1. INSPECTION TECHNIQUES
LECTURE 12 & 13

2. Introduction
The process of determining the physical cause of an IC failure is typically a combination of sample preparation and inspection.
The inspection process for IC failures takes advantage of a number of forms of microscopy.

5. Following decapsulation, internal examination can be achieved by the use of optical microscopes or the Scanning Electron Microscope(SEM)
Optical microscopy has historically played a key role in integrated circuit inspection due to its ease of use and interpretation.
With shrinking feature and defect sizes, came the demand for higher resolution, which has been provided by the Scanning Electron Microscopy(SEM). 

7. Optical Microscopy
Optical microscopy has a number of advantages such as:
(a) Ease of use (no vacuum is required, making sample loading simple)
(b) The images are easily interpreted
(c) Versatility–allows viewing through the transparent thin film dielectrics films used

9. However, optical microscopy has also disadvantage
such as, limited resolution and depth of field at high
magnifications (This has become more significant as feature sizes have decreased and the number of metallization layers has increased).

10. Scanning Electron Microscopy-SEM
(a)Principle of Operation
A Scanning Electron Microscope (SEM) in principle is a microscope generating an electron beam scanning fort hand back over a sample.
Due to the interaction between the beam and the sample, several different signals are produced providing the user with detailed information about the surface structure, or information about the elemental content (SEM attached to EDX-energy dispersive x-ray spectroscopy).

11. Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. This examination can yield the topography, morphology, composition and crystallographic information.
•Topography
–The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties (hardness, reflectivity...etc.)
•Morphology
–The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity...etc.)

12. Composition
–The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties (melting point, reactivity, hardness...etc.)
•Crystallographic Information
–How the atoms are arranged in the object; direct relation between these arrangements and materials properties (conductivity, electrical properties, strength...etc.)

14. When the electrons hit the specimen, several phenomena occur in the SEM.
The 5 most import ones are listed below:
The specimen itself emits secondary electrons
b. Some of the primary electrons are reflected
(backscattered electrons).
c. Electrons are absorbed by the specimen.
d. The specimen emits X-rays.
e. The specimen sometimes emits photons (= light).

15. In SEM imaging, primarily secondary and backscattered electrons are detected.
These signals are collected by detectors to form images of the sample displayed on a cathode ray tube screen.

19. (b) Sample Preparation
To view a sample under SEM, the sample must be electrically conductive in order to get rid of the unused electrons by grounding.
If the sample is not electrically conductive, the electron bombardment will result in charging of the sample (accumulation of electrostatic charge) which in return will affect the image quality.

20. In other words, sample preparation is carried out to prevent charging of the specimen, to facilitate conduction and to increase signal and surface resolution.
All metals are conductive and require no preparation to be viewed using SEM.
In order to view non-conductive samples such as ceramics or plastics, the sample need to be covered with a thin layer of a conductive material.
Typically, gold (Au) and graphite are being used.

21. However, the coating of the specimen could also leads to the issue of artifact introduction, and difficulty to remove the coating layer for further deprocessing.
The impact of this contamination can be reduced
by good vacuum practices and minimizing unnecessary SEM rastering in the areas of interest.

22. (c) Depth of Field
An advantage of SEM over optical microscope is its
depth of field at lower magnification.
This is particularly important in package evaluations
(For example, many evaluations of bonding require a
higher depth of field).

23. SEM could not only provide information about the surface appearance or topography of the sample but also its composition in the form of material contrast
However, it is limited to surface imaging and so requires delayering of films between inspection steps.

27. Scanning Probe Microscopy-SPM
(a)What is SPM? SPM techniques involve moving a very sharp tip over the surface in a raster pattern and measuring displacements of the tip to maintain some parameter constant.

32. (i) Scanning Tunneling Microscope (STM)
The STM has a metal needle that scans a sample by moving back and forth over it, gathering information about the curvature of the surface.
The needle doesn`t touch the sample, but stays about the width of two atoms above it.

33. The STM takes advantage of what`s called the tunnel effect:
If a voltage is applied to the tiny distance between the needle and the sample, electrons are able to tunnel or “jump” between the needle and the sample, creating an electric current.

34. The feedback electronics keep the distance between the needle/ tip and the sample constant, in order to keep the constant tunneling current.
In short, the STM scans a surface while forcing a constant tunneling current to the surface and measuring the displacement of the probe.
STM is generally used with samples that conduct electricity.

36. (ii) Atomic Force Microscopy (AFM)
Like the STM, the AFM uses a probe to scan back and forth over the surface of a sample.
But instead of using an electrical signal, the AFM relies on forces between the atoms on the tip and in the sample.

39. The probe of the AFM is a flexible cantilever – with a tip attached to its under side.
As the tip scans the sample, the force between them is monitored.
A feedback mechanism is employed to adjust the
tip -to-sample distance to maintain a constant force
between the tip and the sample.

41. In addition to gathering information about the topography of a sample, AFM can measure the friction between the tip and the sample, and it can also measure the elasticity, or softness of a sample.
AFM can operate in either:
(a) constant-contact mode
(b) non-contact mode
(c) tapping mode

42. Constant-contact mode
In contact mode, the probe tip actually is in the
repulsive region of the Van der Waals force regime and is touching the surface.
The most common contact mode is the constant-
force mode. The force of the tip on the sample is
maintained at a constant point using feedback from
the deflection of the cantilever.

43. Non-contact mode
In non-contact mode, the height of the tip above the sample is maintained in the attractive region of the Vander Waals force curve.
The probe tip is vibrated, typically on a stiff cantilever, near its resonant frequency (usually KHz) with an amplitude of tens or hundreds of Angstroms.

44. Tapping mode
In tapping mode, the tip intermittently tapping gently on the sample.
This approach works well with soft samples that might be harmed if the tip stayed in contact.

48. AFM vs. SEM AFM has several advantages over the SEM such as:
AFM provides a true 3-D surface profile, unlike a 2-D projection by SEM.
(ii) Sample viewed using AFM does not require special treatment (such as metal/carbon coating) that would irreversibly change or damage the sample.

49. (iii) Does not require expensive vacuum environment for proper operation. (iv) AFM has higher resolution than SEM

50. However, AFM has also its disadvantages such as:
(i) AFM can only image a maximum height on the order of micrometers and a maximum scanning area of around 150 x 150 µm (as compared to SEM which can image an area in an order of mm x mm).
(ii) AFM cannot scan images as fast as an SEM, requiring several minutes for a typical scan, while an SEM is capable of scanning at near real-time (although at relatively low quality).