (and in some implementations, the second interlayer dielectric layer 116) and the ability to route the gate runner below the source pad 120 allows the size of the source pad to be maximized which can enable better performance of the semiconductor device. First passivation layer 122 is presented around the perimeter of the source pad 120. In this implementation, the material of the first passivation layer 122 is a hard passivation material like a silicon nitride. Second passivation layer 124 is also formed over the first passivation layer 122, and in this implementation is a polyimide material to assist with providing compliant material adjacent to the subsequent interconnect.

The source pad system 106 is ready for processing using the foregoing methods of forming an interconnect like those disclosed herein including depositing a barrier layer and seed layer and then electroplating copper, nickel, and gold/palladium layers. Because the interlayer dielectric layers have been planarized and electroplating has been used to form the rest of the interconnect structures, the resulting source pad and interconnect can handle high continuous operation temperatures and operating voltages like those disclosed in this document.

The source pad system implementations disclosed herein are formed using various implementations of a method of forming an interconnect. Referring to FIG. 28, silicon carbide substrate 108 is illustrated with pads 110 and gate runner 112 formed thereon. All of the device layers below metal 1 are omitted for the purposes of easier illustration in FIGS. 28-33. The pads 110 may be made of aluminum silicon copper, aluminum silicon, or aluminum and may have a range of thicknesses of about 3 microns to about 5 microns. In a particular implementations, the thickness of the pads 110 may be between about 3 microns to about 4 microns. To form the first interlayer dielectric layer around the pads and gate runner 112, the material of the interlayer dielectric layer is first deposited using a process consistent with the material used. Referring to FIG. 29, if chemical vapor deposition is used to form the layer forming the first interlayer dielectric layer 114, then the film will tend to conformally cover the pads, creating raised potions in the film. The same result can occur if another material like spin-on-glass is used using a coating process. In various implementations, the material of the first interlayer dielectric layer is made of silicon dioxide. To make the first interlayer dielectric layer 114 planar, a planarization process is used such as, by non-limiting example, chemical mechanical planarization, grinding, polishing, lapping, or any other flattening process consistent with the material used. Following the planarization process, a patterned layer of photoresist is formed over the first interlayer dielectric layer 114 leaving the pads 110 covered and an etching process is used to etch the remaining dielectric material away forming the structure illustrated in FIG. 29 where the first interlayer dielectric layer 114 is formed just around the pads 110 and gate runner 112.

Referring to FIG. 30, the silicon carbide substrate 108 is illustrated following formation of second interlayer dielectric layer 116 over the first interlayer dielectric layer 114. While in this implementation, the use of two interlayer dielectric layers is illustrated, in some method implementation, only one interlayer dielectric material may be employed. The material of the second interlayer dielectric layer 116 may be the same as or different from the material of the first interlayer dielectric layer 114. The process of forming the second interlayer dielectric layer is consistent with the material used (deposition, spin coating, etc.). In the implementation illustrated in FIG. 30, the material of the second interlayer dielectric layer 116 is silicon nitride. Following the formation of the second interlayer dielectric layer 116, a patterned layer of photoresist is formed covering the pads 110 and gate runner 112 and an etching process is carried out. As a result, the material of the second interlayer dielectric layer 116 remains only over the first interlayer dielectric layer 114. Because the first interlayer dielectric layer 114 was planarized prior to forming of the second interlayer dielectric layer 116, no additional planarization of the second interlayer dielectric layer may need to be carried out. However, in some method implementations, a planarization process like those disclosed herein may be carried out on the material of the second interlayer dielectric layer 116 prior to the patterning and etching process.

Referring to FIG. 31, a plurality of vias 118 are then formed through the second interlayer dielectric layer 116 and first interlayer dielectric layer 114. In various method implementations, the plurality of vias are formed by first patterning and etching via openings in the two layers 114, 116 followed by metal deposition into the vias. A wide variety of materials may be used in the vias including, by non-limiting example, tungsten, copper, titanium, any combination thereof, or any other suitable material for a contact or via structure. The process of forming the vias may involve a chemical mechanical planarization process where sputtering/chemical vapor deposition is used. Where electroplating is used, then no planarization process may be needed.

FIG. 31 also illustrates source pad 120 formed over vias 118. Source pad 120 may be formed by sputtering or electroplating (or a combination of sputtering and electroplating) process in various implementations following by a patterning and etching process designed to leave only the material of the source pad 120 in place. This particular method implementation has been designed to allow the gate runner 112 to pass beneath it while being electrically isolated from the pads 110 and source pad 120. The material of the source pad 120 may be any pad material disclosed herein including only aluminum, aluminum silicon copper, aluminum silicon, or copper.

Referring to FIG. 32, the silicon carbide substrate 108 is illustrated following formation of a first passivation layer 122 over the source pad 120. As illustrated, the first passivation layer 122 is windowed, or contains an opening that extends around and over a portion of the source pad 120. This first passivation layer 122 was formed by first depositing the material of the first passivation layer 122 over the source pad and then patterning and etching the material of the layer from around the pad and in the area of the opening, exposing the source pad 120. In the implementation illustrated in FIG. 32, the material of the first passivation layer 122 is silicon nitride, though other passivation materials could be used, including a polyimide or other polymeric material in some method implementations.

Following formation of the first passivation layer 122, FIG. 33 illustrates the silicon carbide substrate 108 following formation of a second passivation layer 124 thereon. As with the first passivation layer 122, the second passivation layer 122 is windowed and extends over a portion of the perimeter of the source pad. In the implementation illustrated in FIG. 33, the material of the second passivation layer 124 is a polyimide material to introduce a compliant material into the interconnect system adjacent to the source pad 120. In various method implementations, the method includes forming a backmetal layer 126 on a backside of the silicon carbide substrate 108 opposing the side with the source pad 120. The backmetal layer may be formed using any of the methods disclosed herein and may be formed of any material(s) disclosed herein for use as a backmetal in various method implementations.

At this point, the source pad 120 is ready for the formation of the interconnect structure. In a particular method implementation, the method of forming the interconnect includes forming a barrier layer over the source pad 120 and the second passivation layer 124, forming a seed layer over the barrier layer, and patterning a layer of photoresist with an opening exposing the source pad 120. The method also includes forming a copper layer over the source pad 120 by electroplating into the opening and forming a nickel layer over the copper layer. The method also includes forming a gold layer or a palladium layer over the nickel layer and removing the layer of photoresist. The method also includes etching the barrier layer and the seed layer to form a copper-containing interconnect.

While the source pad system 106 can be used with the interconnects disclosed herein, it may also be used with other interconnect types, or used for direct bonding using a wirebond or solder in various implementations. The principles disclosed herein can be used to construct various interconnect systems for source pads. Furthermore, the principles disclosed herein can be applied to full thickness or thinned silicon carbide or gallium nitride substrates where the thinning takes place prior to formation of the interconnects or afterward. The thickness of the thinned silicon carbide substrates may range between about 75 to microns to 125 microns in various implementations. In particular implementations, the thickness may be about 100 microns. The gallium nitride substrates may be utilized at a full thickness of the underlying substrate (silicon, sapphire, etc.) or may be thinned as desired to improve heat transfer or mechanical size.

In places where the description above refers to particular implementations of copper pad metallization systems and implementing components, sub-components, methods and sub-methods, it should be readily apparent that a number of modifications may be made without departing from the spirit thereof and that these implementations, implementing components, sub-components, methods and sub-methods may be applied to other copper pad metallization systems.