Michael Aquilino Microelectronic Engineering Department

Development of a Deep-Submicron CMOS Process for Fabrication of High Performance 0.25 m Transistors
Michael Aquilino Microelectronic Engineering Department Rochester Institute of Technology April 21, 2005

Motivation Enable the Microelectronic Engineering department to continue the semiconductor industry trend of fabricating high performance transistors that have faster switching speeds, increased density and functionality, and ultimately a decrease in cost per function. Push the limits of the SMFL in all areas from design to fabrication to test Create a baseline process that can be used to integrate strained silicon, metal gates, high-k gate dielectrics, and replacement gate technologies at RIT 2

Outline RIT/Industry Scaling Trends Gate Control Fundamentals
Short Channel Effects Deep-Submicron Scaling Unit Process Development Layout Integration/Fabrication/Test Time Line Questions 3

RIT/Industry Scaling Trends
Table 1: RIT/Industry Scaling Trends [3] Over last 30 years, transistors have gotten smaller and faster 110x reduction in gate length 36,000x increase in switching speed Integrated circuits have become much more complex 54,000x increase in number of transistors on an IC 4

Table 1: RIT/Industry Scaling Trends [3] In May of 2004 an NMOS transistor with LPOLY = 0.5 µm was developed This is RIT’s smallest transistor with LEFFECTIVE = 0.4 µm This is the same technology as the Intel Pentium from 9 years earlier 5

Table 1: RIT/Industry Scaling Trends [3] By September of 2005, this process will yield CMOS transistors with LPOLY = 0.25 µm and LEFFECTIVE = 0.18 µm This technology was used in high volume manufacturing of the Intel Pentium III at 600 MHz only 6 years ago The gap between industry and RIT is rapidly shrinking 6

Gate Control Fundamentals
NMOS Transistor 4 Terminal Device Gate Source Drain Body To turn transistor on: Apply positive charge to Gate, QG A depletion region in the p-type body is created as positively charged holes are repelled by gate, exposing negatively charged acceptor ions, QB As the gate charge is further increased, electrons from the source diffuse into the channel and become inversion charge, QI QG = QB + QI Figure 1: Schematic of NMOS Transistor [4] 8

The source and body terminals are grounded A positive voltage is applied to the drain, VDS Inversion charge moves from source to drain and is the current in the device Figure 1: Schematic of NMOS Transistor [4] Equation 1 shows the classical derivation for drain current in a transistor by integrating the inversion charge along the channel (Eq. 1) 9

A depletion region from the drain is created by the reverse biased body-drain p-n+ diode The positively charged donor ions in drain support some of the negative inversion charge This is known as “Charge Sharing” as the gate does not have full control over the inversion channel For large gate lengths, the contribution of the drain in controlling the inversion layer is small compared to the gate contribution As transistors are scaled smaller in gate length, the drain has a larger percentage contribution in supporting inversion charge in the channel Figure 1: Schematic of NMOS Transistor [4] 10

Figure 2 : VT Roll-off Effect [1]
Short Channel Effects “Gate Control” is the most important concept in scaling of transistors The consequence of the drain taking control away from the gate is short channel effects such as: VT Roll-off Drain Induced Barrier Lowering (DIBL) Punch-through Threshold voltage decreases as gate length decreases For small enough gate lengths the devices will be on at 0 V Figure 2 : VT Roll-off Effect [1] 12

Figure 3: Drain Induced Barrier Lowering (DIBL) [1]
Short Channel Effects “Gate Control” is the most important concept in scaling of transistors The consequence of the drain taking control away from the gate is short channel effects such as: VT Roll-off Drain Induced Barrier Lowering (DIBL) Punch-through Increased off-state leakage with increase in drain voltage If gate were in full control, these curves would be one on top of the other Figure 3: Drain Induced Barrier Lowering (DIBL) [1] 13

Figure 4: Lateral source/drain Punch-through [4]
Short Channel Effects “Gate Control” is the most important concept in scaling of transistors The consequence of the drain taking control away from the gate is short channel effects such as: VT Roll-off Drain Induced Barrier Lowering (DIBL) Punch-through At high drain bias, the drain takes control of current through the device Excessive heating can occur and cause device failure Figure 4: Lateral source/drain Punch-through [4] 14

Short Channel Effects “Gate Control” is the most important concept in scaling of transistors The consequence of the drain taking control away from the gate is short channel effects (SCE) such as: VT Roll-off Drain Induced Barrier Lowering (DIBL) Punch-through Transistors with short channel effects are undesirable These SCE must be minimized to yield long-channel transistors 15

Deep-Submicron Scaling
The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Figure 5: Physical Scaling Guidelines [7] 17

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Figure 5: Physical Scaling Guidelines [7] Decrease gate oxide thickness Gate is closer to the channel More control in switching device off Cox  since Cox = A/tox or Cox =Q/V ID and gm  since  Cox 18

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Figure 5: Physical Scaling Guidelines [7] Decrease junction depth of source/drain Depletion region from gate dominates depletion region from the drain Trade-off is sheet resistance  and ID  ND must  which will cause RS  19

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Figure 5: Physical Scaling Guidelines [7] Use sidewall spacer to create deeper source/drain junction called the contact Typically ~2x as deep as shallow LDD Doped heavily to reduce RS 20

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Figure 5: Physical Scaling Guidelines [7] Increase doping of channel to decrease the depletion region from the source/drain Use a retrograde profile where doping is low at the surface and higher sub-surface Mobility of carriers  ID and gm  since both are  mobility 21

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] As TOX , gate leakage current  VDD must  to reduce this leakage VT must  so (VGS-VT) , this is “Gate Overdrive” and is  ID Want ION  since gate delay  ION-1 22

The goal in deep-submicron scaling is to maximize the gate control for switching the device on and off by scaling physical and electrical parameters Table 2: 0.25 µm Scaling Parameters from NTRS Roadmap [6] Want IOFF to be low to reduce standby power The industry standard metric is 1 nA/µm of off-current This results in 5.75 decades between ION and IOFF There is 500 mV swing between 0 V and VT SS of 85 mV/decade is needed to turn the device off Theoretical limit ~ 60 mV/decade @ 300K 23

0.25 µm Device Technology Unit Processes will be developed to yield cross-section shown in Figure 6 Figure 6: CMOS Cross Section showing NMOS (left) and PMOS (right) Transistors Shallow Trench Isolation Dual Doped Poly Gates 50 Å gate oxide w/ N2O Low Doped Source/Drain Extensions Titanium Silicide Super Steep Retrograde Twin Well Nitride Sidewall Spacers Rapid Thermal Dopant Activation 2 Level Aluminum Metallization Back End CMP 25

Unit Process Development
Shallow Trench Isolation Channel Engineering Uniformly Doped Twin Well Super Steep Retrograde Well Ultra Thin Gate Oxide Gate Formation Lithography Resist Trimming Reactive Ion Etch (RIE) of Gate Source/Drain/Gate Doping Poly re-ox Low Doped Source/Drain Extensions Sidewall Spacers Source/Drain Contacts Dual Doped Poly Rapid Thermal Dopant Activation Titanium Salicide Contact Cut Etch 2 Level Aluminum Metallization Figure 7: Shallow Trench Isolation 26

Shallow Trench Isolation
Replacement of LOCOS as preferred isolation technology LOCOS suffers from “Bird’s Beak” effect Field oxide encroaches into active region, reducing W Reduction in Width leads to reduction in drive current To recover this loss, transistors must be made wider which takes up more real estate Small geometries are not possible since Bird’s Beak oxide encroaches from both sides of active area With STI, WACTUAL ≈ WDRAWN Increased packing density Higher drive current for devices with same WDRAWN Decreased topography early in the process 27

Shallow Trench Isolation
Grow Pad Oxide Deposit Nitride by LPCVD Pattern and Etch Silicon Trench Grow liner oxide to repair silicon and round off sharp corners Fill with PECVD TEOS Oxide CMP trench oxide Use Nitride as etch stop Remove Nitride in H3PO4 acid Figure 8: STI Process after trench etch Figure 9: STI Process after trench fill Figure 10: STI Process after CMP 28

Shallow Trench Isolation Channel Engineering Uniformly Doped Twin Well Super Steep Retrograde Well Ultra Thin Gate Oxide Gate Formation Lithography Resist Trimming Reactive Ion Etch (RIE) of Gate Source/Drain/Gate Doping Poly re-ox Low Doped Source/Drain Extensions Sidewall Spacers Source/Drain Contacts Dual Doped Poly Rapid Thermal Dopant Activation Titanium Salicide Contact Cut Etch 2 Level Aluminum Metallization 29

Channel Engineering Uniformly Doped Twin Wells NMOS built in p-well
Figure 11: Uniformly Doped Twin Wells Figure 12: Super Steep Retrograde Wells Uniformly Doped Twin Wells NMOS built in p-well PMOS built in n-well Set Field VT to stop parasitic conduction channels from forming between adjacent devices Super Steep Retrograde Wells 2nd step in well formation process Lower doping at surface which transitions to higher doping sub-surface Control short channel effects Set active VT 30

Uniformly Doped Twin Well
In industry, wells are implanted through the trench oxide with energies of 1 MeV to place the peak 1 µm below the surface The limit of the Varian 350D Ion Implanter is 190 KeV which is not high enough to implant through 3000 Å of trench oxide Also, there will be non-uniformities in silicon trench depth and TEOS oxide fill. For packing density purposes, multiple active regions are built in the same well with 1 common contact It is therefore required for the well doping to be continuous under the trench oxide regions For this process, the wells will be implanted before the trench oxide fill in the STI process to ensure the wells are continuous in the field Figure 11: Uniformly Doped Twin Wells 31

Uniformly Doped Twin Well
Table 3: Field Region VT Figure 11: Uniformly Doped Twin Wells A drive-in thermal step will create uniform wells with: ND = 1x1017 cm-3 XJ-WELL = 1 µm With a 2.0 V supply, the field-VT is sufficiently large The junction depth of the wells must be large enough to prevent vertical punch-through between the reverse-biased drain and complimentary doped starting wafer A junction depth of 1 µm will provide more than enough tolerance 32

Super Steep Retrograde Well
Figure 12: Super Steep Retrograde Wells Increased carrier mobility due to lower channel doping at surface Increased drive current due to increased mobility Suppression of short channel effects (SCE) such as VT roll-off, punch- through, and drain induced barrier lowering (DIBL) due to the higher peek doping sub-surface Latch-up is suppressed because the base regions of parasitic BJTs are doped higher, therefore reducing their gain. 33

Figure 12: Super Steep Retrograde Wells Increased carrier mobility due to lower channel doping at surface Increased drive current due to increased mobility Suppression of short channel effects (SCE) such as VT roll-off, punch- through, and drain induced barrier lowering (DIBL) due to the higher peek doping sub-surface Latch-up is suppressed because the base regions of parasitic BJTs are doped higher, therefore reducing their gain. 34

Peak channel concentration Lower surface doping concentration Uniform well for Field VT Figure 13: Super Steep Retrograde Profile [10] Implanted after uniformly doped wells go through drive-in In Industry, heavy ions that are slow diffusers are used: Indium (In) is used for p-well of NMOS Antimony (Sb) or Arsenic (As) are used for n-well of PMOS At RIT, Boron and Phosphorous are available Boron (B) will be used for p-well of NMOS Phosphorous (P) will be used for n-well of PMOS Are more susceptible to transient enhanced diffusion (TED) Won’t produce profiles as steep as In or Sb but by keeping thermal budget low, can still produce retrograde profiles 35

50 Å Gate SiO2 with N2O Incorporation
Figure 14: Ultra Thin Gate Oxide NTRS Roadmap suggests 40 – 50 Å, target for this process is 50 Å ox = 4 MV/cm, this is at limit of Fowler-Nordheim tunneling Nitrogen is incorporated into oxide to block diffusion of boron from p+ poly gate into channel of PMOS transistor Surface Charge Analysis can be done to determine density of interface trap states. 37

Shallow Trench Isolation Channel Engineering Uniformly Doped Twin Well Super Steep Retrograde Well Ultra Thin Gate Oxide Gate Formation Lithography Resist Trimming Reactive Ion Etch (RIE) of Gate Source/Drain/Gate Doping Poly re-ox Low Doped Source/Drain Extensions Sidewall Spacers Source/Drain Contacts Dual Doped Poly Rapid Thermal Dopant Activation Titanium Salicide Contact Cut Etch 2 Level Aluminum Metallization Figure 15: Poly Gate Formation 38

Gate Formation - Lithography
Figure 16: ASML DUV Stepper Figure 17: Canon i-line Stepper ASML PAS 5500/90 stepper 248 nm KrF excimer laser Capable of 0.25 µm lines/spaces Tool is currently down, laser Canon FPA-2000i1 stepper 365 nm i-line source Capable of 0.5 µm lines/spaces Standard resist coat/develop track Capable of 0.05 µm overlay error 39

Gate Formation – Resist Trimming
Figure 18: Resist Trimming Process Resist trimming can be used to make 0.5 µm line a 0.25 µm line for gate Isotropically etch 1250 Å of resist of sides and top of 0.5 µm line Resist thickness reduced from 10,000 Å to 8750 Å This is still sufficiently thick to protect poly during gate plasma etch Line width reduced to 0.25 µm target in photoresist This technique will be explored if over-exposing/developing is not successful 40

Gate Formation – RIE Poly
Polysilicon thickness is reduced for smaller gate lengths 2500 Å target will provide 1:1 aspect ratio for plasma etch Will block up to 50 keV Phosphorous and 30 keV Boron Want high selectivity of poly to oxide since oxide is very thin Reactive Ion Etcher will be used since it etches anisotropically Etch rate is higher in the vertical direction compared to horizontal If etch was isotropic, enough under-cut would occur due to lateral etching that 0.25 µm would be etched away and remove the photoresist masking layer Figure 19: Under-Cut of Gate Mask 41

Figure 20: Poly Re-Oxidation
After plasma etch of gate there is damage to edges of gate oxide A 250 Å oxide will be thermally grown to: Repair damage to gate oxide from plasma etch of the poly gate Create a thicker screening oxide for source/drain extension implant Make a thicker etch stop for sidewall spacer etch process Form an off-set region for lateral diffusion of shallow s/d extension Want to reduce gate overlap of s/d to reduce Miller Capacitance This capacitance will reduce the cut-off frequency of the device Need 15 – 20 nm overlap or drive current will degrade [11] Figure 20: Poly Re-Oxidation 43

Figure 21: Gate Overlap and LEFFECTIVE Calculation
Poly Re-Oxidation XJ = 75 nm 52.5 nm LPOLY = 250 nm LEFFECTIVE = 180 nm Figure 21: Gate Overlap and LEFFECTIVE Calculation Gate Overlap = 52.5 nm – 25 nm = 27.5 nm ≥ 15 – 20 nm requirement Process is designed for LPOLY = 0.25 µm and LEFFECTIVE = 0.18 µm 44

Low Doped Source/Drain Extensions
Shallow junctions implanted after poly re-ox step A portion of VDS is dropped across the LDD so a lower effective voltage is across the channel (Eq. 2) The advantage of this is reduced hot carrier effects since carriers see lower horizontal electric field The trade-off is there will be a reduction in drive current & switching speed Figure 22: Low Doped Source/Drain Extensions Figure 23: Equivalent Circuit with Parasitic Resistance Vchannel = VDS – 2*IDS(Rterminal + Rs/d + RLDD) (Eq. 2) 45

Low Doped Source/Drain Extensions
Table 4: NTRS Guidelines for LDD Scaling [6] Low end of range is chosen for XJ and RS so if more diffusion occurs then is anticipated, the devices will still operate properly The implant energy must be selected to place the peak of the implant at the Silicon surface. With a 300 Å screening oxide layer: B11 of 10 keV P31 of 25 keV The Varian 350D is capable of implant energies as low as 10 keV In industry, Silicon (Si) or Germanium (Ge) are implanted before the shallow S/D implants to reduce channeling of dopant ions 46

Silicon Nitride Sidewall Spacers
Figure 24: Nitride Sidewall Spacers Figure 25: SEM Micrograph of Nitride Spacer Sidewall spacers are formed after the LDD implants to create an offset region where a deeper source/drain contact region can be implanted. Figure 26: Sidewall Spacer Process 47

Silicon Nitride Sidewall Spacers
In a perfectly anisotropic etch the Lspacer = tpoly = 0.25 µm Parasitic LDD resistance is controlled by length of sidewall spacer = 4108 µm (Eq. 3) VDD = 2.0 V |VT| = 0.5 V Tox = 50 Å (Eq. 4) Table 5: Sidewall Spacer Length Effect on Drive Current [9] NTRS Guidelines require < 10% reduction in drive current due to parasitic source/drain extension resistance Source/Drain contact resistance must also be taken into account 48

Figure 27: Source/Drain Contact and Gate Doping
Source/Drain contacts implanted after sidewall spacer formation and are self-aligned to the gate Figure 27: Source/Drain Contact and Gate Doping Contact is doped higher and junction depth deeper to decrease parasitic source/drain resistance Silicide thickness must account for < ½ contact junction depth or increased leakage current will result 49

Table 6: NTRS Guidelines for Source/Drain Contact Scaling [6]
Midpoint of XJ range chosen so XJ-CONTACT ~ 2*XJ-LDD The implant energy must be ~ 2x higher then shallow LDD implants: B11 of keV P31 of keV The sheet resistance of this region must be decreased to decrease the reduction in drive current The polysilicon gates are doped n+ for NMOS and p+ for PMOS at the same time the contacts are doped. 50

Dual Doped Poly Gates Dual doped poly is used to create surface channel PMOS transistors by engineering a better matched Metal-Semiconductor work function, MS (Eq. 5) (Eq. 6) (Eq. 7) Retrograde profile will be designed to set appropriate Na for the VT equation in Eq. 5 51

Figure 28: Super Steep Retrograde Profile
Dual Doped Poly Gates Gate depletes body to a depth of WDMAX Na transitions from 1x1017 cm-3 at surface to 1x1018 cm-3 at the peek The peak of each profile must be set such that the effective Na the gate depletes is what is required for |VT| = 0.5 V Na(effective) for NMOS =6.75x1017 cm-3 Na(effective) for PMOS = 6x1017 cm-3 Lower surface concentration, higher sub-surface channel doping and proper VT are all benefits of using a super steep retrograde well Figure 28: Super Steep Retrograde Profile With WDMAX [10] 52

Rapid Thermal Dopant Activation
Need to repair damage to Si lattice caused by ion implant Electrically activate dopant atoms Transient enhanced diffusion will cause dopants to diffuse at accelerated rate and cause deeper then desired junction depths Rapid Thermal Processing is used to rapidly heat the wafer for a short amount of time Thermal budget for 0.25 µm device is only °C Figure 29: Thermal Budget for p+ Junctions 54

Titanium Silicide Formation (TiSi2)
Figure 30: Titanium Silicide Formation Want to reduce sheet resistance of source/drain contact regions from 50 – 75 /sq to 4 /sq Want high drive current for fast switching speeds Titanium Silicide was widely used at the 0.25 µm node and will be used in this process 56

Figure 31: Transistor Cross section with Parasitic Resistances [14] 57

Require < 10% reduction in drive current due to RPARASITIC Sample calculation shown in Table #, assume: RS-LDD = 400 /sq LSPACER = 0.25 um RS-Silcide = 4 /sq LSILICIDE = 0.75 um Table 7: Reduction in Drive Current due to Parasitic Resistance [14] All Resistances are calculated for a nominal 1 µm width As width is increased, the total resistance components will decrease but the ratio for drive current reduction will remain the same Drive current is only reduced by ~5% by integrating silicide 58

Table 8 shows properties for Titanium Silicide reactions Table 8: Titanium Silicide Properties [14] 45 nm of Si is consumed by 20 nm of Ti to produce 50 nm of TiSi2 in C49 phase The C49 phase is a higher resistivity phase created after a 500- 700°C rapid thermal step The unreacted Ti is removed by wet chemistry and a 2nd thermal step is performed at °C to form lower resitivity C54 phase 50 nm of TiSi2 in the C54 phase should yield an Rs ~ 4 /sq 59

Titanium Silicide suffers from a narrow line width effect where Rs increases as line width is decreased This is why the industry transitioned to CoSi2 for sub-0.25 µm CMOS Intel reports an RS of 4 /sq for their 0.25 µm CMOS process, although it is not reported if this is for the source/drain regions only, or gate too Test structures will be designed on the test chip to observe this effect Figure 32: Narrow Line Width Effect [9] 60

Contact Cut Etch In a fully scaled 0.25 µm process, contact cut dimensions are 0.25 µm x 0.25 µm Since Canon FPA-2000i1 stepper will be used, the smallest contact cut dimensions will be 0.5 µm x 0.5 µm Contact cuts below 2 µm x 2 µm must be dry etched since wet chemistry acid will not go into small holes Some devices will be designed with the smallest contact cuts that must be plasma etched, while other devices will be designed with larger contact cuts (2 µm or 5 µm) that can be wet etched Loading is a fabrication phenomena where features with small areas will take longer to clear then large areas. It is desirable to have all contact cuts for a given design generation to have the same area so that over-etch of smaller contacts will not destroy regions with large areas that clear earlier 62

Aluminum Metallization
Multi level metal is necessary to build complex logic circuits that require extensive routing while minimizing real estate Figure 33: 2 Level Aluminum Metallization PECVD TEOS will be deposited as inter-level dielectric layer Aluminum will be sputter deposited to a thickness ~ 0.5 µm, then patterned and etched by RIE in Chlorine chemistry A 2nd PECVD TEOS layer will be deposited to insulate Metal 1 from Metal 2, this layer may undergo CMP to reduce topography Vias will be patterned and etched by RIE in Fluorine plasma Metal 2 will be sputter deposited to thickness ~ 1.0 µm, then patterned and etched similarly to Metal 1 64

Table 9: Test Chip Design Layers
Layout of Test Chip Table 9: Test Chip Design Layers A new test chip will be designed and mask set fabricated There are 9 design layers which will be used to create 10 photo masks These 10 photo masks will be used for 14 lithography levels In addition to transistors, logic circuits will be designed, as well as capacitors for CV analysis, Van-der-pauw test structures for sheet resistance measurements and Cross-Bridge Kelvin Resistor test structures for measurement of contact resistances 66

Integration/Fabrication/Test
Unit processes will be integrated into a full CMOS manufacturing flow All fabrication will be done in the Semiconductor and Microsystems Fabrication Laboratory (SMFL) at RIT When fabrication is completed, the devices will be tested in the Semiconductor Device Characterization lab at RIT On and off-state performance will be characterized to qualify this process Electrical parameter extraction will be performed to develop SPICE models for this process which can be used in future circuit designs The fabrication limits of the SMFL tool set will be pushed so that custom design rules can be created for future layouts 68

Figure 34: Masters Thesis Time Line
This time line constitutes a reasonable plan of study to design, fabricate and test 0.25 µm CMOS transistors Ample time is left for thesis writing, review, and defense 70

References 72

Michael Aquilino Microelectronic Engineering Department

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