NTHU ESS5810 Micro System Fabrication and Lab

F. G. Tseng Lec12, Fall/2001, p13

Lecture 12 Process Testing

u  Mask Alignment accuracy and lithography resolution

u  Optical microscopy

u  Line width measurement

u  Resistivity measurement

u  Film thickness measurement

u  Surface profile measurement

u  Mask Alignment accuracy and lithography resolution

1. Mask alignment accuracy

Alignment accuracy: larger than 4 mm

2. Lithography resolution

Lithography resolution:

u  Optical microscopy

1. Basic

Three broad categories for optical measurements:

a. Photometric measurements (amplitude change of reflected or transmitted light)

b. Interference measurements (phase change of reflected or transmitted light)

c. Polarization measurements (ellipticity change of reflected light)

Four main optical techniques (reflection, emission, absorption, transmission):

2. Optical microscopy

Compound microscope

Overall magnification M=Lateral magnification of the objective ´ angular magnification of the ocular

3. Resolution, Magnification, Contrast

Diffraction at a circular aperture of diameter d, the angular position and the first minimum (by Airy in 1834):

a. Resolution (two objects can be distinguished when the central maximum of one coincides with the first minimum of the other, Raleigh)

NA=numerical aperture, expressing the resolving power of the lens and the brightness of the image. However, increasing NA decreases depth of field and shallow working distance.

Most NA in air is around 0.4-0.8, to increase NA:

i. Increasing angle q.

ii. Immersion objective in oil (increasing n)

iii. Using short wave length light (green light, also most sensitive to human eye)

b. Magnification

Magnification above this is empty magnification!!

Usually M»400-500´ with air as the immersion medium.

c. Contrast

i. The ability to distinguish between parts of an object.

ii. Contrast of surface-height variation can be improved by dark-field microscopy—using condenser lens and light impinging on the sample at an angle to see the reflect light from the steps.

iii. Smaller steps (down to 30 Armstrong) can be seen by Differential interference contrast.

u  Line width measurement

Line width usually called critical dimensions (CD); measurement error should be 3-10 times smaller than the process error.

1. Optical method

Usually use optical microscopy, resolution is around 0.5 mm. Optical images are determined by diffraction, aberrations, focus position, the spectral bandwidth of the illumination, numerical aperture.

Diffraction effect

2. Electrical method

a. SEM: resolution 0.005 mm.

u  Resistivity measurement

Resistivity measurement by two probes

Rc: contact resistance and Rsp spread resist can not be accurately determined thus Rs can not be accurately extracted from the measured resistance.

1. Four point probe

Separate current probe from voltage probe to reduce current spread and contact effect by reducing current flow into voltage probe (with large impedance).

Thus, the potential V at distance r from the electrode carrying a current I in a material of resistivity r is:

For probes resting on a semi-infinite medium with current entering probe 1 and leaving probe 4, the voltage V between any two point on the same line become:

If s=s1=s2=s3,

Optimum probe spacing is on the order of 0.5-1.5mm, and varies with sample diameter and sample thickness.

For arbitrarily shaped sample, the above equation need to be modified by thickness effect, edge effect, and probe placement effects:

i. For thin film, t<s/2

ii. For sample diameter d>40s, edge effects become negligible

iii. For 3 to 4 probe spacing or more to wafer boundary, the probe direction will not effect measurement.

u  Film thickness measurement

1. Reflectivity measurement (Nanospec)

For insulating layers and epitaxial semiconductor films:

i: complete cycle from l0 to lI

For thinner film (<2000 A), it is difficult to find the first minimum, thus less accurate—using ellipsometry instead.

2. Ellipsometry

i. Measure the thickness of thin dielectric films on highly absorbing substrates but also be used to determine the optical constants of films or substrates. It allows thickness measurements at least an order of magnitude smaller than interferometric methods. (10 A possible)

ii. Ellipsometer is based on measuring the state of polarization of polarized light. (when light is reflected from a single surface, the amplitude is reduced and phase is shifted, which is related to index of the material)

To measure film thickness (cyclical nature of thickness measurement):

u  Surface profile measurement (Surface profiler)

Reference:

1. Dieter K. Schroder, “Semiconductor Material and Device Characterization”, John Wiley & Sons, Inc., chapter 1 and 9, 1990.

2. Dektak3ST surface profile measuring system manual, VeeCO Metrology Group.