Laser-induced Forward Transfer for Flip-chip Packaging of Single Dies

1. LIFT-assisted Flip-chip Bonding

NOTE: There are three stages involved in realizing the LIFT-assisted flip-chip assemblies, namely-micro-bumping of the substrates using the LIFT technique, attaching the optoelectronic chips to the bumped substrates using thermo-compression flip-chip bonding method, and finally encapsulation of the bonded assemblies. Each of these stages is discussed in the following sections:

1. Micro-bumping using LIFT:
   1. For donor preparation, deposit a thin film of the donor material onto a laser-transparent carrier substrate. For this experiment, evaporate a 200 nm thick film of indium metal on top of glass carrier substrate with dimensions: 2 inch diameter x 0.05 cm thickness.
      NOTE: Donor preparation method depends on the phase of the donor material, e.g., use evaporation and sputtering for solid phase donor materials and spin-coating and doctor-blading for liquid-phase donors.
   2. For receiver preparation, use glass substrates with dimensions of 5 x 5 x 0.07 cm³ as the receivers. Pattern these substrates with the metallic contact pads for bonding the OE chip and fan-out probing structures using photolithography. For this experiment, pattern 4 µm thick Ni-Au bond pads and fan-out probing tracks onto glass receiver substrates.
   3. Next, place the donor in contact with the receiver and mount the donor-receiver assembly onto a computer controlled X-Y translation stage.
      NOTE: Depending on the phase of the donor material (e.g., solid (indium) or liquid (ink/paste)) and its thickness, the donor and the receiver substrates are placed at an optimum separation that can be easily controlled (e.g., by using metallic spacers).
   4. Focus the incident laser beam at the carrier-donor interface employing an objective lens of 160 mm focal length and scan the beam (20 µm spot size) across the donor substrate for transferring donor micro-bumps onto the receiver bond-pads. Use a picosecond laser source of 355 nm wavelength and 12 psec pulse duration to LIFT indium bumps onto the receiver bond-pads at a fluence of 270 mJ/cm².
      NOTE: The laser properties such as energy, no. of pulses, objective lens height, coordinates of the precise location on the receiver substrate for printing donor micro-bumps and the desired pattern to be transferred are accurately controlled by a computer program. Key experimental parameters (e.g., transfer fluence) need to be optimized in case of using another laser source.
   5. For thicker bumps move the donor to a fresh area and repeat step 1.1.4 several times. For example, repeat step 1.1.4 six times to get a stack of 6 indium bumps printed on top of each other for this experiment. The final LIFTed bumps have an average height of ~ 1.5 µm and a diameter of 20 µm (Figure 2).
      NOTE: For these experiments the surface profile and thickness of the bumps were measured using an optical profilometer. It was examined that the bumps had a convex/dome morphology with an average thickness of 1.5 µm, averaged over the bump diameter (as marked yellow in Figure 3). The reason for this is attributed to the fact that the donor melted in the laser irradiated zone and the transferred pellet then re-solidified upon reaching the receiver surface (Indium has a low melting point). The advantage of this is that it results in good adhesion of the printed bump to the VCSEL contact pads.
2. Chip to substrate thermo-compression bonding (Figures 4-6):
   1. Use a semiautomatic flip-chip bonder for bonding the optoelectronic chips to the bumped substrates.
   2. Load the bumped receiver and the chip to be bonded onto their respective vacuum plates of the bonder. Place the chip in a flipped position, i.e., with its active area facing down.
   3. Use a suitable pick-up tool and align it on the center of the chip. Use a needle-shaped tool as shown in Figure 5. Next, pick the chip using this pick-up tool.
   4. Align the chip bond-pads with the corresponding contact pads on the receiver substrate using a camera-alignment system.
   5. Once aligned place the chip on the substrate.
   6. Apply heat (~ 200 °C) and pressure (12.5 gf/bump) simultaneously to realize chip to substrate electrical and mechanical interconnections.
3. Encapsulation of the bonded assemblies (Figures 4-6):
   1. Dispense an optically transparent adhesive around the edges of the bonded assembly using a syringe needle. The encapsulation increases the mechanical reliability of the bonded assemblies. Use a single component UV curable adhesive such as NOA 86 for encapsulating the bonded chips.
   2. Cure the adhesive using a UV lamp for ~ 30 sec.

2. Characterization of the Bonded Vertical-cavity Surface-emitting Lasers (VCSELs)

NOTE: After fabrication the next step is to evaluate the electro-optical performance of the bonded assemblies. The light-current-voltage (LIV) curves of the devices are recorded post-bonding using a probe station. The following steps are involved for the testing:

1. Place the flip-chip bonded device onto a custom-made transparent stage. The stage has a hole drilled in the center for easy access to the light emitted by the VCSELs.
2. Place a photodetector (PD) underneath the transparent stage and align its active area with the bonded chip using a microscope.
3. Precisely position the probing needles on the Ni-Au probing pads using a microscope.
4. Inject up to 10 mA of current and measure the voltage drop across the VCSEL and the light emitted by it using a current/voltage source-meter unit and a power meter respectively.