Cleaning in Semiconductor Processing Technology PDF

1 Cleaning Courtesy: Prof. Chad Young Semiconductor Processing Technology Prof. Walter Hu ...

1 Cleaning Courtesy: Prof. Chad Young Semiconductor Processing Technology Prof. Walter Hu  2 Cleaning is the name of the game!  Modern IC manufacturing is a battle against contaminants  Clean Room (filtered air, machines designed to minimize particle production, ultra-pure chemicals/gases) UTD TI https://newsroom.intel.com/video-archive/video-intel-clean-room-b-roll/#gs.dp8g0l  Clean wafers (remove particles, organics and metals from wafer surface)  Gettering (collect unwanted impurities into noncritical part of wafer) Semiconductor Processing Technology  3 Clean Room (CR)  "Class" of CR https://www.controlledair.com/cleanrooms-chambers/  Defined by number of 0.5 um or larger particles in certain volume (ft3) of air.  UTD CR class 10,000 (in reality, < class 1,000)  Intel CR class 10 (most area, < class 1)  Particles 10 nm ~ 10 μm suspend in air  Sources of particles? www.prudentialuniforms.com  Humans (emit ~10 million particles per min.)  Robots handle wafers in IC fab.  Equipment (CR specific dust reduction equipment used)  Materials (ultra-pure materials used)  Air filtration using HEPA filter  DI (de-ionized) water rs-ness.com Semiconductor Processing Technology  4 Wafer cleaning  Why is wafer cleaning important?  Vth affected by trace level of Na+, K+ in the gate oxide 2𝜀 𝑞𝑁 2𝜙 𝑞𝑄 𝑉 =𝑉 + 2𝜙 + + 𝐶 𝐶  e.g., for 10 nm oxide, 6.5 x 1011 ions/cm2 (~10 ppm) shifts Vth by 0.1 V. Was a huge problem back in 1960s.  e.g., DRAM, impurity should be ppb-level, cleaning wafer not enough, gettering used  Wafer cleaning procedures have lasting impact on IC fab. because of residual chemicals  RCA cleaning is a standard cleaning in IC fab. Semiconductor Processing Technology  5 RCA cleaning  The RCA clean is a standard set of wafer cleaning steps which need to be performed before high-temperature processing steps (oxidation, diffusion, CVD) of silicon wafers in semiconductor manufacturing  Werner Kern developed the basic procedure in 1965 while working for RCA, the Radio Corporation of America.  It involves the following chemical processes performed in sequence:  Removal of the organic contaminants (organic clean + particle clean)  Removal of thin oxide layer (oxide strip, optional)  Removal of ionic contamination (ionic clean) Semiconductor Processing Technology  6 RCA cleaning http://studytronics.weebly.com/fabrication-basics.html http://www.ultrat.com/precision-cleaning-technology.html RCA Cleaning removes organics/impurities chemically Ultrasonic cleaning (brushing off) physically remove particles Semiconductor Processing Technology  7 Step 1 & 2, Organic/oxide removal  Photoresist residue is one of the largest sources of organic contamination. There are two main ways to remove organic residue.  Ozone/ashing → dry process that uses very strong oxidizing agents. Essentially you burn the organic residue converting everything to CO2  Second method is a wet chemical process (also known as Piranha Clean)  H2SO4:H2O2 at 120°C  Both processes leave oxide on silicon which must be removed  Oxide on Si  Dissolved O2 in DI water enhances oxide growth (~15Å of oxide in 3 hours)  Clean wafer exposed to CR air will grow ~6.7Å in one week Semiconductor Processing Technology  8 Cleaning model  Metal atoms on the surface of the wafer  Converted into metal ions which are soluble  M → Mz+ + ze- (oxidation) M ← Mz+ + ze- (reduction)  Cu, Ni, Fe are most common  H2O2/H2O solution makes most reaction "oxidation" Oxidant/ Standard Oxidation-Reduction Reaction Reductant Oxidation Potential (volts) Mn2+/Mn 1.05 2  Mn  Mn  2e SiO2/Si 0.84   Si  2H 2 O  SiO2  4H  4e 3  Cr 3 /Cr 0.71 Cr  Cr  3e Ni 2 /Ni 0.25 Ni  Ni2  2e   Fe 3 /Fe 0.17 3 Fe  Fe  3e H 2 SO 4 /H2SO3 -0.20   H 2 O  H2 SO3  H2SO 4  2H  2e 2 -0.34 2  Cu /Cu Cu  Cu  2e O 2 /H2O -1.23 2H2O  O2  4H   2e  Au3  /Au -1.42 3 Au  Au  3e  H2 O2 / H2O -1.77   2H 2O  H2 O2  2H  2e O3 /O2 -2.07 O2  H2O  O3  2H  2e  Semiconductor Processing Technology  9 SC1 & SC2  Sequential cleaning process is based on H2O2 at high and low pH  SC1 (standard cleaning 1)  NH4OH-based high pH solution (H2O2 is a very strong oxidizing agent)  The solution oxidizes the organic films into water soluble compound (CO2, H2O)  The solution also convert metals into water soluble complexes for common metals like Cu, Ag, Au, Ni, Cr and Co  SC2 (standard cleaning 2)  HCl-based low pH solution Primarily used to remove heavy metals that cannot be removed by SC1 Create water soluble complexes of Al, Fe, Mg Remove incompletely removed metals by SC1 (e.g., Au,...)  Chemical reaction follows Arrhenius equation (strongly depending on temp.)  The higher the wafer cleaning solution temp., the faster the cleaning Semiconductor Processing Technology End of the slide set