FRONT END PROCESSES - CLEANING, LITHOGRAPHY, OXIDATION ION IMPLANTATION, DIFFUSION, DEPOSITION AND ETCHING Cleaning belongs to front end processes and is an important part of fabrication. Reference - ITRS Roadmap for Front End Processes (class website). Chapter 4

Semiconductor Manufacturing Clean Rooms, Wafer Cleaning and Gettering Importance of unwanted impurities increases with shrinking geometries of devices. 75% of the yield loss is due to defects caused by particles (1/2 of the min feature size) Crystal originated ( nm) particles (COP) ~1,000Å=void with SiO x -> affect GOI -> anneal in H 2 -> oxide decomposes and surface reconstructs! & oxide precipitates from deep depth in Si. Yield -> 90% at the end -> each step

Historical Development and Basic Concepts Contaminants and their role in devices (various elements, various films) Na +, Ka + X OX ~10nm Q M ≈ 6.5x10 11 cm -2,   V TN =0.1V (equivalent to 6.7*10 17 cm -3 or 10 ppm contaminations) !! !! Life time killers Poly-Si, silicides Particles cause defects QMQM

SEMICONDUCTOR MANUFACTURING - CLEAN ROOMS, WAFER CLEANING AND GETTERING Modern IC factories employ a three tiered approach to controlling unwanted impurities: 1. clean factories 2. wafer cleaning 3. gettering Contaminants may consist of particles, organic films (photoresist), heavy metals or alkali ions ITRS Front End processes - see class website Up till 2018

Dynamic Random Access Memory Leakage currents discharge the capacitor (mechanism SRH) refresh the charge storage (time ~ a few msec) Deep-level traps (Cu, Fe, Au etc.) Pile up at the surface where the devices are located. Lifetime must be > ~ 25 µsec Use gettering to keep N t  G ≈100µsec write, read V th ~10 7 cm/sec  ~ cm -2

Role of Surface Cleaning in Processing Oxide thickness [Å] Residual contaminants, layers affect kinetics of processes. Surface effects are very important (MORE) in scaled down devices

Level 1 Contamination Reduction: Clean Factories Air quality is measured by the “class” of the facility. (Photo courtesy of Stanford Nanofabrication Facility.) Factory environment is cleaned by: Hepa filters and recirculation for the air, “Bunny suits” for workers. Filtration of chemicals and gases. Manufacturing protocols.

Level 1 Contamination Reduction: Clean Factories Class 1-100,000 mean number of particles, greater than 0.5  m, in a foot of air Particles ---> people, machines, supplies suitsMaterial filtersChemicals, water (use DI) Small particles remain in air (long) coagulate  large ones precipitate quickly and deposit on surfaces by (small) Brownian motion and gravitational sedimentation (larger). Use local clean rooms from Ex. Class 100 -> 5 particles/cm,  >0.1 µm in 1hr.

Level 2 Contamination Reduction: Wafer Cleaning Front End Process Back End Oxygen plasma Organic strippers (do not attack metals) 5 H H 2 O 2 + NH 4 OH SC1 Oxidizes organic films Oxidizes Si and complexes metals 6H 2 O : H 2 O 2 : HCl SC2 Small content reduces Si etch (0.05%) Removes alkali ions & cations Al 3+, Fe 3+, Mg 3+ (insoluble in NH 4 OH - SC1) H 2 S0 4 +H 2 O 2 Oxygen plasma Ultrasonic and now megasonic cleaning for particulates removal (20-50 kHz) Good clean for high T stepsLow T - less critical DI water is necessary: H 2 O H + +OH - [H + ]=[OH - ]=6x cm -3 Diffusivity of:H + ≈9.3x10 -5 cm 2 s -1 -> µ H+ =qD/kT=3.59cm 2 V -1 s -1 of :OH - ≈5.3x10 -5 cm 2 s -1 -> µ OH- =qD/kT=2.04cm 2 V -1 s -1

Level 2 Contamination Reduction: Wafer Cleaning RCA clean is “standard process” used to remove organics, heavy metals and alkali ions. Ultrasonic agitation is used to dislodge particles. with all contaminants -> H passivation (or F!) NH 4 OH small -> reduce surface roughness Not removed by SC1 HF dip added to remove oxide

Level 3 Contamination Reduction: Gettering Gettering is used to remove metal ions and alkali ions from device active regions. For the alkali ions, gettering generally uses dielectric layers on the topside (PSG or barrier Si 3 N 4 layers). For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk. Backside = extrinsic gettering. Bulk = intrinsic gettering.

Gettering Concepts: contaminants freed  diffuse  become trapped Fast Diffusion of Various Impurities Metal contaminants will be trapped by dislocations and SF (decorate) and far away from ICs PSG (for alkali ions Na +, K + and metals) affects E fields (dipoles in PSG) and absorbs water leading to Al corrosion (negative effects) or Si 3 N 4 Closer to devices than to a backside layer -> high efficiency metals

Heavy metal gettering relies on: Metals diffusing very rapidly in silicon. Metals segregating to “trap” sites.

“Trap” sites can be created by SiO 2 precipitates (intrinsic gettering), or by backside damage (extrinsic gettering). In intrinsic gettering, CZ silicon is used and SiO 2 precipitates are formed in the wafer bulk through temperature cycling at the start of the process. SiO 2 precipitates (white dots) in bulk of wafer. Intrinsic Gettering Oxygen ~ cm -3 ; ppm O i >20ppm -> too much precipitation-> strength decreases and warpage increases O i no precipitation-> no gettering denuded zone = oxygen free; thickness several tens of µm µm in size Slow ramp 1-3 nm min size of nuclei, concentrations ≈ cm -3 >> D dopants but D 0 << D metals

Intrinsic Gettering Due to Oxide Precipitates Precipitates (size) high T Density of nucleation sites low T The largest & the most dense defects -> the most efficient gettering

Measurement Methods Clean factories = particle control. Detect concentrations < 10/wafer of particles smaller than 0.1 µm Unpatterened wafers (blank) Count particles in microscope Laser scanning systems -> maps of particles down to ≈ 0.2 µm Patterned wafers Optical system compares a die with a “known good reference” die (adjacent die, chip design - its appearance) Image processing identifies defects (SEM) Test structure (not in high volume manufacturing)

Test Structures Trapped charge Q T  V TH change Dielectric breakdown due to particles, metals etc. Water – measure water resistivity  Deionized Water  =18.5 M  H 2 O  H + + OH - Models relate type of defects (typical for processes) with yields

Monitoring the Wafer Cleaning Efficiency Concentrations of impurities determined by surface analysis Primary beam – e - good lateral resolution Detected beam – e – good depth resolution and surface sensitivity X-raypoor depth resolution and poor surface sensitivity ions (SIMS)excellent ions (RBS)good depth resolution, reasonable sensitivity (0.1 atomic%) works with SEMHe MeV O + or Cs + sputtering and mass analyses Excite Identify (unique atomic signature) Count concentrations emitted

Electrons in Analytical Methods Inelastic collisions with target electrons, which are then emitted from the solid Elastic collision of incoming electrons with atoms (reflected back) ~ the same energy as for the incoming electrons ~ 5 eV (as in SEM)

Analytical Techniques kicked out a core electron This scheme is for lighter elements (Z=33 as is crossover b/w Auger and X- Ray Several keV X-Ray Electron Spectroscopy Electron Microprobe If X-Ray is at the input: el. Emitted= X-ray Photoelectron Spectroscopy (XPS) X-ray emitted= X-ray Fluorescence (XRF) XPS usually more dominant for lighter elements, XRF for heavier Auger El. Spectroscopy The core electron energy levels Several keV

Monitoring of Gettering Through Device Properties and Dielectric p – n leakage, refresh time DRAM junction and dielectric breakdown,  of n-p-n Material properties :  G (>>  R ) in the bulk and on the surface Photoconductive Decay Measurements ∆n=g op  G Carriers are generated due to light Decrease resistivity Recombine emission capture

= = + Carrier Generation Lifetime Deep Depletion - Return to Inversion via Carrier Generation (measure  G ) and surface recombination (s) DL inversion Zerbst technique: if plotted vs. (C min /C)-1 s=f(N ST,  )  =Capture cross section

Lifetime Measurements: Open Circuit Voltage Decay Diode switched from ON V D when carriers recombine Measurements include surface and bulk recombination t=0 ≈0.7V off Use also DLTS: identifies traps (E t ) and concentrations Thermal or photoexcitation processes in voltage modulable space-charge region (Schottky Diode, p-n junction, MOS Capacitor) Measured: capacitance, currents or conductance for t  >4

Excess Carrier Concentrations Decays: minority carriers  x=  tL/t d Experiment to calculate the diffusion constant D p, (n) for minority carriers (  p n ) -> µ p, (n) oscilloscope screen Pulse v d -> µ diffusion Drift: v d =L/t d µ p =v d /E Drift&diffusion Mobility of minority carriers