At the annual Imec Technology Forum USA, imec, in collaboration with Soitec, reported that they successfully demonstrated a sequential 3D front end integration process by stacking two device layers on one another on a 300mm wafer.
This vertical integration of sequentially processed device layers, also named sequential 3D integration (S3D), is perceived as a promising alternative to continue the benefits offered by semiconductor scaling, overcoming the constraints of geometrical scaling while maintaining the benefits of functional scaling through the vertical 3D integration.
The most critical challenge of sequential-3D integration is the management of the fabrication thermal budget. To avoid or limit thermally induced device degradation issues of the bottom device layer, the top device layer must be processed at low temperatures below 525°C.
The top thermal budget needs to be reduced to avoid degradation of the bottom devices, the bottom interconnects, and the bonding interface. These limitations are overcome with the implementation of junction-less transistors on the top layer which decreases the fabrication complexity and provides sufficient device reliability.
A high device density per chip area can be realized because the alignment accuracy only depends on the lithography stepper performance and not on the accuracy of the wafer alignment, as is the case for alternative 3D-integration technologies using through-Si vias. Moreover, the sequential processing of the device layers promises a reduced length of the interconnection lines and a reduced RC delay, as compared to a 2D implementation.
For efficient sequential device stacking, imec worked with Soitec’s Smart Cut technology to transfer a thin layer of crystalline material from one substrate to the substrate processed with the first layer of devices, overcoming physical limitations and opening innovative opportunities in 3D IC product design.
Anne Vandooren, Senior Researcher at imec, stated: “Demonstrating a good performance of the top tier device using a low temperature process is an important breakthrough in our aim to develop sequential 3D as a valid option to further increase power-performance-area-cost in advanced technology nodes beyond 5nm. To realize this, we had to use very advanced technologies in some of the most critical process steps including back-end-of-line, contacts, and gate stacks.”
imec demonstrated a junction-less top device layer featuring a low fabrication complexity, which does not require junction formation, channel doping and activation performed prior to layer transfer. The top tier device was processed at a temperature below 525°C achieving good device performance (SS =72mV/dec, DIBL=80mV/V Ioff=1pA/um Ion=220uA/um at VDD=1V), without impact from layer transfer.
Moreover, a junction-less device presents the additional advantage of a reduced oxide electric field at on-state condition, which enables sufficient NBTI/PBTI reliability (ten year reliable operation at VG=Vth+0.6V, 125C), even without performing the customary high temperature ‘reliability anneal’.
Bich-yen Nguyen, Senior Fellow at Soitec, added: “This exciting R&D collaboration between imec and Soitec is paving the way for global innovation. Together we are leveraging the maturity of Soitec’s Smart Cut process, excellent quality and thickness control, and imec’s advanced device and integration know how, to develop another method to meet the PPAC (Power, Performance, Area and Cost) in scaling.”