Flip Chip Hybridization Using Indium Bump 

Technology at ARL 

by Kimberley A. Olver 

ARL-TN-283 July 2007 

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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TN-283 July 2007 

Flip Chip Hybridization Using Indium Bump 

Technology at ARL 

Kimberley A. Olver 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Flip Chip Hybridization Using Indium Bump Technology at ARL 

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Kimberley A. Olver 

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U.S. Army Research Laboratory 

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Adelphi, MD 20783-1197

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Flip chip hybridization bonding is a microelectronics packaging technique which directly connects an active device to a 

substrate facedown, eliminating the need for peripheral wirebonds. Solder material is used as the conductive link between the 

two parts. Soldier bumps are directly deposited onto the active regions of the device and substrate. The main type of solder 

bump used at the Army Research Laboratory is the indium solder bump. Indium bump technology has been a part of the 

electronic interconnect process field for many years. This report discusses the techniques of flip chip hybrid bonding using 

indium bumps. 

15. SUBJECT TERMS

Flip chip hybridization, microelectronic packaging, indium solder bumps 

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Kimberley A. Olver 

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Contents 

List of Figures iv

1. Introduction 1

2. Process Overview 2

2.1 Indium Bump Placement.................................................................................................2 

2.2 Flip Chip Hybridization...................................................................................................3 

3. Conclusion 4

References 5

Glossary 6

Distribution List 7 

iii
List of Figures 

Figure 1. Orientation of chip and substrate for hybrid bump bonding. ..........................................1 

Figure 2. Processing steps for evaporating indium bumps onto Au pads.......................................3 

Figure 3. Alignment of the chip and substrate in the bonder with a split view microscope and 

camera system............................................................................................................................4 

iv
1. Introduction 

Flip chip hybridization is a microelectronics packaging and assembly process which directly 

connects an individual chip (device) to a substrate (readout) facedown, eliminating the need for 

peripheral wirebonding. Conductive connections are made between the two parts using 

interconnect bumps consisting of a solder material. Both parts are placed into a Flip Chip 

Hybrid Bonder and, using thermo-compression as the bonding technique, “flipped” together. 

Flip chip assembly is also known as Direct Chip Attach (DCA), because the chip is directly 

attached to the substrate via conductive bumps (see figure 1). 

Flip chip hybridization allows for lower lead resistance due to very short conductive bonds, and 

is a very reliable and robust technique due to the solder joint connections. It is capable of high 

density connections with a very low profile. 

Chip 

Substrate 

Solder 

Bumps

Figure 1. Orientation of chip and substrate for hybrid bump bonding. 

The main type of conductive interconnect solder bump used by the Army Research Laboratory 

(ARL) for direct connection of active devices to substrates is the indium bump. Indium bump 

technology is a unique process used mainly for flip-chip hybridization of semiconductor 

components, and has been part of the electronic interconnect process for about 40 years as part of 

a low cost assembly process. Because of its cryogenic stability, thermal and electrical 

conductivity, self-adhesive (ductile) nature and relative ease of application, indium is a good 

material for these applications. 

1
2. Process Overview 

2.1 Indium Bump Placement 

In the indium bump process, indium is deposited as the last step in the photolithography/ 

metallization processing of a device wafer. Indium bump masks are designed for both top and 

bottom parts. If commercial readouts are a preference, they should be purchased with indium 

bumps already in place. For a 5 to 6 micron high indium bump, an 8 to 9 micron thick 

photoresist layer is used. A vacuum-thermal evaporation of 300Å chromium for adhesion 

followed by several microns of indium is completed, a metal lift-off is done, and indium bumps 

are the result. The height and shape of the bumps are determined by the photoresist used and the 

mask design. 

A lift-off of the extra material produces indium bumps several microns in height. Both top and 

bottom components are processed with indium bumps, and using a flip chip bonder as illustrated 

in figure 2, the parts are either compression or thermo-compression bonded together. Bump area 

and the total number of bumps determine the amount of pressure needed for successful bonding. 

If a substantial amount of indium is being used (e.g., for thermal conductivity) thermocompression bonding will be necessary. 

In designing indium bump masks for the device and submount system, the indium bump masks 

for top and bottom parts have to be mirror images of each other for flip chip bonding purposes. 

The mask design of the actual indium bump is variable. Indium is a soft metal, and because it is 

evaporated onto the wafer, the shape of the bump can be chosen to best suit the application. 

Round pillars, square pads, and ovals are some of the shapes of bumps used. The height of the 

indium is also variable simply by using a thinner or thicker photoresist and evaporating less or 

more material. 

2
Gold pad on device

Photoresist layer

Photoresist exposure and develop

Cr/In deposition

Lift off and cleaning

Figure 2. Processing steps for evaporating indium bumps onto Au pads. 

2.2 Flip Chip Hybridization 

Connections are made between two parts using solder bumps made of Cr/In. The two parts are 

mounted onto the upper and lower chucks of the bonder with bumps facing each other. (See 

figure 3). An optical borescope system connected to a camera is moved into place between the 

two parts, and the bumps are aligned in the x and y direction. A second separate optical 

collimation system is then brought into place, and using a reflected beam of visible light, the 

rotation angle (theta) and planarity are corrected on the lower vacuum chuck. The borescope is 

removed, and the bottom part moves up to the top part while keeping all alignments constant. 

The bumps make contact, and through pressure (and sometimes heat), the parts are connected. 

The amount of pressure and (if used) the heat needed to melt the indium vary with the materials 

being used in the system, as well as with the size and distribution of the indium bumps. After 

some experimentation, it was found that for 20 micron square bumps, a compression force of 0.5 

grams per bump works well. For example, the total compression force required to hybridize a 

chip containing an 8 x 8 array of 20 micron square indium bumps would be 32 grams (0.5 grams 

per bump times 64 bumps), and a large format array (1000 x 1000 array) would require a total 

bonding force of 500 kilograms. 

3
Visible light source 

Upper vacuum chuck 

Figure 3. Alignment of the chip and substrate in the bonder with a split view microscope 

and camera system. 

3. Conclusion 

Flip chip hybridization, also known as DCA, is a well proven packaging technique used for the 

direct connection of processed device chips to readout devices and submounts. A grid of solder 

bumps on the surface of the active area on the device chip is joined directly to a corresponding 

set of solder bumps on the substrate. The main advantage of flip chip hybridization, compared 

with other packaging techniques, is that peripheral wire bonding is avoided and the very short 

electrical connections allows for lower lead resistances. It is a robust, reliable technique due to 

the solder joint connections. 

The main solder material used by ARL for flip chip hybridization is indium. Indium bump 

technology is a process used to place indium bumps on to the active sites of the device chip and 

corresponding submount when bonding the two parts together facedown. There are advantages 

of using indium as the solder material. It is relatively inexpensive, it has good thermal and 

electrical conductivity, it is ductile, and it is cryogenically stable. A disadvantage, however, is 

that indium bumps are not easily reworked. 

Vertical split view microscope 

Lower vacuum chuck 

Substrate with indium 

bumps 

Chip with indium bumps 

Turning mirror 

Upper and lower 

Camera 

Borescope 

Prism 

4
References 

1. Micro and Nanotechnology Commercialization Education Foundation (MANCEF) 

www.mancef.com. 

2. Research Devices, Inc. Flip Chip Aligner Bonder Model M8-A User’s Manual, Piscataway, 

NJ 08854,(now part of Laurier Inc), Londonderry, New Hampshire, 03053. 

3. Course Material written for 2002 class titled “Flip Chip Assembly Initiative, The National 

Training Center for Microelectronics, Northampton Community College, Bethlehem, PA. 

18020 www.northampton.edu. 

4. Olver, Kimberley A. “Evaporated Indium Bumps for Flip Chip Assembly”, Tutorial #38 

www.flipchips.com, February 2004. 

5. Paterson, Deborah S.; Elenius, Peter; Leal, James A. Wafer Bumping Technologies: A 

Comparative Analysis of Solder Deposition Processes and Assembly Considerations. SUSS 

Technical Bulletin, Karl Suss America, 1999. 

6. Riley, George A. “Introduction to Flip Chip; What, Why, How”, tutorial #1, 

www.flipchips.com, October 2000. 

5
Glossary 

Flip Chip Hybridization – A packaging technique introduced by IBM, where the chip is attached 

to the plastic or ceramic substrate facedown without using cumbersome peripheral wire bonding. 

A grid of solder balls (bumps) on the surface of the active area of the die is joined directly to a 

corresponding set of solder pads on the substrate. An integrated circuit designed for facedown 

mounting is attached by controlled-collapsed solder pillars on input/output pads of the device. 

DCA – Direct Chip Attach – a chip directly attached to a substrate, board or carrier by 

conductive solder bumps. 

Borescope – an optical device (as in a prism or optical fiber) used to inspect an inaccessible 

space (i.e., an engine cylinder). 

Hybridization – the act of producing something that initially has two different types of 

components, and bringing them together in order to perform a function. 

Readout device – any microelectronic device that presents data output for immediate use. 

6
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