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

The overall goal of this procedure is to demonstrate the use of the laser induced forward transfer lift technique for flip chip packaging of optoelectronic components. This is accomplished by depositing a thin film of metal, the donor onto a transparent glass substrate, the carrier. The second step is to pattern another glass substrate the receiver using photolithography with metallic contact pads and probing structures onto which donor micro bumps are lifted.

Next, the donor and receiver are put in contact and a laser pulse is focused at the carrier donor interface. The pulse forward transfers the donor microbus from the irradiated zone onto the receiver contact pads. The final step is to align an optoelectronic chip to the lifted receiver substrate and perform thermal compression flip chip bonding, followed by encapsulation of the bonded assembly.

Ultimately, the bonded optoelectronic chips are electrically, optically, and mechanically characterized to show the successful functioning of the lift assisted bonded chips. The main advantages of the lift technique over the existing methods, such as stencil printing, evaporation, electro plating, and electroplating, is that it's a simple process. It's fast, it's cost-effective, it's a low temperature process, and above all, it provides the flexibility that most of these standard techniques lack.

The implications of this technique extend towards applications requiring chip level bumping and bonding high accuracy and fine pitch for high density interconnections. The first step is to prepare substrates for the experiment. This video begins with prepare two inch and five centimeter by five centimeter glass substrates for the donor and receiver respectively.

The donor substrate has a 200 nanometer thick indie metal film evaporated onto its surface. The receiver substrate has been patterned with photolithography. The pattern includes four micrometer thick nickel, gold bond pads and fanout probing structures.

Once the substrates are ready, move them to the laser induced forward transfer or lift set up. The lift setup consists of a 355 nanometer wavelength laser and upholster of 12 picoseconds and an objective lens. First, the receiver substrate is placed onto the translation stage vacuum table.

Then the printing position on the receiver substrate is defined using a camera vision system, place the donor on top of the receiver substrate. The donor and receiver should be in contact with the donor first in the optical path of the laser. Use the camera vision system to adjust the focus and focus the laser beam onto the carrier donor interface.

In the laser control software, enter the desired pattern for the indium bumps to be transferred onto the receiver bond pads and begin the scan. Here is an example of the receiver bond pads after bumps have been transferred. Thicker bumps can be created by moving the donor to a fresh area and repeating the scan.

The printed bumps are inspected using the camera system of the software. After removing the donor substrate from the top to proceed, retrieve the bumped receiver from the translation stage. Take a bumped receiver to a semi-automatic flip chip bonder to have it bonded to an optoelectronic chip.

Place the receiver onto the appropriate vacuum plate. Then obtain a vixo optoelectronic chip to bond to the bumped receiver substrate. Here is an image of a vixo chip, which is about 1000 micrometers by 350 micrometers.

Load the vle onto a vacuum plate of the bonder with its active area facing down. Now begin the bonding process. First, select the needle shaped pickup tool and align it on the center of the vle chip.

Pick up the chip and use the camera alignment system to align the chip bond pads with the corresponding bumped contact pads on the receiver substrate. Once the pads are aligned, place the chip on the substrate bond the two by applying 12.5 gram force per bump at a temperature of about 200 degrees Celsius for about five minutes. With the bonding complete, use a syringe needle to dispense an optically transparent UV curable adhesive around the edges of the assembly.

Cure the adhesive with a UV lamp for about 30 seconds. Then remove the assembly to characterize it After fabrication, the bonded assembly is ready for evaluation of its electro optical performance. Make use of a probe station to record light current voltage curves.

Equip the station with a stage with a hole in its center for the passage of the light emitted from the bonded vle and mount the flip chipped substrate onto it. Ensure that the bonded vle is aligned to the hole. Next place a photo detector beneath the stage.

Use a microscope to align its active area with the bonded chip and then continue to precisely position the probing needles on the nickel gold probe pads. This is an image of a properly aligned chip with probes in position for a measurement. During the measurements, inject up to 10 milliamps of current and measure the voltage drop across the vle and the power of the emitted light.

The solid blue and the dotted red lines are data for current voltage measurements for the optoelectronic chip before and after encapsulation. The solid black curve and the dotted black curve are data for optical power emitted by the optoelectronic chip before and after encapsulation. This data verifies the successful functioning of the VIX Excels post bonding and that the ENCAPSULANT had no effect on chip functionality.

In this plot recorded current voltage curves for flip chip assemblies bonded using different pressure are compared with those recorded from a bear die. This supports a conclusion that there is negligible additional resistance due to laser induced forward transfer printed bumps. The bonded vertical cavity surface emitting laser chips were age tested in a climate chamber at 85 degrees Celsius and 85%relative humidity for 400 hours.

The average DC resistance in red shows no change over time for the average optical power, a change of less than 0.3 decibel was recorded in blue. Once you have optimized the parameters for lift printing and thermo compression bonding, you can perform the packaging in less than 10 minutes. After watching this video, you should have a good understanding of the use of the lift technique for flip chipp packaging of single optimal chronic components, which can further be scaled up to wafer level.