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SJSU EE 225A - Mask Process Wafers

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1Mask Documentation of the 4 Mask Process WafersD. W. ParentSJSU2Overview• The 4 mask NMOS process mask has many test structures:– Materials (Sheet resistance, Junction depth)– Diodes– MOS Capacitors (Long channel VT, Qss, NSUB– MOSFETS (KN, VT, γ, λ)– Circuits (Inverters, Current Mirrors, and Ring oscillators.)3Overview Continued• These test structures provide data that the circuit design engineer uses to select W and L of a transistor to meet some circuit specification.4Materials• These structures are used to measure – Sheet resistance of the aluminum or n-diffusion– Contact resistance between the aluminum and n diffusion– Mask registration Errors– Oxide Thickness– Junction depth5Diodes• There are diode structures that can be used to measure – Junction capacitance – Side wall capacitance– Breakdown voltage–Io– Ideality factor (this is actually very hard)6MOS Capacitors• MOS Caps are used to measure–Qss– Substrate doping– Sodium contamination7MOSFETS• These are used to measure– KN, VT, γ, λ– Effective channel length and width8Circuits• The inverter is used to measure:– Gain, Vinth, noise margins• The current mirror is used to measure how well transistors on the same wafer match each other.• The ring oscillator is used to measure– The ultimate speed of the MOSFETs– How many circuits can be integrated without a device failure.910Process Windows and LegendMask NamesMask NamesAlignment MarksMask Names11Alignment MarksLegend“Normal” Alignment MarksVernier Alignment Marks12Normal Alignment MarksBoxes CrossesM2 aligned to M1, M3 aligned to M1, M4 alignedto M3.13Vernier Alignment MarksOnly one box in the center should be perfectly centered on the previous mask.14Cell Map15Test Cell Structure16Test Cell Map17Gate Oxide MOSFET ArrayWidth Length15 x 5BodyDrainGate100 x 100Length Constant in Rows.Width Constant in Columns18MOSFET Process Sequence19.5um Box Vernier Test Structures200.5um Regular Vernier Test Structures21Find the two Bars that line up perfectly.Count how many bars from the center to the perfectlyaligned bar. Multiply this by .5μm. This is your registration error.In this casethe erroris zero(plus orminus .5μm)22In this case the bar most perfectly aligned is this one.3 2 1 Error in X=-3*.5μm=-1.5μm23PL Test Structures24Needle PL Test Structures25PL/Resistor Test Structures26Inductor Test Structures27RS Test StructuresForce a current through A and D.Measure the voltage difference from B to C.ADCBSIVVR−= 54.4For a more accurate value you canRotate the measurement and average the two RS values.ABCD28Contact Resistance (RC) Test Structures2lIVVADCBC×−=ρl=10x10-4cmThis will vary across chip. This is the as drawn length of the contact. Force a current through A and D.Measure the voltage difference from B to C.ABCD29Planar Capacitor Test StructuresGap 2.5um30MIS Capacitor Test StructuresField OxideGate OxideMeasure TOX with nanospec here.Measure TOX with nanospec here.Use the MTI CV meter to extract VT,NSS, and NSUB. Use the TOX measurednext to the MOSCAP. The as drawn diameteris 1000μm. You might have to scratch theback of the wafer and deposit AL.31Hall Effect/ RS for NDIFF Test StructuresForce a current through A and D.Measure the voltage difference from B to C.ADCBSIVVR−= 54.432Field Oxide MOSFET Test StructuresNo Gate Oxide on Purpose!These MOSFETS use the field oxide for the gate oxide. Since the Field oxide is much thicker that the active oxide layer, the VT should be much higher. However since we have a SOG layer over the field oxide, which has a Qss, the VT might be Negative! The μnx Coxwill be much smaller in either case.33Diode Test StructuresWidth of NDIFFHeight of NDIFFTwo Diodes per Test Structure34There are 4 DC spice parameters for a diode IS Saturation current (Given in Amps)N Emission Coefficient (No Units)RS Parasitic resistance (Given in Ω)BV Breakdown Voltage (Given in Volts)IBV Breakdown Current (Given in Amps)GMIN is a small resistanceTo prevent a value of zero current formFlowing. Usually set to 10-12S35Full Diode Response36Test the diode of a NMOSNote there are two diodes being tested.37Forward Bias DiodeLinear0.00E+002.00E+004.00E+006.00E+008.00E+001.00E+011.20E+011.40E+011.60E+010.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 1.20E+00ID(A)VD(V)Series138Take the RS value from the Linear part of the diode curvey = 77.327x - 62.472R2 = 10.00E+002.00E+004.00E+006.00E+008.00E+001.00E+011.20E+011.40E+011.60E+010.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.020.00E+002.00E+004.00E+006.00E+008.00E+001.00E+011.20E+011.40E+011.60E+010.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 1.20E+00ID(A)VD (V)39Forward Bias Log Scale(RS position omitted)0.00E+002.00E+004.00E+006.00E+008.00E+001.00E+011.20E+011.40E+011.60E+010.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 1.20E+00ID(A)VD(V)y = 38.664x - 31.543R2 = 1-30-25-20-15-10-500.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80Do not use linearRegion of Diodeor too close to 0V.The RS value and GMINwill skew the results.40Super MOSFET Test Structures41Current Mirror Test Structures42Small Gate Length MOSFET Test StructuresL from 1um to 5um (You never know!)43Super MOS Current Mirror Array Test Structures44MIS Capacitor Test Structures45Solar Cell Photo DiodeN+P46Simple Beam Test Structures47Finding VT for a MOSFET• Set VDS to less than the Vbi of the Drain/Body Junction (.2-.5V)• Sweep VGS from below VT to around 4-5 volts– If you are unsure of what VT is you can start at –5Volts• Measure ID• Plot gm(ΔID/ΔVG) vs. VGS• Find the xvalue for the maximum gm• Find the tangent the of ID-VGS at the value point• VT is where the line formed by the tangent intersects the x-axis.48IDIDgmgm-maxTangent of ID atthe xvalue that give gm-maxVT49VT NumericalIDmax=The ID at the point of maximum gmVGmax=The gate voltage at the point of maximum gmgmmax=The largest value of ΔID/ΔVG()maxmaxmaxmaxgmVGgmIDVT×−−=50KN• Since Mobility varies with the electric field from Gate to body and source to drain KN=μnCox varies as well. This means KN is not a constant. One could take the average of the gm plot or use:eUDStTGScoxSiVUVVUtoxKNnewKN⎟⎟⎠⎞⎜⎜⎝⎛−−⋅⋅=⋅)()(εεThis requires extensive testing and a regression.One could also report just gm-max.51Body Effect (γ)Do a ID, BGS plot for different VSB voltages.52Body Effect


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