The overall goal of this technique is to print thin film-like features that are highly congruent in shape and size, to an incident laser spot. This additive manufacturing technique can print spanning interconnects, microbridges, and high aspect ratio structures. This is a direct write method based on laser-induced forward transfer, or LIFT.
Unlike traditional LIFT, our process uses a high-viscosity silver nano paste, that minimizes any wedding interactions with the substrate. The main advantage of this technique is that it is a non-contact, nozzle-free process that can transfer shapes congruently, with dimensions ranging from a few microns to over 100 microns. Begin with spreading about 10 milligrams of prepared silver nanopaste into one side of a two micrometer deep well etched into a glass slide.
This nanopaste is the donor substrate. Next, using a metal blade, spread the paste into the well uniformly, and without any thin spots. A prepared small well with paste functions as an ink ribbon.
Wipe away the excess paste around the well. Next, dry the ink ribbon in a box filled with trinitrogen. A low humidity environment is required.
After two or more hours at room temperature, the ink should be ready for printing. For storage up to one month, place the well face down onto a glass slide, and keep this assembly in a trinitrogen environment. To test ribbon viscosity, attempt to laser transfer some square voxels.
The word voxel"refers to a single volumetric element of material. The square shape is selected because it serves as a good test for congruent laser transfer. First, prepare the receiver substrate, in this case, a diced silicon wafer.
Attach it to a vacuum truck on an X, Y translation stage. Next, place shims made of the same material and of the same height as the receiver substrate, on both sides of the substrate. Now, activate the vacuum pump to secure the receiver substrate and shims to the chuck.
Then, place the donor substrate, ink side down, upon the reciever substrate. Select an aperture of the desired voxel shape, and position it in the beam path. In this example, a square aperture is used.
Using a camera that is parfocal with the laser spot, focus on the ink layer in the well, through the back side of the ribbon. Now, attempt to laser print some voxels. First, fire a single laser pulse onto the donor substrate.
Begin somewhere between 40 and 60 millijoules per square centimeter. If the focus position is correct, and if the energy is sufficiently high, a voxel will be ejected. Check for a square hole in the ribbon ink layer, which indicates a voxel was ejected.
Then, move aside the donor substrate and inspect the ejected voxel. If the hole isn't visible or clear, adjust the beam focus by repositioning the objective. If the focus is correct, but no voxel appeared, then gradually increase the energy of the laser.
If a hole appears, but seems to quickly disappear, it could be refilling with ink. This occurs when the ink viscosity is too low. Try drying the ribbon for another 30 minutes.
Optimization of ribbon ink viscosity is the most important step in the LIFT process. If the ink viscosity is too low, you will not achieve well-defined shapes. And if it is too high, the voxels will crack or shatter.
Once a clear and sharply-defined square voxel can be made, try printing an array of voxels, to ensure the repeatability of the transfer process. If everything is well optimized, a well-defined array of voxels should be transferred, which is evidenced by an array of square holes on the ribbon. Many advanced structures can be printed using this high-viscosity ink, that are impossible to print with low-viscosity ink, such as a bridge.
A bridging structure can be created by ejecting voxels over a microchannel or gap in the substrate. Pyramid structures can be created by stacking voxels upon one another, in a staggered fashion. High aspect ratio stacks can also be created by repeatedly transferring voxels to the donor substrate at the same location.
If any structure will be taller than five microns, insert spacers between the substrates periodically, so the voxels don't contact the ink ribbon. Do not forget that the optics must always be refocused after adding spacers. Even more complex voxel shapes can be printed, by using a DMD chip in lieu of an imaging aperture.
An image of the desired voxel shape must be prepared beforehand, in the commercially available DMD software. This process is described more thoroughly in the text protocol. Based on the image selected in the software, the DMD chip will reflect the incident laser beam to create a corresponding image on the donor substrate.
If performed properly, a voxel in the shape of the image should be ejected. Using this method, it is relatively simple to print any variety of complex voxel shapes with 10 to 20 micron resolution. After deposition, the silver paste can be oven cured in atmospheric conditions, to improve the functional properties.
To oven cure the printed voxels, transfer the substrate to a 180 degree Celcius furnace. Let the sample cure for up to two hours. Then, characterize the sample.
A significant volume reduction occurs when the voxels are cured. A 50 to 60%volume reduction is common. Usually, this volume reduction manifests as a reduction of voxel thickness, and is not a decrease in the lateral dimensions.
Viscosity of the silver paste has a huge impact on the voxel qualities. When the paste viscosity is low, surface tension will cause the voxels to be more rounded, losing their original shape. If the viscosity of the paste is too high, the voxels tend to fracture.
When the paste viscosity is just right, voxel edges are sharply defined, and there is no fracturing. Voxel chains can be made into long conductive lines using an end to end technique. Simply connecting rectangular voxels, however, is not terribly reliable, and links are often broken after furnace treatment.
By printing interlocked voxels in an end to end manner, this problem can be averted, and seamless lines can be made reliably. Laser induced forward transfer is a unique direct write process, which can congruently transfer high-viscosity nanopastes, while retaining their intrinsic functional properties. This high congruence between the laser spot and printed voxel enables the fabrication of novel structures and surfaces, for microelectronic applications.
With enough practice, you should be able to reliably transfer simple shapes in one to two hours.
