Design Considerations
Designing for highest yields for today’s Advanced Technology nodes (1Xnm and below) requires specific process architectures and an understanding of the physics of semiconductor processing. From the perspective of the architecture, we consider thermal transport mechanisms (conduction, convection and radiation) and how to minimize uncertainties, errors and variables. When we consider a patterned wafer from the perspective of the physics of processing, the wafer employs many materials which means variation of thermal effects from emissivity and thermal expansion. In addition, there are many different pattern periodicities (similar to diffraction gratings of different spacings) which couple to many frequencies in the visible spectrum. These facts give rise to what we observe as “the pattern effect”, thermal non-uniformity of processing, and are exacerbated when photons (lamps) are used as the main heating mechanism.
How WaferMasters Achieves Superior Yields and Performance
Superior on-Wafer REPEATABILITY is the Key to Yield Improvement. Once the process is qualified, REPEATABILITY = YIELD… Compact Isothermal Process Module Design Is the KEY – The Process Module provides a very large, stable thermal mass which efficiently transfers heat to the wafer. The process module is maintained at a consistent temperature. The thermal characteristics of the module remain invariant. Thus on-wafer results are REPEATABLE wafer to wafer, week to week and month to month!
Excellent Uniformity – The Process Module presents a nearly Isothermal environment to the wafer which translates to excellent on-wafer uniformity.
Minimum Added Stress – The Isothermal process environment minimizes thermal stress by keeping the wafer in thermal equilibrium as much as possible.
Minimum Pattern Effect – No Lamps. WaferMasters minimizes radiation transfer using conduction and convection as thermal transfer mechanisms. There is never any thermal overshoot!
Reliability by Design – MTBF > 5000hr (greater than 7 months!). WaferMasters’ design leaves nothing to chance. Moving parts, interlocks and unnecessary complexity are minimized.
WaferMasters Leaves Nothing to Chance – Minimum Variables, Minimum Uncertainty = Maximum Repeatability. WaferMasters’ Un-Compromising Design Rules reduces uncertainty and errors in every detail.
WaferMasters has developed robust platforms to provide a near Isothermal environment for rapid thermal processing of wafers. This provides truly differentiated results compared to lamp based RTP for two reasons:
1. Consistent, uniform, predictable and reliable on-wafer processing at advanced technology nodes because the process module is essentially invariant throughout the manufacturing run.
2. Since radiation is a small component in the Isothermal Process Module design, processing is largely without the harmful effects of the Pattern Effect. This is demonstrated in some of the graphics below:
Wafer Level
At the Wafer Level, the benefits of Isothermal Processing can be seen in this comparison of detailed sheet resistance measurements following WSi anneal carried out on wafers processed in 3 different systems; a WaferMasters SRTF, a honeycomb Lamp RTP system and a linear Lamp RTP system. All wafers have been characterized in detail, with 625 point measurements. In the 2D and 3D representations below, the excellent within wafer uniformity is confirmed in the SRTF. The advantage of the Isothermal Process Module is designed in, and, with the advantage of rock solid reproduceability, wafers are produced with essentially identical results over months of continuous processing, with no interruptions or maintenance, and uptimes of > 99%.
Device Level
The schematics of the STI structures on the left illustrate the way Isothermal Processing and lamp RTP differ at the device level. The Isothermal Process Module of the SRTF/SAO conveys heat to the structures evenly (there is no physics mechanism for differential treatment of the circuit geometries). However, for photon based processing, structures act as gratings and diffraction effects, as well as other localized thermal transfer mechanisms, have measureable impact, causing non-uniformities. Within the trench, for example, due to the periodic trench structures, radiation bends and we can get localized sidewall heating and selective film buildup that are a cause of leakage. At the feature sizes of advanced technology nodes, strong coupling exists between the circuit periodicities and the many spectral lines of halogen lamp radiation, exacerbating this phenomenon to meaningful levels. In the hot wall, Isothermal Process Module of the SRTF/SAO, there is no similar mechanism. The graphic at the right shows how edge to center thickness can vary, depending on the choice of thermal process system.
Data from: H. Itokawa, H. Akutsu, A. Nomachi, H. Oono, T. Iinuma and K. Suguro, Ext. Abst. 2006 Int. Conf. on Solid State Device and Materials (Yokohama, 2006) 342.