November_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 4 10 DHEM: OHMIC CONTACT AND HIGH-MOBILITY CHANNEL ENGINEERING AND CHARACTERIZATION FOR ICS Abhijeet Joshi and Bulent M. Basol Active Layer Parametrics (ALP), Scotts Valley, California ajoshi@alpinc.net EDFAAO (2020) 4:10-16 1537-0755/$19.00 ©ASM International ® INTRODUCTION Development of next-generation semiconductor tech- nologies comeswithescalated costs due toever increasing technical challenges and extended development cycles needed to meet such tasks. Dependable, high-resolution electrical characterization techniques are key for efficient film/device development efforts as well as for prediction anddeterminationof device failure. Traditional techniques such as four-point probe (4PP), secondary ion mass spectrometry (SIMS) and Hall effect measurements are not adequate to characterize semiconductor layers with electrical properties that may vary as a function of depth through the film. SIMS provides chemical composition profile data which can only be translated into electrical property information in cases where all the dopants are fully activated in the layer. This is clearly not the case in advanced device structures where, for example, ohmic contact formation for source/drain regions of transistors necessitates introduction of very high concentration of dopants (as much as 4-8%) much of which may not be electrically activenear the surface. Relyingon4PPandHall effect measurements, which provide average resistivity and effective mobility values, can be quite misleading as will be explained later. Recognition of the need for depth profiling electrical parameters gave rise to development of scanning spread- ing-resistance microscopy (SSRM) and electrochemical capacitance-voltage (E-CV) techniques. [1,2] However, one limitation of these approaches is the fact that they mea- sure either the resistivity or carrier concentration values, without providing mobility depth profiles. This neces- sitates assuming a mobility value for the thin-film, which is problematic for various reasons. The silicon mobility models are based on a 1981 publication, [3] while the ger- manium models are mostly taken from a 1961 paper. [4] These models may not be absolutely relevant to mate- rials produced today using modern processes such as atomic layer deposition and advanced epitaxial growth techniques, which may introduce very high concentra- tions of dopants in the films and utilize non-equilibrium approaches such as laser annealing. Furthermore, models do not directly expand to include new materials such as Si-Ge alloys with varying amounts of Ge. There is also limited electrical data for thin films of III-Vmaterials grown by different approaches. Therefore, a technique that can directly measure depth profiles of carrier concentration and mobility through layers at high depth resolution is highly valuable. DIFFERENTIAL HALL EFFECT METROLOGY Differential Hall effect metrology (DHEM) provides depth profiles of all critical electrical parameters through semiconductor layers at nanometer-level depth resolu- tion. [5,6] It is based on the differential Hall effect (DHE) method. [7] The traditional DHE method makes succes- sive sheet resistance and mobility measurements on a layer using Hall effect and Van der Pauw techniques as the thickness of the layer is reduced through successive processing steps, typically involving thermal, chemical or electrochemical etching or oxidation. The data obtained as a function of thickness removed can then be used to determine the depth profiles of carrier concentration, resistivity, and mobility. DHE has not been widely used in the semiconductor industry, because it is a slow laboratory technique requiring access to wet chemical processing facilities for etching and cleaning the substrates multiple