(Invited) Activation and Deactivation in Ultra-Highly Doped n-Type Epitaxy for nMOS Applications
Authors: Ye Zhiyuan , Chen Miao-Chun , Chang Flora , Wu Chen-Ying , Li Xuebin , Dube Abhishek , Liu Patricia , Chopra Saurabh , Chu Schubert 
Published: December 2020

In last 15 years, n-type doped selective epitaxy in source and drain (S/D) for nMOS have been heavily investigated. Initial interests for nMOS S/D epitaxy were focused on carbon and phosphorus doped Si:CP film for channel strain engineering and was adapted in production. In 2012, another type of ultrahigh phosphorus doped Si:P was proposed as a replacement for Si:CP for tensile engineering. With as high as 4.5E21 doping level, such Si:P film showed comparable strain as Si:CP with 2% substitutional carbon, but with much simpler process design and quickly accepted by the industry. These ultra-high doped Si:P film also promoted interests in the mechanism for such high doping level and high strain level. In last couple years, phosphorus vacancy complexes, particularly P4V, or pseudo-cubic Si3P4, is widely accepted as the key mechanism. Although still under debate, formation of high concentration, phosphorus stabilized vacancy, up to 1E21 or 2%, is also believed to be responsible for the observed high strain. Along the same line, it was quickly discovered that ultrahigh arsenic or antimony doped films are also possible (Fig 1). And high strain Si:As is also observed with a similar strain level as Si:P, with even higher doping level, likely due to the formation of As4V complex. A group V and vacancy complex, V4V complex, is electrically neutral therefore would not contribute to carrier activation. V4V interaction with carrier would be essential to understand activation and deactivation in ultrahigh n-type doped epitaxy film.

P-V interaction have been observed in early studies of bipolar device or solar device. It is also extensively studied in late 1990s to understand the USJ formation and activation. To compare with these typically implanted and annealed or diffused data, high doped Si:P and Si:As epitaxy films were deposited at different temperature and annealed with different thermal budget. Our data showed that selective Si:P as grown resistivity v.s. deposition temperature closely follow the equilibrium model based on literatures, indicating selective Si:P film as grown being near thermal equilibrium from active carrier point of view, despite one order magnitude higher P concentration (Fig.2). Selective Si:As, however, showed a different trend, with lower deposition temperature having lower resistivity. To understand the mechanism, non-selective Si:P with fast deposition rate was deposited and then annealed at deposition temperature for different time. it reveals that with fast deposition rate, initial film is activated well beyond thermal equilibrium. But it quickly deactivated within 90sec to near thermal solubility (Fig 3.). Si:As is deactivated at a much slower rate and lead to a different as grown resistivity trend. The energy favored V4V structure requires a minimum temperature to dissolve and activate the carrier. 800C is found not sufficient for Si:P and Si:As requires even higher temperature and thermal budget.

To investigate the activation and deactivation, HRXRD and 4-point probe were used to study the strain and carrier concentration evolution during activation and deactivation. Unlike Si:As that always observed higher strain during deactivation from HRXRD, Si:P seems to have multiple paths for deactivation. It is found that depending on active carrier history; for example, low temperature deactivation of active carrier from random capture during epitaxy growth could involve more P4V formation therefore associated with a strain gain (Fig. 3); while for carriers gained from millisecond laser high temperature anneal, post spike annealing sometime involves strain loss during deactivation, possibly via significant interstitial P, on top formation of P4V. P4V formation during deactivation provides a model for transient diffusion enhancement in Si:P with some anneal conditions. As shown in Fig 4, only over-activated sample showed enhanced tail diffusion during 900C spike anneal, probably due to P4V silicon interstitial kick-out effect.

To further investigate Si:P activation and deactivation, differential Hall effect measurement (DHEM) was used to obtain the carrier depth profile of different samples. As shown in Fig 5, 1.5E21 to 3E21 total doping level, there is minimum effect of on carrier concentration. One distinct feature on the data is the carrier degradation toward surface. It is possible during the film cool down post deposition, the surface deactivated faster, assisted from the surface flux of interstitial or vacancy. Understand the surface effect on Si:P and Si:As activation deactivation is essential to improve the contact resistance for future nodes.

Figure 1