Modelling Pumpdown of Vacuum Systems

1
Modelling Pumpdown of Vacuum Systems

• M.A.Galtry
• IoP Vacuum Group Meeting 26/10/05

2
Introduction

• The building blocks of a pumpdown modeller
• Steady models
• Dynamic models
• Gas species
• Temperature effects

3
The Building Blocks

• Chamber Details
• Volume
• Outgassing properties
• Often badly specified, but can be crucial
• Pipework
• Pipework dimensions
• Conductance modelling algorithms
• Turbulent, Laminar, Transitional, Molecular, and
  Compressibilty are all significant.
• Pump Data
• From simple speed curves to mechanism models
• System Modeller

4
Steady Models

• Calculate the effect of pipework by assuming
  steady states in the system
• Assumes throughput leaving the chamber
  throughput entering the pump
• Assumes falling pressure at the pump
• Steps from pressure to pressure and calculates
  time between steps.

5
Steady Models
Chamber
P2.i
Downstream pressure, P1,i and throughput, Qi,
enables calculation of pipe upstream pressure,
P2,i and pumping speed at the chamber, Si.
Downstream pressure, P0.i, and throughput, Qi,
enables calculation of pipe upstream pressure,
P1,i
P1.i
P0.i
Pump
Step from initial pressure to base pressure in
uniform pressure steps, P0,i. Pump Speed curve
gives throughput Qi
6
Steady Models

• With pressures and pumping speeds at the chamber
  known, the time between the pressure steps can be
  calculated using
• This can be modified to allow for process flows,
  leaks and outgassing.

7
Steady Models

• Steady models are very fast, stable and can be
  very accurate for simple systems.
• Allowances can be made for system features that
  violate the assumptions
• E.g. equalisation with pipework, soft starts
• Not suitable for systems with dynamic pumps
• Pump/Booster slows down at high inlet pressures.
  
• Behaviour strongly dependent on system details,
  no speed curve for the pumpdown available.
• Systems with inherently transient behaviour can
  stretch the fixed throughput assumption too far.

8
Transient Models

• Makes fewer initial assumptions about system
• Calculates pipe flows based on pressures
  throughout the system
• Modifies pressures by conserving throughput over
  a short time step
• Far more versatile in system modelling
• Multiple chambers, multiple pumps
• Time-dependent behaviour
• Allows modelling of dynamic pumps
• Evolution of pump rotational speed can be
  calculated along with evolution of pressures

9
Challenges of Transient Models

• Far slower than steady models
• Tens of thousands of calculations rather than
  hundreds
• May require in-line calculation of pump/booster
  performance rather than reference to a speed
  curve
• Subject to serious stability problems
• The same calculation may involve massive changes
  in pressure over milliseconds (equalisation) and
  very slow changes in pressure (outgassing
  dominated systems)
• Adaptive time-stepping is required
• Stability has to be reliable before model can
  enter mainstream use

10
Transient Models
Chamber
P2
2
P1
1
P0
0
Pump
11
Comparison of the Two Models

• Test Case
• 10 l Chamber
• 2 m of 70 mm diameter foreline
• EPX500 pump
• Steady Model PumpCalc
• Uses speed curve for EPX500
• Transient Model TransCalc
• Models EPX500 rotational speed and performance

Transient model pumps faster initially as pump is
spinning at full speed
12
Gas Species

• Some applications are sensitive to specific gas
  species
• Water vapour in EUV Lithography
• Fluorine in some ALD applications
• Prediction of partial pressures needs analysis of
  diffusion of contaminants through the bulk gas
• Not practical in the general case
• Specific cases have been analysed

13
Temperature Effects

• Previous models are ISOTHERMAL
• Gas in chamber cools rapidly during pumpdown
• Temperatures of -30C have been recorded
• Expansion of gas entering the pump reduces
  effective pumping speed, making predictions
  optimistic
• A feature of modelling very fast pumpdowns, where
  heat transfer into the gas is poor.
• Very small (3 l) single wafer loadlocks
  pumpdown in 3 s
• Very large (2 m³) LCD loadlocks pumpdown in
  30 s
• Requires detailed treatment of heat transfer in
  system
• Under development in BOCE

14
Investigation of Thermal Effects in Pumpdown