EMT 4803 RELIABILITY FAILURE ANALYSIS Lecture 10 Inspection

EMT 480/3: RELIABILITY & FAILURE ANALYSIS Lecture 10: Inspection Techniques

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

Observability Scales X-ray diffraction analysis Reproduced from http: //web. utk. edu/~prack/MSE%20300/SEM. pdf

Microscopy and its resolution

q Following decapsulation, internal examination can be achieved by the use of optical microscopes or the scanning electron microscope (SEM) q Optical microscopy has historically played a key role in integrated circuit inspection due to its ease of use and interpretation

q With shrinking feature and defect sizes, came the demand for higher resolution, which has been provided by the Scanning Electron Microscopy (SEM). q Ultimately, even SEM resolution has proven inadequate for some applications leading to the expanded use of SPM (Scanning Probe Microscope)

Microscopes

2. Optical Microscopy q (a) (b) (c) (d) (e) Optical microscopy has a number of advantages such as: Ease of use (no vacuum is required, making sample loading simple) The images are easily interpreted Versatility – allows viewing through the transparent thin film dielectrics films used Direct imaging with no need of sample pre-treatment, the only microscopy for real colour imaging. Fast, and adaptable to all kinds of sample systems, from gas, to liquid, and to solid sample systems, in any shapes or geometries.

q q However, optical microscopy has also disadvantages such as, limited resolution and depth of field at high magnifications. Resolution - Optical microscopes have lower resolution. Usually down to only sub-micron or a few hundreds of nanometers, mainly due to the light diffraction limit. Because the refracted light waves are spread out, the resulting image is blurred. Even instruments that provide additional lenses for increasing magnification do very little to improve image resolution. Roughly optical resolution can be estimated as wavelength λ/2 NA (NA is the numerical aperture of lens, usually ~ 1. 0): for white light, average wavelength is around 500 nm, the best resolution is thus a few hundreds nm.

Figure showing two points at the limit of detection

3. Scanning Electron Microscopy-SEM (a) Principle of Operation q A Scanning Electron Microscope (SEM) in principle is a microscope generating an electron beam scanning forth and back over a sample q 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 spectrosocpy)

l l 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 following 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. ) 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. )

Electron-Solid Interaction (Reproduced from http: //web. utk. edu/~prack/MSE%20300/SEM. pdf

q When the electrons hit the specimen, several phenomena occur in the SEM. The 5 most important ones are listed below: a. 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).

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

Schematic representation of SEM

(a) (b) Photo of blood corpuscles taken by means of (a) light-optical microscope; and (b) electron microscope (same magnification)

(b) Sample Preparation q To view a sample under SEM, the sample must be electrically conductive in order to get rid of the unused electrons by grounding. q 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

q In other words, sample preparation is carried out to prevent charging of the specimen, to facilitate conduction and to increase signal and surface resolution q All metals are conductive and preparation to be viewed using SEM q 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. require Typically, gold (Au) and graphite are being used no

q 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 q The impact of this contamination can be reduced by good vacuum practices and minimizing unnecessary SEM rastering in the areas of interest

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

q 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 q However, it is limited to surface imaging and so requires delayering of films between inspection steps

Small screw (about ¼ inch) when viewed using (a) optical microscope; and (b) SEM (Reproduced from http: //www. azom. com)

SEM image showing the defective gate oxide films of transistors

SEM images conducting and semiconducting materials

SEM images of bio-species

TRANSMISSION ELECTRON MICROSCOPY (TEM) q q IN TEM, A TEM works much like a slide projector. A projector shines a beam of light through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide. These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide. TEMs work the same way except that they shine a beam of electrons (like the light) through the specimen(like the slide). Whatever part is transmitted is projected onto a phosphor screen for the user to see. A more technical explanation of a typical TEMs workings is as follows (refer to the diagram below):

l l l l The "Virtual Source" at the top represents the electron gun, producing a stream of monochromatic electrons. This stream is focused to a small, thin, coherent beam by the use of condenser lenses 1 and 2. The first lens(usually controlled by the "spot size knob") largely determines the "spot size"; the general size range of the final spot that strikes the sample. The second lens(usually controlled by the "intensity or brightness knob" actually changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam. The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons (those far from the optic axis, the dotted line down the center) The beam strikes the specimen and parts of it are transmitted This transmitted portion is focused by the objective lens into an image Optional Objective and Selected Area metal apertures can restrict the beam; the Objective aperture enhancing contrast by blocking out high-angle diffracted electrons, the Selected Area aperture enabling the user to examine the periodic diffraction of electrons by ordered arrangements of atoms in the sample The image is passed down the column through the intermediate and projector lenses, being enlarged all the way The image strikes the phosphor image screen and light is generated, allowing the user to see the image. The darker areas of the image represent those areas of the sample that fewer electrons were transmitted through (they are thicker or denser). The lighter areas of the image represent those areas of the sample that more electrons were transmitted through (they are thinner or less dense)

Advantages and disadvantages of TEM

4. 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

What is SPM? o STM (Scanning Tunneling Microscope) o AFM (Atomic Force Microscope) - LFM (Lateral Force Microscope) - MFM (Magnetic Force Microscope) -EFM (Electric Force Microscope)

q A probe is scanned near the sample surface

SPM Principle

(i) Scanning Tunneling Microscope (STM) q 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 q The needle doesn’t touch the sample, but stays about the width of two atoms above it

q 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

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

Basic concept of STM (Reproduced from articles by NISE Networks on “Seeing Atoms”)

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

Principle of AFM observation

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

Basic concept of AFM (Reproduced from articles by NISE Networks on “Seeing Atoms”)

q 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 q AFM can operate in either: (a) constant-contact mode (b) non-contact mode (c) tapping mode

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

Non-contact mode q In non-contact mode, the height of the tip above the sample is maintained in the attractive region of the Van der Waals force curve q The probe tip is vibrated, typically on a stiff cantilever, near its resonant frequency (usually 100400 KHz) with an amplitude of tens or hundreds of Angstroms

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

Surface topography of sputter coated Ti. N layer on Si. C wafer (Reproduced from http: //www. newi. ac. uk/wrighta/Consultancy. html)

Rubber They are the image which observed the special rubber which change phase in normal temperature (left), and the image observed after heating for 5 minutes at 100 degrees C (right). Change of the surface shape by heating is observed well (Using sample heating unit)

Quantum dots on Ga. As substrate

5. AFM vs. SEM q AFM has several advantages over the SEM such as: (i) 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

(iii) Does not require expensive vacuum environment for properation (iv) AFM has higher resolution than SEM

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 m (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)

Revision l a) What is the importance of fault localization in Failure Analysis? l b) Give ONE (1) example of a fault localization technique and describe briefly its operation principle by using relevant illustration.

Q 2 l Metallographic cross-sectioning refers to a FA technique used to expose the internal plane of interest in an object or package for examination. l Describe briefly the steps involved in a cross-sectioning process for an encapsulated sample. l (b) Give TWO (2) purpose of encapsulation in preparing sample for cross-sectioning. l (c)Name THREE (3) molding compound that are commonly used in encapsulating dies for failure analysis. From the three molding compound stated above, which one is widely used in semiconductor applications and why?

Q 3 l a) In die exposure failure analysis technique, the die is usually being exposed through delidding or decapsulation process. What is the difference between both processes? l (b) Chemical decapsulation refers to a technique for opening IC plastic packages. By using relevant illustrations, describe briefly the difference between manual cavity etching and jet etching.

Q 4 l Access to the die is a fundamental requirement for physical failure site isolation techniques. Briefly explain the THREE (3) process steps involved in backside die preparation.

Q 5 l In Scanning Electron Microscope (SEM), the sample preparation involves coating the sample with conductive materials. What is the purpose for this action and what possible inadvertent effect that may arises?