The key challenge to solve for high-throughput electron beam lithography is to get sufficiently high levels of current onto the wafer while avoiding the Coulomb interactions, or electron repulsions, that blur the pattern. Mapper’s solution is to deploy many thousands of low-power parallel electron beamlets, and use arrayed electron optics to guide these beamlets in groups of 49 from source to wafer. The electron optics includes a transmissive blanking module that can switch each beam independently on or off at very high speeds. After modulation, an array of electron optical micro-lenses focuses the beamlets in Gaussian spots of 25 nm each. The wafer is brought into the focal plane and scanned underneath.
The FLX is Mapper’s 3rd generation platform. The first models come with 65,000 beamlets. A huge step up from the 110 beamlets in the previous generation, and even that number is still unmatched. The FLX is designed with the future in mind: it supports extendibility paths in resolution, overlay, throughput and substrate types. The column design extends to 650,000 beamlets. Its footprint and service access are prepared for clustering, so that 40 wph or more from an e-beam litho-cell is a reality not too far away.
how it works
The electron column comprises of three modules. Using a 5 kV acceleration voltage the beam generator creates a wide electron beam, approximately 3 cm in diameter. A single electron source is used for uniformity. The blanker module splits this beam in 65,000 beamlets. Each beamlet passes a pair of MEMS deflectors. The deflector array is made out of standard CMOS technology and post-processed using Mapper's advanced process capabilities and know-how. The deflector voltages are controlled by high-frequent signals that are fed into the system through hundreds of optical fibers, all mounted on the same chip. After passing the blanking module the electrons enter the final module, the projection optics. The core of the projection optics is a micro-lens array with thousands of electrostatic lenses. Undeflected beamlets pass through the center of the micro-lenses, while deflected beamlets are stopped on top of the array and don’t reach the wafer. Macro deflectors scan the beamlets over a 2 µm range so the whole area can be covered. The micro-lenses demagnify each beamlet to a 25 nm Gaussian spot, focused on the wafer that is only tens of micrometers below. The wafer is coated with a standard DUV type of chemically amplified resist. The typical dose needed is just 20 to 30 µC/cm2, much lower than needed in systems with high landing energies.
The wafer is scanned under the column while the beamlets each write their own part of the IC pattern. As the pattern originates from a large computer memory, it can be repeated several times in a scan from one side of the wafer to the other. Before each exposure scan, alignment targets are measured to calibrate the exact position of the wafer with respect to the beamlets. For compatibility the wafer target measurement system use an optical readout. Height measurement are done during the fly back between exposure scans.
While the wafer is moved under the column, the beamlets are deflected at a high frequency perpendicular to the scan direction. The deflection range is slightly over 2 µm. With this deflection each group of 49 beamlets covers a stripe of 2 µm wide and 300 mm long. There is some overlap to make the pattern from one stripe seamlessly connect to the pattern of the adjacent stripes.
The FLX imaging capability is designed to match the requirements for the 28 nm logic technology node. As there is no low-k1 diffraction limit, the FLX is more flexible at these resolutions than optical methods. The full-2D capability means that there are no design rules restricting the periodicity, orientations and dimension variations in a layer. For example, contacts and vertical interconnects can print at the same pitch as lines, and 45 degrees routing is no more difficult than 1-dimensional layouts.
In close collaboration with CEA-LETI, Mapper has developed a process stack containing the same materials as in use today for optical scanners. The resists are typical chemically amplified DUV type of resists. The collaboration has yielded a complete package of know-how, ready to use: data preparation, proximity effect corrections, tool monitoring, metrology, etch transfer processes and matching strategies have all been developed, characterised and fine-tuned.
extendibility of 5 keV
Although the first FLX systems are built with the 40/45 nm and 28 nm nodes in mind, 5 keV has a comfortable extendibility path ahead of it, both for resolution and for throughput. Resolution can be scaled by decreasing the spot size from 25 nm to a smaller value. The feasibility of which was demonstrated on Mapper's pre-alpha platform.
Without countermeasures this would reduce the total current and throughput. Throughput is therefore maintained by increasing the total number of beamlets. The FLX electron optics are pre-dimensioned to provide for increasing both beamlet density and column filling. The >100 mA source supports the full development.
The productivity path for the FLX series is therefore straightforward: the first steps will increase the total number of beamlets per unit by a tenfold: to 650,000 individually controllable electron beamlets. From there, several expose units can be clustered and connected to a track to form a litho cell supporting well over 40 wafers per hour. This evolution will come about on the same platform, as the FLX design is prepared for such a future.