Instrumentation tools exclusively for thin films

instrumentation

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Properties of thin films like its electrical property, thickness, defects, surface topography, imaging etc are measured and analysed by special instrumentation tools.

Electrical Properties:

Resistance and resistivity
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Four point probe:

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A known value of current (I) is passed between the two outer probes and the potential difference (v) developed across the two inner probes is measured. This approach avoids dealing with the effect of contact resistance. The spacing between probes, s, should be less than the wafer diameter and less than the film thickness. The resistivity of the film sheet is related to the four-point probe current and voltage by:
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Reflectrometry (or Reflection spectroscopy)

Reflectometry is the measurement of reflected light. Spectroscopic reflectometry uses multiple wavelengths (colors) of light to obtain an array of data for analysis of film thickness. Today’s semiconductor manufacturers still depend on spectroscopic reflectometry for most film metrology applications. For film metrology, a wavelength spectrum in the visible region is commonly used.

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Based on the relationship of how light reflects off the top and bottom surface of the film layer, reflectometry can be used to compute the film thickness.

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Requirements:

  1. The layers and the substrate must have a difference in refractive index to get any reflection at the interface at all.
  2. The layers must be transparent or at least semi-transparent in the used spectral range to let the light through. E.g. 100µm thick silicon membrane can be measured very well as silicon itself gets transparent above 900nm wavelength.
  3. The substrate and the layer surface must be smooth enough to get enough directly reflected light back. E.g. layers on standard white paper are not measurable because of the highly diffuse reflection of the paper surface, while e.g. layers on rough aluminium plates can still be measured perfectly

Ellipsometry

  • For thin film thickness and optical properties
  • Nondestructive, noncontact optical film-thickness measurement technique primarily used to measure thin, transparent films

linearly polarized laser light source that, when reflected from the sample, becomes elliptically polarized.

Ellipsometer image

After reflection on a sample surface, a linearly polarized light beam is generally elliptically polarized. The reflected light has phase changes that are different for electric field components polarized parallel § and perpendicular (s) to the plane of incidence. Ellipsometry measures this state of polarization.

Properties of interest for an ellipsometry test:

  • Film thickness

  • Refractive index

  • Surface roughness

  • Interfacial mixing

  • Composition

  • Crystallinity

  • Anisotropy

  • Uniformity.

Scatterometry

  • Refers to the use of scattered light to determine the dimensions (for periodic structures)
  • Wavelength and angle of incidence are variables
  • single wavelength–multi angle scatterometry measurement of critical dimension
  • The intensity maximums of diffracted light occur at angles that depend on the shape and width of the lines making up the grating structure.

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Operation modes

  1. Single wavelength–multi angle scatterometry
  2. multi wavelength–single angle scatterometry measurement

In both methods, intensity of the reflected light is compared to a library of scattering patterns that simulate linewidths and feature shapes.

Advantage:

  • No optical limitation (lens aberration)
  • Non contact (no damage/contamination)

Disadvantage:

  • No imaging possible

Photoacoustic Technique

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This noncontact technology is based on light-induced sound pulses that generate an acoustic pulse that is directed toward the film stack. When the acoustic pulse strikes the surface and underlying film interface, an echo is created that bounces back toward the surface. This echo causes a slight change in reflectivity that is detected at the wafer surface.

The time it takes for the pulse echoes to bounce back is used to calculate the film thickness.

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This technique has a spot size of < 8 μm, which because of its small size, makes it capable of probing structures on patterned wafers.

It can measure film stacks with an individual layer thickness down to < 20 A, which is critical as device scaling requires smaller structures for increased performance.

an acoustic wave is first generated by the thermal expansion caused by sub-picosecond laser light pulse absorbed at the surface of a the metal.

The temperature increase (typically 5–10°C) is a function of sample depth which results in a depth dependent isotropic thermal stress which gives rise to a sound wave which propagates normal to the sample surface into the bulk.

This wave is partially reflected at the interface between the film and substrate.

For an acoustic wave, the reflection coefficient RA depends on:

acoustic impedances Z (Z =density x sound velocity) of the film and substrate materials

When a reflected wave (or “echo”) returns to the free surface after a time t, it causes a small change in the sample optical reflectivity, ∆R. This change is monitored as a function of time by a second laser probe. Based on the sound velocities of the materials making up the sample (which for most materials are known from bulk measurements) and the echo time, the film thickness may be evaluated from the simple relation

Properties of this technique

  • a high precision

  • sampling speed (2 to 4 s/pt)

  • spatial resolution (less than a 10 mm diameter spot size)

  • applicable to single and multilayer metal deposition processes.


Commonly used optical thickness measuring methods are ellipsometry and reflectometry. In ellipsometry, the complex reflection ratio and phase change are measured in a single measurement, and film thickness can be calculated when substrate optical constants are known from independent measurement. In reflectometry, a wavelength scan is made (e.g., 300–800 nm) and this is fitted to a reflection model. For very thin films, uncertainty is introduced because optical constants are not really constants, but depend on film thickness. Xray reflection (XRR) can be used to measure film thickness. Unlike optical methods, XRR is insensitive to refractive index change. Measurement time, however, is in minutes or even hours, compared with seconds for optical tools.


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