Novel approach to patterning in atomic layer deposition


By Matthias Carnoy - Microtechnology Engineer 

Semiconductor microdevices are nowadays built using a usually long alternating series of additive (vapor deposition, sputtering) and subtractive (dry or wet etching) processes, where patterning is typically defined by photolithography (exposing photosensitive material to UV light through special masks to define a pattern). This manufacturing process needs to be done in particle-free environments (cleanrooms) as these would compromise the design of the devices being manufactured.

In order to be performant, these cleanrooms must be able to filter the air within the facility extremely well, typically contain direct lines of distilled water, pressurized air as well as other gases such as argon, nitrogen and sometimes hydrogen, chemical waste storage for the organic and inorganic liquids routinely used for etching and cleaning of samples. Needless to say, these facilities are marvels of engineering and logistics, but represent massive financial investments and are extremely energy intensive.

Other manufacturing industries have been recently subjected to an alternative view to the traditional fabrication models: additive manufacturing, aka 3D printing, has been growing as a new way to make all sorts of products from microscopic plastic-based replacement parts to massive things like boats, houses, and is even applicable to complex and unexpected domains like biotechnology, where scientists are able to 3D print with live cells.

Up until recently though, the scale at which additive manufacturing can be performed, and the materials which can be used for the process, have been two main limitations preventing the potential development of additive manufacturing equipment for micro- and nanofabrication purposes. Some laser-based, electron beam-based or inkjet-based techniques can reach micrometer ranges, which is still far from the nanometer ranges photolithography is now able to reach.

Photolithography-based processes have been around for sixty years and as such have been developed to be able to reach such extreme resolutions. Another limitation of the current micrometer-scale additive manufacturing options is that they are often limited to a specific range of materials, and thus cannot perform deposition of all the many materials used in micro- and nanodevices. These can involve combinations of oxides, nitrides, metals, sulfides, selenides, and even complex organic compounds.

In the broad range of thin film deposition techniques that are used in semiconductor foundries and FABs, atomic layer deposition (ALD) is a specific case of chemical vapor deposition process that produces thin films of materials with a maximum of one atomic layer at a time. ALD has been around since the 1970s and up until today, over 450 processes have been developed for this technology, meaning that ALD can be used to deposit a vast range of different materials used in micro- and nanofabrication.

ATLANT 3D Nanosystems has developed a microscale ALD reactor which, combined with advanced mechatronics (i.e., a moving stage in the X, Y and Z axes), is capable of harnessing the wide range of ALD-compatible materials, and depositing these materials in a selective way, thereby inherently patterning materials used for micro- and nanodevices. This microreactor’s size is what defines the lateral feature size of the deposited material, while the vertical feature size is defined by the ALD process itself, i.e. one monolayer per pass of the printhead. We call this fabrication technology Microreactor Selective Area Direct Atomic Processing (μSADALP™).

Fig.1: Illustration of the µSADALP process; the printhead on top deposits gases onto a moving stage underneath

This concept has been proven to work now for more than a year and a half; by integrating this microreactor in a working prototype, the printing of lines of materials with varying thicknesses has been achieved with several materials, by adapting regular ALD processes to function with the microscale reactor. As an example, we can show here the patterned deposition of platinum on a thermal silicon oxide wafer. More specifically, the control of the thickness is visible on the thickness mapping of the ATLANT logo, as expected for an ALD process the thickness is dependent on the number of passes. 

Fig.2: SEM picture of the ATLANT print
Fig.3: Measured Pt thickness of the ATLANT print

The μSADALP™ technology also allows for selective patterning on top of non-flat surfaces, which typically poses big problems or is even sometimes impossible, when using traditional lithography-based fabrication processes: corrugated surfaces are very difficult to handle in photolithography because the resist used to define the patterns is usually spun on the surface of the wafer, which cannot be done evenly if the surface is not flat. Deposition done through μSADALP™ conformally deposits material on top of the corrugations, which has been tested on microchannels up to 20 µm deep.

Fig.4: Deposited Pt on top of microchannels

The possibility for direct selective area materials deposition gives this technology a wide range of applications: from advanced materials innovation to prototyping and even manufacturing of MEMS and sensors as well as functional, optical, protective and encapsulation coatings.

Material innovation is one of the key application of this technology, as processes that would be developed using a μSADALP™ machine would require fewer iterations and materials (being able to deposit multiple times on one wafer as opposed to multiple times over multiple wafers). Moreover, the speed at which the prints can be achieved is very beneficial for rapid prototyping in general, since being able to quickly print one device to the next using different parameters allows for a much faster way to perform R&D on devices. 

Another interesting application for this technology is for in-Space use; μSADALP™ is theoretically suitable for use in micro or even 0 gravity environments, meaning it would be able to function in Space. Contrary to the common ALD systems, the use of the microreactor from ATLANT 3D Nanosystems can fulfill the harsh requirements for a launch at 0 gravity i.e., a fast deposition time, a low weight and low volume. The proof of concept for the growth of functional material by ALD in such conditions is therefore opening outlooks for micro- and nanofabrication beyond Earth.

Additive manufacturing being used for the future of space exploration is an ongoing hot topic, whether it is to be used for building habitat, equipment, or spare parts. The ability to manufacture the required equipment on-site instead of bringing it from earth is a game-changer. Moreover, space itself is a unique environment with ultrahigh vacuum and zero gravity that can foster development and manufacturing of novel technologies of the future. 

Currently, additive manufacturing solutions do not address the fabrication of advanced materials and microelectronics on which much of our modern technology relies: if even a simple microsensor on a space station or even in a living pod on Mars fails because of gas leak or low temperature, what will you do? What about the solar cells you need to power your space station with? You cannot wait for 6 months for simple items to be delivered – you need them now. These devices are vital to space exploration, and as such, a way to fabricate them rapidly on-site is a necessity. Moreover, certain fragile devices or structures can risk being damaged during their departure from Earth. Fabricating such sensitive structures while in orbit or after landing would automatically imply higher performance. 

Nowadays’ micro- and nanofabrication facilities are immensely complex and expensive, so sending everything required for such a facility is not an option and even impossible. Sending large stocks of microdevices would be the alternative but would add to the weight of the rocket, provide a finite amount of replacement devices, and take a long time to deliver to distant locations. 

A compact μSADALP™ machine could both work on the ground on Earth and Mars settlements or within the context of a Space station, opening opportunities to establish in-Space nanofactories. By using the combination of additive manufacturing and advanced microfabrication techniques, the same technology platform could automatically manufacture micro- and nanocomponents such as microelectronics, sensors, energy devices, and more.

Discover the story behind our work with NASA on the first-ever compact microreactor selective area direct ALD machine able to work at harsh conditions, including micro and zero gravity, and direct write ALD quality films and patterns – Nanofabricator Lite

The ATLANT 3D Team!