The Exaddon CERES µAM (additive micromanufacturing) system prints complex metal structures with submicron resolution via additive manufacturing. Exaddon has developed a surface mapping functionality that scans and prints on existing structures in one seamless workflow.
This is performed by using the unique technology of their iontip printing probes to enable microscale metal objects to be printed on contact lines and pads with submicrometer precision.
Figure 1. UNIQUE TECHNOLOGY. The iontip of our CERES μAM print system enables a surface to be mapped prior to printing on it. A user wants to print on an existing structure (A) with submicrometer precision. The mapping operation produces a visual representation of the structure (B), and the user selects the desired print location (C). The new object (D) is printed, in this case a 500 nm Ø pillar. Image Credit: Exaddon AG
Exploring New Possibilities
Exaddon’s unique iontip is at the heart of their CERES µAM print system. It is a hollow atomic force microscope (AFM) cantilever from which an aqueous solution containing metal ions is dispensed. These ions are then reduced into solid metal atoms through electrodeposition, allowing the printing of individual voxels of pure metal.
There are twofold benefits to using a hollow AFM cantilever; the ability to scan a surface as per a conventional AFM and the ability to additive manufacture complex metal structures in microscale dimensions with submicrometer precision.
This topographical analysis capability, defined by Exaddon as “mapping”, is a recent breakthrough that was attained after extensive research and testing. The aim of this process was to print on contact lines and pads to allow ultra-precise modification of semiconductors and other devices with the unique capabilities of metal µAM.
What New Functionality Does This Bring?
In conventional AFM, the cantilever is employed to physically trace the surface of the material in order to create an ‘image’ of its surface features. This process enables much higher resolution than the optical refraction limit, and it was this functionality which Exaddon sought to exploit.
In addition to printing objects directly onto blank substrates, they also wanted to enable their CERES system to print microscale objects upon existing structures, like connectors between contact pads on a wafer. This is a very demanding operation that needs incredible precision to locate and print on structures with submicrometer accuracy.
In the majority of microscale uses, identification of the exact print location is usually done using the high-resolution optical cameras which are part of the CERES system (these permit XY accuracy to around 4 µm). However, the extreme precision needed to locate contact pads presented a potential challenge; the cameras do not enable sufficient accuracy to locate pre-existing flat or patterned structures <5 µm in diameter and <1 µm in height with the necessary precision.
Accordingly, Exaddon performed extensive testing and research to develop a surface mapping functionality that enables the system to map the anticipated print area before printing. This is all performed in a single, seamless workflow, which is crucial for print accuracy. This is explained best through an example use case, which is detailed below.
Example Use Case: Locating and Printing on Contact Pads.
A user has an integrated circuit (IC) which has preexisting conductive traces and contact pads on it, and the user needs to print microscale connectors upon these pads. In the CERES printer software, the user defines the size of the grid needed by creating a square grid over the area within which the contact pads are located.
Each intersection of this grid denotes a point where the iontip actually moves down and touches the surface; the tighter the grid, the higher the mapping resolution. The user begins the mapping workflow once this grid has been defined, and the system starts to map the area as defined, locating the contact pads exactly within that area.
The software outputs a plot representing the mapped area which is essentially a height map of Z coordinates from within this grid, as seen in Fig. 2.
Figure 2. NANOSCALE APPLICATION. Our mapping workflow outputs a visual representation of the detected surface topography, such as the two contact pads displayed here. Image Credit: Exaddon AG
Full User Customization
One key feature of the mapping functionality is that it is fully customizable; the user is able to define the speed and resolution of mapping required. For example, a 40 x 40 µm grid with 1 µm increments would be suitable for a very accurate scan of a small area, i.e. it would result in a high-resolution map.
In an alternative case, the user could also set the grid to be hundreds of micrometers wide in both X and Y stages with bigger increments, for instance, every 1 µm, 5 µm, or 20 µm. It is crucial to remember that this functionality is not intended for full 3D objects, but for mapping what can be considered 2D or “2.5D” structures, like pads or conductive traces.
It is also important to note that the mapping process is slower with more increments: the time taken increases linearly with the number of grid points. For instance, a 40 x 40 µm grid with 1681 points takes around 6 minutes to be mapped, with a set iontip retract height of 5 µm in between each point.
Accuracy and Durability
This functionality does not affect print accuracy, as Exaddon’s iontip was originally based on an AFM cantilever. Repeated map – print – map – print operations are possible without influencing the alignment or precision of the system to enable multiple print operations to be performed on a single wafer.
Figure 3. EXADDON IONTIP. The printing tip of our CERES machine utilizes a hollow AFM cantilever, called an iontip. Image Credit: Exaddon AG
In terms of detection capabilities, this iontip was designed to address the requirement to locate pre-existing structures on wafer surfaces. The mapping function can detect structures with heights as small as 200 nm. Exaddon was able to locate and print upon conductive lines 500 nm in height in their test scenarios.
Printing at an extremely specific location on a silicon chip patterned with a copper or gold layer is a real-world application of this procedure. The mapping function is able to detect and locate tracks in the pattern to ensure the exact placement of printed structures with a resolution of below 1 µm.
This mapping functionality will be integrated with the latest release of Exaddon’s print system operator software, with complete notes within the user manual for simplicity of use. This mapping functionality creates the basis of a series of new developments which will extend the already unique capabilities of the CERES system.
This information has been sourced, reviewed and adapted from materials provided by Exaddon AG.
For more information on this source, please visit Exaddon AG.