1 HOT CARRIERS AND RELIABILITY IN MOS DEVICES The term “hot carriers” refers to electrons or holes in the substrate of a MOS device that have energies significantly above average. Hot carriers may be present due to a variety of circumstances. When they are produced as a result of very high fields in the drain region of a MOSFET, they may compromise operation of the device by generating charged defects in the oxide layer, and by degrading the oxide and the Si-SiO2 interface. These effects constitute a reliability problem. Hot carriers also generate unwanted current components. References - Sabnis, VLSI Reliability, in “VLSI Electronic Microstructure Service”, vol 22 Academic Press, N. Y., 1990, Chapter 6 - Yue, Reliability in ULSI Technology, C. Y. Cheng and S. M. Sze, eds, Mc Graw-Hill N. Y., 1996 Chap 12 - Chenming Hu, Hot carrier Effects, in “VLSI Electronic Microstructure Science”, Vol. 18, Academic Press N. Y., 1989, Chapter 3 HOT CARRIER GENERATION We consider here the process of hot carrier generation near the drain. Muller and Kamins, Fig. 10.9 Processes listed in Fig. 10.9: (1) Injection of electrons into the gate (2) Avalanche pair production (3) Substrate current Isub which can generate a voltage in competition with VBS2 (4) Forward biasing of source due to voltage drop caused by drift of holes from process (2) (5) Collection of electrons emitted from source Processes 2, 4, and 5 may become self-sustaining in which case we have Avalanche Breakdown. Process (4) is essentially DIBL, where the field generated by VDS is sufficient to reduce the barrier to injection of electrons from the source. Process (5) is punch-through. Avalanche Breakdown at the Drain For large VDS, we risk avalanche breakdown at the drain. Technically, this is a hot carrier effect since avalanching requires high electric fields. Note that an increase in NB will reduce the breakdown voltage. Tsividis Fig. 4-23 Snapback Breakdown An interesting effect arising from hot carrier generation near the drain is snapback breakdown. Pierret, Advanced MOS: Fig. 6-16, 6-17 Mechanism Avalanche breakdown in the drain, along with generation of substrate bias via hole drift through the substrate (see Figure above). Circuit model of Fig. 6-17 suggests that we have a “floating base” BJT, which is unstable, and for which snapback breakdown in well known. For the BJT, the progression to breakdown is as follows. (See D. K. Schroder, Advanced MOS Devices, for further details.) Collector current for IB = 0 is I I I I IC E CBO C CBO    . With avalanche multiplication at the drain, this current is increased by a multiplication factor M:3  I M I IC C CBO  Solving for IC gives IMIMCCBO1 Thus IC gets very large (i.e., we have avalanche breakdown) when M = 1/. Since  is not much less than 1, M is not very large, unlike in a reverse-bias pn junction, where M must be very large for breakdown to occur. The multiplication factor is related to the collector-emitter voltage VCE and the breakdown voltage VBD by4  MV VCE BDm11 / where m is a parameter that varies between 3 and 6. We can understand snapback as follows. For low VCE, M is ~ 1, but M approaches 1/ as VCE is increased. Thus breakdown begins and a large IC flows. But for BJTs,  increases as collector current goes up, which means that M must decrease. But a drop in M corresponds to a drop in VCE, so as IC goes up, VCE goes down. In a MOSFET, the formulation is different in detail, but similar in concept. For further information, see F. C. Shu, P. K. Ko, S. Tang, C. M. Hu, and R. S. Muller, An Analytical Breakdown Model for Short-Channel MOSFETs, IEEE Trans. Electron Dev., ED-29, p. 1735-1740 (1982). These authors show that for the MOSFET case, the relation between source/drain current (IS, ID) and M is      IM I V RkM kSD BS    1 0651 1. and the multiplication factor M becomes large as follows.   Mkk 1 1 In these equations, k is the fraction of electrons collected by the drain that cause breakdown, and R is the sum of substrate and external resistances. MOSFET OXIDE DEGRADATION An important issue that we will discuss in detail is the trapping of charge in the oxide layer due to process 1 and generation of oxide charge and interface traps as a result. This is an issue of oxide and Si-SiO2 interface reliability. ULSI Technology Figs. 3, 4 For now, we simply point out that for very high electric fields, near the drain or elsewhere, electrons can enter the oxide via Fowler-Nordheim tunneling (FNT). In this process, electrons enter the oxide conduction band by tunneling through a reduced thickness energy barrier created by application of a high gate voltage. This happens in measurable amounts for electric fields VG/dox larger than 6 MV/cm. Effects of hot carrier injection (HCI) are measured (for MOSFETs) in terms of a degradation in transconductance gm or in mobility. These effects are measured as changes in ID - VD curves.5 n-MOSFET Degradation Due To HCI Oxide charging and interface trap generation cause - degradation in mobility or transconductance (gm )  reduced ID  reduced speed - shift in threshold voltage Engineering Model Based on a review by C. Hu in VLSI Electronics Microstructure Science, we outline a phenomenological model of HCI-induced MOSFET degradation. Important parameters that correlate with hot carrier generation are Isub and Ig (substrate and gate current) VLSI Electronics Microstructure Science Figs. 11, 22 Define the following parameters.67 it = critical energy to generate an interface trap i = critical energy for impact ionization Te = electron temperature d/dt = rate of degradation of some parameter, e.g., VT, gm, … G() = a function describing the dependence of the degradation mode on defects already present (i.e., in the as-fabricated device). The model is ddtAIWG edkTit e1( )/ eikTdsubeIAI/2 Re-arranging... eII ATkII Ai ekTsubde isubd /ln2211 Substituting into the expression for d/dt gives ddtA GIWeA GIWII Adk kd subdit iit i 112( )( )/ /( / )ln(...)/   or, ddtBGIWIId subdit i( )/  where B = A1/A2 is a constant. If we assume a specific form for