Optical endpoint detection on the dry etch tools at DTU Nanolab

Several dry etch tools at DTU Nanolab are equipped with an endpoint detection system. Out of those systems only one is not of the type optical endpoint detection. The instruments are:

ICP Metal Etch
III-V ICP
Pegasus 1
Pegasus 4

The section below describes the principle behind the optical endpoint detection system.

Optical Emission Spectroscopy

The etch process:
As an example, let's take the etching of silicon by fluorine in one of the dry etchers. The fluorine is supplied to the system by a carrier gas, in this case as SF₆, that is fed to the process chamber using mass flow controllers. Driven by the RF generators (both coil and platen) the plasma will decompose the gas in a series of dissociation and ionisation reactions to form fluorine radicals F^*. In the areas on the wafer that are not covered by a mask, the exposed silicon atoms will be attacked aggressively by the fluorine radical to form volatile SiF. As such, the SiF will desorp from the wafer surface and eventually get pumped away.

The plasma:
Inside the process chamber, the RF field will cause electrons to get pulled off the gas atoms to form ions, radicals and free, fast moving electrons. As the electrons collide with the gas constituents, they latter are pushed into excited states from which they may decay under emission of photons. With a set of characteristic These photons typically have an energy in the visible range and hence the plasma emits light at certain wavelengths

The intensity of the light:
The intensity of the light will depend on whether the molecule is a reactant or an etch product:

Reactant: The concentration in the plasma:

- The carrier gas flow rate
- The RF power (both coil and platen)
- The process pressure

Etch product:

Table with important wavelenghts

Monitored species	Wavelength (nm)	Monitored species	Wavelength (nm)
Al	308.2, 309.3, 396.1	In	325.6
AlCl	261.4	N	674.0
As	235.0	N2	315.9, 337.1
C2	516.5	NO	247.9, 288.5, 289.3, 303.5, 304.3, 319.8, 320.7, 337.7, 338.6
CF2	251.9	O	777.2, 844.7
Cl	741.4	OH	281.1, 306.4, 308.9
CN	289.8, 304.2, 387.0	S	469.5
CO	292.5, 302.8, 313.8, 325.3, 482.5, 483.5, 519.8	Si	288.2
F	703.7, 712.8	SiCl	287.1
Ga	417.2	SiF	440.1, 777.0
H	486.1, 656.5