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Electroplating Innovation Enables Ultrafine Indium Bonding


Flip-chip bonding is essential to hybridization, the process of combining die from differing technologies into a high performance module such as the hybrid pixel detectors found in LiDAR and other imaging applications. The tin solders once used in flip-chip bonding are being replaced by lead-free alternatives including indium. However, creating the indium ‘bumps’ essential to forming bonds is challenging using conventional approaches. The experts at ClassOne Technology believe a new electroplating process solves the indium bump corundum. BY FARZANEH SHARIFI AND BRANDEN BATES, CLASSONE TECHNOLOGY; ELIE NAJJAR, WENBO SHAO, PHD; ERIK YAKOBSON, PHD; AND BRIAN GOKEY, MACDERMID ALPHA ELECTRONIC SOLUTIONS

HYBRID PIXEL DETECTORS are widely used for imaging applications ranging from high-energy physics to military, environmental and medical. Hybrid pixel detectors combine a pixel sensor chip with a readout integrated circuit (ROIC), which allows electronic access to every pixel in the detector. Pixel sensors are made of high-resistivity silicon, while low-resistivity material is required for the ROIC. Hybridization allows each one to be manufactured independently and then later coupled together through a process called flip-chip or bump bonding.

Flip-chip bonding creates a contact that provides high input/output (I/O) density and a short interconnect distance between the sensor pixel and the ROIC, enabling high device performance. During flip-chip bonding, the solder bumps are melted to form this connection. The pixels in a hybrid detector are placed in an array with less than 100-micron (mm) distance, or pitch, between them. This high connection density requires finer, more precise bumps and a very high-yield flip-chip process that ensures each pixel is connected to the IC.

Figure 1a & 1b: Confocal microscopy data (a) topography map of indium features (b) profilometry. The bumps were observed using a scanning electron microscope (SEM) as shown in Figure 2.

Flip-chip evolution

Conventional flip-chip assembly was first achieved using lead-based solder bumps, but those materials have had to be revisited due to the worldwide banning of lead in electronic products due to its toxicity. However, lead-free alternatives such as pure tin or various tin-based lead-free alloys, e.g., SnAgCu (tin-silver-copper, or SAC), also face challenges with pixel detectors, so a viable alternative is a necessity.

Since the readout chips and sensor chips are made of different materials, a low-temperature fabrication process is necessary to reduce the thermal impact on the sensor chips due to a mismatch in coefficient of thermal expansion (CTE). Additionally, the sensors can face environmental extremes, from harsh radiation to cryogenic temperatures. Together, all these challenges require a new solder material with specific characteristics. We propose indium as one such preferred candidate.

Why indium?
Indium is a soft metal (softer than lead) with a low melting point (156oC) that is highly malleable and ductile and retains these properties at very low temperatures, i.e., down to absolute zero (-273oC). This makes indium ideal for cryogenic and vacuum applications.

In terms of chemical properties, indium reacts with oxygen only at higher temperatures, doesn't dissolve in acids, has good adhesion to other metals, and has the ability to wet glass. Its good electrical conductivity, ductility, and low-temperature stability make it an excellent candidate for use in hybrid pixel detectors.

Older approaches
Indium bumps were previously fabricated via thermal evaporation, or sputtering, which yields highly uniform bumps with good bump structure control. However, this method cannot produce small bumps (higher aspect ratio) with a smaller pitch that's suitable for the semiconductor industry's current needs.

Moreover, indium sputtering requires expensive evaporation equipment, is limited to materials with high vapor pressure, requires a complicated fabrication process, isn't well suited to larger wafer sizes because of mask-to-wafer mismatch, is less environmentally safe as it creates more pollution, and is only viable for small-scale production.

In contrast, electroplating bumps with a high aspect ratio, at low cost, and with a simple fabrication process is achievable, especially for mass production. But conventional electroplating needs optimization, as non-uniform bumps cause failures in the fabrication process and reduce the hybrid chip's reliability. Evaporation of indium bumps for ultrafine pitch is difficult and time-consuming. Moreover, the waste of material on the photoresist mask renders the process non-cost-effective, and the smallest pitch size achievable through this approach is 30 mm.