3D Devices

Three Dimensional (3D) Devices is a term applied to semiconductor chip stacking.  Various industry terms such as flip chip and die attach may be also associated with chip stacking but not exclusively so. Hydridization is used as a term which describes the chip stacking process.  As discussed on ARC’s webpage SiC Vacuum Chucks, the FC-150 and now the larger and more accurate FC-300 are the tools of choice for bonding two or more die together, especially for low volume (up to 10,000 per year), high precision projects.  3D Chip Stacking has become ubiquitous as companies try to make thinner devices such as cell phones, memory stacks and as stated on the above ARC webpage, Infrared Focal Plane Arrays (IR-FPAs) and their Read-Out Integrated Circuits (ROICs) for Infrared Cameras.  Other areas of interest for ARC’s customers are that of hybridizing High-Power Laser Diodes and VCELS with Micro Channel Coolers as shown below.  Presented below are additional details about the FC-150 Hybridizer, Bumping Metals, the Hybridization Process, and Laser Die bonding to Micro Channel Coolers.

ARC works with either as-diced die (bare die) or wafers.  For wafers, the company has expertise in precision dicing of standard and compound semiconductor wafers.

The FC-150 Device Hybridizer

ARC FC-150’s have the following combined capabilities:

• 50mm x 50mm Stages Standard • Post Bond Alignment Accuracy: ± 1 micron
• Thermal Bonding: Ambient
(Room Temp) to 450° C
• Force Bonding: Low Force (milli-grams)
and High Force (100 kG)
• Automatic Laser Leveling • Automated Bonding with Cognex
• Epoxy Dispenser • Gas Confinement
• UV Cure Capability
Figure 1 FC150, shown in the background are ARC's thin film sputtering tools in the Class 100
Figure 1 FC150, shown in the background are ARC's thin film sputtering tools in the Class 100

Bumping Metals

Bumping is the term applied to the process of placing solder metal on the contacts of one, or both, of the wafers in preparation for the bonding process.  For example, ARC uses photolithographic lift-off masking and then deposition for Indium or Gold-Tin Eutectic bonding metals, among other sets of materials.

Indium Bumping

Indium is a common Bumping metal due its electrical conductivity, excellent adhesion and malleability which enables chips with different Coefficients of Thermal Expansion (CTE) to be bonded together and then subjected to large temperature change without breaking the bonds.  The Indium bumps give and take as the chips expand and contract while maintaining electrical contact. Indium is a common choice for the IR-FPA’s whose detector wafers are typically exotic compound semiconductors as compared to typically CMOS processed silicon ROIC wafers.

High Density Bumping is shown on the right in Figures 2 & 3 after being reflowed to ensure successful protection from oxidation.  This technology is required for FPA’s and ROIC’s since the number of pixels far exceeds the process capability of Ball Bumping where the number of contacts for a 640 x 512 pixel array is 327,680 pads and can reach 16 million for a 4K x 4K array.

Figure 2 High Density Bumping - 10um pad pitch – Indium bumps after reflow
Figure 2 High Density Bumping - 10um pad pitch – Indium bumps after reflow
Figure 3 Image of patterned Indium bump uniformity after reflow
Figure 3 Image of patterned Indium bump uniformity after reflow

Device Hybridization

A custom hybridization protocol is designed to accommodate a customer’s chip set.  Hybridization process design includes the bonding temperature, bonding pressure, forming gas and timing of the bonding sequence.

Intrinsic to the hybridization process are the design and fabrication of custom SiC Vacuum Chucks. These Chucks are required in order for the FC-150 to pick up, place and securely hold each chip in position as the upper and lower bond arms provide for the thermal-compression or cold compression cycle.

A typical 50mm x 50mm Substrate Chuck is shown in Figure 4 (right upper).  Chucks are often being fabricated for external customer who have their own device bonding equipment.  These Chucks are sold separately to numerous companies who require ultra-high precision fast turn-a-around, and often with ITAR restrictions.

ARC is unique in that is has an internal machine shop which makes SiC vacuum chucks for its own FC-150 equipment suite and can also make them on demand for external customers.

The completed bonding process typical of the IR-FPA is shown in Fig. 5.

In Figure 5, The CAD drawing Illustrates how Five Plates of ARC’s Micro Ion Spectrometer have been hybridized into a 3D stack for lower Ion density plasmas. The completed bonding process typical of the IR-FPA is shown in Fig. 6.

ARC is a customer’s one stop shop for R&D as well as Production for 3D Devices.

For more information on your specific project contact Steve Elision at 651-789-9011, ARC’s Micro Fabrication Manager.

Figure 4 50mm x 50mm SiC Substrate Vacuum Chuck
Figure 4 50mm x 50mm SiC Substrate Vacuum Chuck
Figure 5 Micro Ion Spectrometer stacked plates
Figure 5 Micro Ion Spectrometer stacked plates
Figure 6 A section of a Bonded Infrared Focal Plane Array (IrFPA) and  its Read-Out Integrated Circuit (ROIC)
Figure 6 A section of a Bonded Infrared Focal Plane Array (IrFPA) and its Read-Out Integrated Circuit (ROIC)

Laser Die Attach to Micro Channel Coolers

Optimal cooling for a Laser Diode (LD) is a necessity to maintain the LD Lasing frequency to remain within specification.  To achieve this critical function, its Micro-Channel Cooler (MCC) must meet two likewise critical parameters.  First, a smooth bonding surface to which the LD chip will be hybridized must be achieved.  The second critical parameter is a knife-sharp front edge perpendicular to the bonding surface, a very important characteristic for edge emitting LDs.

ARC pre-finishes all MCC’s by precision lapping the bonding surface to sub-micron smoothness as well as the leading edge to produce the perpendicular knife-edge.

ARC offers various eutectic alloys technology for those customers that require the post bonding temperature before softening is higher than the bonding temperature.  Typically, this is a gold-tin mixture which is deposited onto either the Laser diode chip or the MCC or both.

The laser hybridization process is like the Chip-to-Chip process, Figure 7 (right upper). The bonded assembly is shown on the (right lower).  SiC Vacuum Chucks are also intrinsically a part of the process as explained above as well.

Figure 7 The FC150 with a Customer & ARC Engineer
Figure 7 The FC150 with a Customer & ARC Engineer
Figure 8 A Typical Edge emitting laser diode bonded to its micro-channel cooler
Figure 8 A Typical Edge emitting laser diode bonded to its micro-channel cooler

Wire Bonding

ARC also offers its customers several methods of supplying the needed current level and current uniformity required for uniform lasing in the case of a Laser Diode, as well as hybridized IR-FPA/ROIC assemblies when mounted in a chip carrier.

The options are either a multitude of 0.001 inch gold wires or a series of 0.010 inch gold straps. Since projects often have their own unique requirements, an ARC microfabrication engineer is available to assist.

Figure 9 Wire Bonder
Figure 9 Wire Bonder
Figure 10 Shows wire bonding where a height differential exists as well as more conventional similar height bonding.
Figure 10 Shows wire bonding where a height differential exists as well as more conventional similar height bonding.

Drawings and sketches may be submitted through our contact page. You will be contacted shortly to discuss your project.

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