集成电路中的MOS场效应晶体管ppt课件.ppt
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1、Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-1,Chapter 7 MOSFETs in ICs Scaling, Leakage, and Other Topics,7.1 Technology Scaling - for Cost, Speed, and Power ConsumptionNew technology node every two years or so. Defined by minimum line width-spacing average. Feature sizes ar
2、e 70% of previous nodes. Reduction of circuit area by 2 good for cost and speed.,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-2,International Technology Roadmap for Semiconductors,Vdd is reduced at each node to contain power consumption in spite of rising transistor density a
3、nd frequency Tox is reduced to raise Ion and retain good transistor behaviors HP: High performance; LSTP: Low stand-by power,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-3,7.1.2 Strained Silicon: example of innovations,The electron and hole mobility can be raised by carefully
4、 designed mechanical strain.,N-type Si,Trenches filled with epitaxial SiGe,Gate,S,D,Mechanical strain,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-4,7.2 Subthreshold Current,The leakage current that flows at VgVt is called the subthreshold current.,90nm technology. Gate lengt
5、h: 45nm,The current at Vgs=0 and Vds=Vdd is called Ioff.,Intel, T. Ghani et al., IEDM 2003,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-5,Subthreshold current ns (surface inversion carrier concentration)ns eqs/kT,s varies with Vg through a capacitor network,In subthreshold, s
6、 = constant +Vg/h,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-6,Subthreshold Leakage Current,Subthreshold current changes 10 x for h60mV change in Vg. Reminder: 60mV is (ln10)kT/q,Subthreshold swing, S : the change in Vgs corresponding to 10 x change in subthreshold current.
7、 S = h60mV, typically 80-100mV,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-7,Subthreshold Leakage Current,is determined only by Vt and subthreshold swing.,Practical definition of Vt : the Vgs at which Ids= 100nAW/L=,1/S,Modern Semiconductor Devices for Integrated Circuits (C
8、. Hu),Slide 7-8,Subthreshold Swing,Smaller S is desirable (lower Ioff for a given Vt). Minimum possible value of S is 60mV/dec.How do we reduce swing?Thinner Tox = larger Coxe Lower substrate doping = smaller Cdep Lower temperatureLimitationsThinner Tox oxide breakdown reliability or oxide leakage c
9、urrent Lower substrate doping doping is not a free parameter but set by Vt.,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-9,Effect of Interface States on Subthreshold Swing,Interface states may be filled by electrons or empty depending on its energy relative to EF, i.e., depen
10、ding on Vg.dQint/d (number or interface state per eV-cm2) presents another capacitance in parallel with Cdep,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-10,7.3 Vt Roll-off,65nm technology. EOT=1.2nm, Vdd=1V,K. Goto et al., (Fujitsu) IEDM 2003,Vt roll-off: Vt decreases with d
11、ecreasing Lg. It determines the minimum acceptable Lg because Ioff is too large if Vt becomes too small.,Question: Why data is plotted against Lg, not L? Answer: L is difficult to measure. Lg is. Also, Lg is the quantity that manufacturing engineers can control directly.,Modern Semiconductor Devices
12、 for Integrated Circuits (C. Hu),Slide 7-11,Why Does Vt Decrease with L? Potential Barrier Concept,When L is small, smaller Vg is needed to reduce the barrier to 0.2V, i.e. Vt is smaller.Vt roll-off is greater for shorter L,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-12,Vds
13、dependence,Energy-Band Diagram from Source to Drain,L dependence,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-13,Vt Roll-off Simple Capacitance Model,As the channel length is reduced, drain to channel distance is reduced Cd increases,Vds helps Vgs to invert the surface, there
14、fore,Due to built-in potential between N- channel and N+ drain & source,2-D Poisson Eq. solution shows that Cd is an exponential function of L.,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-14,Vertical dimensions (Tox, Wdep, Xj) must be scaled to support L reduction,Modern Sem
15、iconductor Devices for Integrated Circuits (C. Hu),Slide 7-15,7.4 Reducing Gate-Insulator Electrical Thickness and Tunneling Leakage,Oxide thickness has been reduced over the years from 300nm to 1.2nm.Why reduce oxide thickness?Larger Cox to raise IonReduce subthreshold swingControl Vt roll-offThinn
16、er is better. However, if the oxide is too thinBreakdown due to high fieldLeakage current,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-16,Gate Tunneling Leakage Current,For SiO2 films thinner than 1.5nm, tunneling leakage current has become the limiting factor. HfO2 has sever
17、al orders lower leakage for the same EOT.,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-17,Replacing SiO2 with HfO2-High-k Dielectric,HfO2 has a relative dielectric constant (k) of 24, six times large than that of SiO2. For the same EOT, the HfO2 film presents a much thicker (
18、albeit a lower) tunneling barrier to the electrons and holes. Toxe can be further reduced by introducing metal-gate technology since the poly-depletion effect is eliminated.,(After W. Tsai et al., IEDM03),Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-18,Challenges of High-K Te
19、chnology,The difficulties of high-k dielectrics:chemical reactions between them and the silicon substrate and gate,lower surface mobility than the Si/SiO2 systemtoo low a Vt for P-channel MOSFET (as if there is positive charge in the high-k dielectric). long-term reliability,A thin SiO2 interfacial
20、layer may be inserted between Si-substrate and high-k film.,Question: How can Tinv be reduced? (Answer is in Sec. 7.4 text),Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-19,7.5 How to Reduce Wdep,Or use retrograde doping with very thin lightly doped surface layerAlso, less imp
21、urity scattering in the inversion layer higher mobility,Wdep can be reduced by increasing Nsub,If Nsub is increased, Cox has to be increased in order to keep Vt the same. Wdep can be reduced in proportion to Tox.,Modern Semiconductor Devices for Integrated Circuits (C. Hu),Slide 7-20,7.5 Ideal Retro
22、grade Doping Profile,Compared with uniformly doped body,Assume the body is heavily doped with an undoped layer, Trg thick, at the surface.,Ideal retrograde doping yields a depletion region width (Trg) half as thick as Wdep of a uniform doped body.,Modern Semiconductor Devices for Integrated Circuits
23、 (C. Hu),Slide 7-21,7.6 Shallow Junction and Metal Source/Drain,The shallow junction extension helps to control Vt roll-off.Shallow junction and light doping combine to produce an undesirable parasitic resistance that reduces the precious Ion. Theoretically, metal S/D can be used as a very shallow “
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