DI Reduction in Rinse

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DI Water Reduction in Rinse Processes: 

DI Water Reduction in Rinse Processes Contributions by: Thomas W. Peterson, University of Arizona Andy Hebda, University of Arizona Thomas Roche, Motorola Corp. Eric Hansen, Santa Clara Plastics  1999 Arizona Board of Regents for The University of Arizona

Overview: 

Overview DI water use during rinsing: ~ 103 gallons/wafer ==> ~107 - 108 gallons/year (SEMATECH) Minimal recycle and reuse across the industry Primary objective: CLEAN WAFERS

Indicators of “Clean Wafers”: 

Indicators of “Clean Wafers” ON-LINE Resistivity in the Rinse Tank OFF-LINE Particle Counts on Wafer Surface Elemental Analysis of Wafer Surface Device Characteristics Testing

Terminology: 

Terminology Resistivity Primary measurement of ionic concentration of rinse water. The higher the ion concentration, the lower the resistivity. [=] MW - cm Conductivity The inverse of resistivity. For dilute solutions, conductivity is proportional to ionic concentration [=] 1/ (MW - cm)

Terminology (cont’d): 

Terminology (cont’d) Integrated Conductivity The conductivity in the rinse water integrated over the time period required to “recover” from the chemical burden introduced to the system by the wafers Carryover Amount (moles or grams) of chemical “carried over” with the wafers and the boat from chemistry tank to first rinse tank first rinse tank to second rinse tank

“Typical” DI Water Resistivity (MW - cm): 

“Typical” DI Water Resistivity (MW - cm) Clean water, high flow conditions (greater than 11 gpm): 17 - 18+ Clean water, low flow conditions: 8-17 or lower Rinse bath containing wafers, immediately following chemistry: <<0.01 (mdl for monitor)

Resistivity, Conductivity and Ionic Concentration: 

Resistivity, Conductivity and Ionic Concentration

Integrated Conductivity and “Carryover”: 

Integrated Conductivity and “Carryover”

Resistivity is the Primary Measurement: 

Resistivity is the Primary Measurement

K(t) = 1/ R(t): 

K(t) = 1/ R(t)

Slide11: 

Resistivity and Conductivity

Integrated Conductivity: 

Integrated Conductivity

Slide13: 

Continuous Stirred Tank Model

Slide14: 

CSTR Characteristics

Slide15: 

Evidence of CSTR

Slide16: 

Evidence of CSTR (cont’d)

CSTR vs. PFR Model: 

CSTR vs. PFR Model For a theoretically pulsed input, the response of a PFR reactor is significantly smaller than that of a CSTR reactor. Input Response 0 PFR CSTR

Hypothesis: Wafers can be adequately “cleaned and rinsed” with less water: 

Hypothesis: Wafers can be adequately “cleaned and rinsed” with less water Select the methods of reduction Establish “base-line” water requirements for current processes Alter water usage in rinse bath by various means Examine “Indicators” of wafer cleanliness

Methods for DI Reduction: 

Methods for DI Reduction Pulsed rinse baths HF chemistry Piranha (H2SO4/H2O2) chemistry Decrease in bath volume

Slide20: 

Example Rinse Sequence: SCP 9400 tool Motorola MOS 12 Piranha Super Q Rinse Rinse Rinse Rinse Rinse HF SC1 IPA DRYER

Pulsing Experiments HF Chemistry: 

Pulsing Experiments HF Chemistry 2 minutes in 10:1 HF 6 minute overflow “first rinse” @ 15 gpm 2 minute overflow “second rinse” @ 15gpm** ** Other chemistry steps with “first rinse” baths could precede this step.

Experimental Variations HF Chemistry: 

Experimental Variations HF Chemistry “First Rinse” time: 4, 5, 6 minutes “First Rinse” flow rate: 8 - 15 gpm “Pulsed” flow conditions Period: 10 seconds to 2 minutes High Flow Time: 1 second to 60 seconds Wafer loading: 1 to 50 wafers

Chemical Carryover Results HF Chemistry: 

Chemical Carryover Results HF Chemistry Pulsed flow exhibits an improvement over conventional flow in terms of the volume of water employed to obtain the same level of wafer cleanliness.

Pulsing Experiments Piranha Chemistry: 

Pulsing Experiments Piranha Chemistry 10 minutes in 7.5:1 H2SO4 / H2O2 at 120 °C 5 “quick-dump rinses (“first rinse”) 3 hot water, 2 cold water High flow for ~1 minute, ~5 second drain, ~30 second refill Flow rate ~ 15 gpm 2 minute overflow “second rinse” @ 15 gpm** ** Other chemistry steps with “first rinse” baths could precede this step.

Experimental Variations Piranha Chemistry: 

Experimental Variations Piranha Chemistry ~ 50 experiments conducted “First Rinse” utilized both quick dump and overflow modes “First Rinse” time 3 - 9 minutes “First Rinse” flow rate 8 - 15 gpm “Pulsed” Flow conditions period: 10 seconds to 4 minutes high flow time: 2 seconds to 2 minutes Hot and Cold Rinses “Agitation” (using robot arm)

Effect of Pulsing and Quick-Dump Rinsing: 

Effect of Pulsing and Quick-Dump Rinsing

Effect of Pulse “Sequence”: 

Effect of Pulse “Sequence” “’On’ first” provides a marginally more effective rinse due to the occurrence of convection before diffusion.

Effect of Temperature and Agitation: 

Effect of Temperature and Agitation Hot rinses and cold rinses with agitation reduce the amount of carryover associated with a standard cold rinse. This is possibly due to an increased rate at which chemical contaminants are swept from the wafer surface.

Comparison: Carryover to Surface Loading Piranha Chemistry: 

Comparison: Carryover to Surface Loading Piranha Chemistry “ Clean” Carryover: 0.01 mg H2SO4 ==> 10-7 moles Surface Measurements by XRF: 1013 molecules S/cm2 For 50 200mm wafers ==> 10-7 moles

Slide30: 

Rinse Bath Volume Experiments Bath Comparison

Slide31: 

Comparison of Rinse Tank Performance HF Chemistry, Bare Wafers The lower volume tank exhibits an improvement over the standard tank in rinsing bare Si wafers.

Slide32: 

Comparison of Rinse Tank Performance HF Chemistry, Oxide Wafers For oxide wafers, no significant difference is seen between the standard and Dynaflow tanks.

Slide33: 

Summary Under certain conditions, pulsing has a pronounced effect on rinse efficiency. Combining pulsing sparingly with quick-dumps (in those processes which allow for quick-dumps) can further increase rinse efficiency. A substantial portion of the time during rinsing is spent “cleaning up the water” rather than cleaning up the wafer surface. Moving the characteristic operating conditions of the rinse tank closer to Plug flow conditions can potentially reduce water usage over that required for CSTR conditions. A new rinse bath shape using less water volume can decrease water usage while achieving an effective rinse for bare silicon wafers.

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