Neutral Molecules

• • • 10 E¹⁶ per cm³
    • Do not contribute to the etching process.

Radicals

• • • 10 E¹⁴ per cm³
      
    • Uncharged atoms with unsatisfied chemical bonds that react with the surface to produce volatile products that are pumped away.
      
    • Form more easily and their lifetime is much longer, resulting in a million times more radicals than ions or electrons.

Electrons

• • • 10 E⁸ per cm³
    • Electrons are low mass (9.1 x10^-31 kg) and are rapidly accelerated toward the positively charged anode, on the way they collide into other gas molecules creating new species.

Positive and Negative Ions

• • • 10 E⁸ per cm³
    • Fast moving, heavy molecules that affect the process through energetic (physical) bombardment of the surface.
      
    • Positive ions are much heavier than electrons (Argon mass 6.6 x10^-26 kg), and are accelerated toward the negatively charged cathode.
    • Negative ions are short lived and therefore rare in plasma.

RF Plasma

• • AC voltage overcomes the problem of charge which accumulates on an insulator in the DC system.
    
  • The positive charge which accumulates due to ion bombardment during one half of the AC cycle can be neutralized by electron bombardment during the next half cycle.
  • At frequencies > 100kHz electrons respond and ions do not, ensuring the half period will be shorter than the charge up time of the insulator.

  • Although there are a number of differences in the practical operation of AC plasma, the principles of DC glow discharges can be applied to AC. One simply considers the AC as a rapidly reversing DC plasma.

Reactive Etching

• • • Isotropic process
    • High selectivity
    • Low etch rate

Ion Etching

• • • Anisotropic process
    • Low selectivity
    • Low etch rate

Reactive Ion Etching

• • • Anisotropic process
    • Variable selectivity
    • High etch rate (on the order of 5x faster than either reactive etching or ion etching alone)
      • Proposed Mechanism 1 - Chemically enhanced sputtering due to fluorinated Si surface versus pure Si surface
      • Proposed Mechanism 2 - Lattice damage induced by ion bombardment causes increased chemical reaction rate.
      • Proposed Mechanism 3 - Volatile species are ejected from the surface due to ion bombardment increasing reaction rate.

Capacitive RIE Etch Chamber

• • • Low density plasma
      • n_e≈10⁹ electrons/cm³ 

    • Ionization efficiency ≈ 10^-7

    • RF Power Source (Typ. 13.56 MHz)

Inductive RIE Etch Chamber

• • • High density plasma
      
    • n_e≈10¹³ electrons/cm³ 

    • Ionization efficiency ≈10^-3

    • RF Power Source (Typ. 13.56 MHz)
      

• • • A=(Z-X)/Z

• • • X = 0
    • A = 1

• • • X < Z
    • 0 < A < 1

• • • X = Z
    • A = 0

New chamber walls conditioning and cleaning strategies to improve the stability of plasma processes

Changes in the chamber wall conditions (e.g. chemical composition) are identified as being one important cause of process drifts such as changes in etch rates, etching profiles, etching selectivity or etching uniformity across the wafer. As the reactor wall conditions are modified due to the deposition or removal of etch products, the plasma chemistry changes, which in turn modifies the process performances. Controlling the reactor wall conditions and their stability is of primary concern when trying to create repeatable processes.

In the plot to the right, the first column (a) represents a new, unaltered, Al₂O₃ chamber wall. In the second column (b), we see the effect of Chlorine etching. There is a clear build up of Silicon Oxychloride layers, as well as, Cl+Br bonded to the wall. The third column (c) represents an 80s fluorine etch following the chlorine etch. We see that the Silicon Oxychloride film and Cl+Br bonds have been removed and we are left with a thin fluorinated aluminum layer. Column (d) represents subsequent fluorine etches totaling 20 minutes of plasma exposure. We see that the fluorinated aluminum layer is building but surface chemistry has not changed.

This will lead to several issues including flake off of AlxFy particles on the wafer and process drifts (due both to the progressive growth of AlF material on the SiO2 windows and to the release of F atoms from the chamber walls during the etching process). 

Plasma chemistry can be used to control the conditioning of the chamber. Transitions from chlorine to fluorine and visa versa require different approaches, see below for details.

Proposed Mechanism for Effects

The transient effects of oxychloride film buildup persist until it has achieved a steady state creation/loss ratio. At this point the Chlorine etch conditions become constant. In the paper referenced, it is suggested that transient effects are eliminated after approximately 300 seconds of Chlorine etching.

The chamber seasoning alters recombination rates at the walls, which affects both the neutral species density and the plasma density as shown in the plot to the right. The plot shows that SiCl₄ is low and increasing at the start of first Chlorine etch after a Fluorine etching. SiCl₄ is the primary etch product of Si etched in Cl₂ chemistry and is a good indicator of real time etch rate.

Recommended Chlorine Etch Chamber Preparation

In our shared facility, it is often difficult to know exactly what has happened in the chamber before you arrived to use the tool. In addition, most users are more concerned with final depth of their etch rather than efficiency of time usage. For these reasons, it is best to return the chamber to a known condition before you begin your processing. Since the transient effects discussed above are consistent and predictable, we can generate a recipe that will bring the chamber to a stable condition before Chlorine etching.

Before beginning your Chlorine etch, it is best to strip the oxychloride film from the chamber walls and bring the chamber walls back to a known starting point. Do this by running the clean and coat recipe available at the tool.

Once returned to the known starting point, you may proceed with your Chlorine etch as usual.

Example Recipe for Panasonic E620 (other tool recipes may vary from this but purpose is the same)

Observed Effects

During Chlorine plasma etches a thin film of silicon oxychloride grows on the chamber walls. Fluorine etch chemistry will strip away the thin oxychloride film from chamber walls resulting in significant transient effects in plasma density and real-time etch rate when transitioning back to fluorine etch chemistry. The root cause of these transients is assumed to be a result of dynamic oxychloride layer stripping processes at the chamber walls and subsequent build up of Al-F layers.

The oxychloride stripping effects of fluorine etch plasma are short lived and typically last less than a minute. The subsequent build up of Al-F layers can take a few minutes to stabilize.

Proposed Mechanism for Effects

The transient effects of oxychloride film stripping persist until the film has been removed. At this point the fluorine etch conditions become constant. In the paper referenced, it is suggested that transient effects are eliminated after approximately 30 seconds of fluorine etching.

The chamber seasoning alters recombination rates at the walls, which affects both the neutral species density and the plasma density as shown in the plot to the right. The plot shows that SiCl₄ is low and increasing at the start of first Chlorine etch after a Fluorine etching. SiCl₄ is the primary etch product of Si etched in Cl₂ chemistry and indicates low etch rate due to plasma losses.

In our shared facility, it is often difficult to know exactly what has happened in the chamber before you arrived to use the tool. In addition, most users are more concerned with final depth of their etch rather than efficiency of time usage. For these reasons, it is best to return the chamber to a known condition before you begin your processing. Since the transient effects discussed above are consistent and predictable, we can generate a recipe that will bring the chamber to a stable condition before fluorine etching.

Before beginning your fluorine etch, it is best to strip the oxychloride film from the chamber walls and bring the chamber walls back to a known starting point. Do this by running the chamber clean recipe available at the tool.

Once returned to the known starting point, you may proceed with your fluorine etch as usual.

Example Recipe for Panasonic E620 (other tool recipes may vary from this but purpose is the same)