EMT 480/3: RELIABILITY AND FAILURE ANALYSIS

1. EMT 480/3: RELIABILITY AND FAILURE ANALYSIS by Noraini Othman noraini_othman@unimap.edu.my Edited by Dr Hasnizah Aris Lecture 8: Die Deprocessing

2. Introduction • Deprocessing (also called delayering), as the name implies, is a systematic process of removing the thin film layers of the die after it has been exposed or removed from the package. • It refers to a semiconductor failure analysis technique of removing thin film layers / passivation layers of the die to expose the defect site that is buried underneath these layers.

3. The most common application of parallel delayering is for I.C. delayering, but it can also be used to thin PCBs, compound semiconductors and a variety of other materials.

4. Introduction • The purpose is to provide the analyst more visibility and accessibility to areas below the surface of the die where much of the electrical activity takes place. • Accessibility is typically defined in terms of electrical access to signals for failure site isolation.

5. Introduction • On the other hand, visibility provides access to defects for physical and chemical characterization. • Therefore, deprocessing is a critical step in the failure analysis process since improperly or hastily implemented deprocessing procedures can result in the loss of key pieces of information needed to understand the physical cause of failure.

6. Die delayering is usually done as a sequence of steps, removing a layer or two at a time.   • Since each layer is chemically and physically different from the others, the delayering steps are different from each other as well.   • Deprocessing can be viewed as a reversal of the wafer fab process. Thin film layers are removed sequentially in reverse order of application in the wafer fab (reverse engineering)

7. Example of deprocessing steps

8. Many die delayering techniques exist, e.g., plasma etching, reactive ion etching, wet chemical etching, etc.   • A typical die delayering sequence starts with either plasma or reactive ion etching to remove the nitride passivation on top of the die surface, followed by a series of wet chemical etching steps to remove the rest of the die layers.  

9. The device shown is fabricated using a planarized process with five layers of metallization (shown in blue)

10. Thin-film layer / passivation layer is a non-conducting layer used to prevent physical damage as well as the penetration of moisture and other contaminants • Passivation is usually composed of layers of oxide, nitride, polyimide or some combination thereof

11. Deprocessing can be performed on a die while still in a package or on a die removed from the package with fundamentally the same methods. After removal of top glass layer, the aluminum is accessible (Reproduced from http://www.semitracks.com/reference/FA/die_level/ deprocessing/delayering/delayering.htm)

12. The purpose of deprocessing is to provide the analyst more accessibility and visibility to areas below the surface of the die where much of the electrical activity takes place (a)Accessibility is typically defined in terms of electrical access to signals for failure site isolation (b) Visibility provides access to defects for physical and chemical characterization

13. 1.When a defect has been identified and characterized so that it is known to cause the observed failure, delayering can be used to document the anomaly. 2.Second, sometimes layers such as the passivation need to be removed to allow further electrical characterization. Removal of the passivation allows a DC voltage contrast technique to be used in ant SEM and enables the mechanical probing capacity (which allows for signal injection as well as signal monitoring on exposed conductors)

14. 3.And third, delayering is appropriate when suspected problems exist that are not easily characterized through non-destructive methods. For example, substrate problems such as stacking faults or doping issued often do not create unique signal on the ICs external connections.

16. Orange = Nitride, Blue = Metal, Yellow = Oxide, Green = Poly, Red = Diffusion, and Gray = Substrate

17. Deprocessing or delayering usually implies the removal of an entire layer across an IC. • However, with more sophisticated analysis, it can be useful to perform selective deprocessing to supplement global deprocessing procedures. • Selective deprocessing refers to removal of passivation layer at selected region on the IC and so on • Etch processes may be classified by their rate, selectivity, uniformity, directionality (isotropy or anisotropy), surface quality and reproducibility

18. All etching processes involve three basic events: (i) movement of the etching species to the surface to be etched; (ii) chemical reaction to form a compound that is soluble in the surrounding medium; and (iii) movement of the by-products away from the etched region

19. Many die delayering techniques exist such as : (a) wet chemical etching – inexpensive, good selectivity (b) dry etching (plasma etching) – expensive, bad selectivity (c) mechanical polishing

20. These techniques are not mutually exclusive. • For example, many deprocessing schemes include plasma etch of dielectrics and wet etch of metals. • Similarly, mechanical polishing is commonly combined with chemical deprocessing techniques.

21. In many cases, the same techniques and tools developed for etching materials during wafer fabrication can be adapted for failure analysis deprocessing. • The etchants used in the wafer fab provide, at a minimum, an excellent starting place for development of deprocessing etchants.

22. Type 1. Wet Chemical Etching • Wet chemical etching is the oldest method for IC deprocessing. • It involves the application of liquid solutions to the die surface to remove one or more layers of materials or to highlight defects. • The primary issues with chemical wet etching are selectivity and its isotropic nature.

23. The chemicals used during wet etching depends on the etching selectivity desired (Etch selectivity : selected material is etched at the much higher rate than others). • In many cases, good selectivity is desired so the etchant is selected based on its ability to etch the desired material while minimizing damage to the underlying layer and other critical features.

24. Control of etch rates is primarily accomplished by temperature, time, and concentration of the etchant solution. • Wet chemical etching, when used· for failure analysis, is normally a global etch technique although photoresist masking has been employed to etch out only selective areas. • Without selective masking, wet chemical removal of material occurs across the entire IC.

25. Selectivity of metal etches to most dielectrics is very good and etch-through is frequently halted by barrier/adhesion layers and tungsten plugs in vias, making it less of an issue during deprocessing. • Silicon etching and crystal defect decoration are still commonly performed using wet etches.

26. For example, HF is not selective i.e. it can be used to etch out almost all the layers on the die surface • On the other hand, hydrogen peroxide is highly selective and would etch out only the TiW layer on the die surface

27. Wet etching is generally limited to devices with large feature sizes • Wet chemical etching is superior than dry etching in terms of: (i) effectiveness (ii) simplicity (iii) low cost (iv) low damage to the wafer (v) high throughput

28. However, wet etching produces isotropic etching. Isotropic means that the etching rate is the same in both horizontal and vertical direction

29. The disadvantage of isotropic etching is it produces unwanted undercutting (as shown below) Undercutting

30. As a result of undercutting, narrow metal lines will have the tendency to lift off the surface (lifting off metal interconnect tracks) • Other disadvantage include: - Isotropic etching also leads to etching of materials through holes such as vias, known as “etch through” - It is incapable of patterning submicron features - The needs for disposal of large amounts of corrosive and toxic materials

31. Etch through led by isotropic etching

32. Some results of etching (a) (b) • SEM photo of an IC after wet nitride etch (b) SEM photo after HF dip to remove interlayer dielectric • (Reproduced from http://www.accleratedanalysis.com/NitrideEtch1.html )

33. Some results of etching Nitride etch followed by pyrhana (Reproduced from http://www.accleratedanalysis.com/NitrideEtch2.html )

34. Applications of wet etching

35. Type 2: Dry Etching • The main purpose of developing dry etching is to achieve anisotropic etching (etching rate is different in horizontal and vertical direction)

36. There are 2 different types of dry etching (a) Plasma based (b) Non-plasma based • Uses radio frequency (RF) power to drive the etching Definition: Plasma is a distinct phase of matter, separate from the traditional solids, liquids, and gases. It is a collection of charged particles that respond strongly and collectively to electromagnetic fields, taking the forms of gas-like clouds or ion beams. Since the particles in plasma are electrically charged (generally by being stripped of electrons), it is frequently described as an “ionized gas”

37. Plasma-based etching: (a) Physical Sputtering • Involves momentum transfer. • Based on physical bombardment with ions or atoms

38. Plasma is used to energize a chemically inert projectile so that it moves at high velocity when it strikes the substrate • Momentum is transferred during the collision • Substrate atoms are dislodged if projectile energy exceeds bonding energy • Highly anisotropic • Etch rates for most materials are comparable (i.e. low selectivity) • May result in redeposition

39. (b) Chemical Etching • It uses chemically reactive gases or plasma • Purely chemical etching

40. Plasma is used to produce chemically reactive species (atoms and radicals and ions) from inert molecular gas • Highly selective • Isotropic in nature • An evacuated and pumped chamber is continuously backfilled with a pure gas or mixture of gases

41. The plasma is generated when sufficient energy is applied across two electrodes to form neutral free radicals and charged ions from the gases in the chamber • Etching occurs when radicals strike the sample, reacting with the material on the surface to form volatile by- products that are then pumped out of the chamber • Plasma etching offers the flexibility to choose either highly anisotropic or isotropic etching, enabling a very clean and accurate removal of the layers

42. (c) Reactive Ion Etching (RIE) • Refers to combination of chemical reactions and physical bombardment

43. Reactive Ion Etching (RIE) is similar to plasma etching, except that it involves bombardment of the surface being etched with accelerated reactive ions • These accelerated ions sputter material off the substrate as they hit its surface (achieving material removal by sputtering) , as well as react with the substrate material • Thus, with RIE, etching is accomplished by two processes: sputtering and chemical reaction • RIE can only easily etch certain materials and while the etches tend to be fairly selective, RIE processes act anisotropically

44. Plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions • The ions are accelerated towards, and reacts at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching (Reproduced from http://www.memsnet.org/mems/processes/rieetch.jpg_)

45. Gas Selection: • React with the material to be etched • Result in volatile byproduct with low vapor pressure

46. In addition to sputter-removal, the bombarding ions used in RIE were chosen so that they will chemically react with the material being bombarded to produce highly volatile reaction byproducts that can simply be pumped out of the system. This is the reason why RIE is widely used in wafer fabrication – it achieves the required anisotropy (by means of sputter-removal) and the required selectivity (through chemical reactions).

47. Non-plasma based • Uses spontaneous reaction of appropriate reactive gas mixture • Isotropic etching • Highly selective to masking layers • Highly controllable via temperature and partial pressure of reactants • Particular advantage over RIE is that there is no back sputtering of removed material, which may be very disruptive

48. Dry Etching (Plasma-based) Comparison RIE is best of both world

49. Other benefits of dry etching are: (i) the reduced chemical hazard and waste treatment problems, and (ii) the ease of process automation and tool clustering

50. Anisotropic etching: Etchants: KOH (Potassium hydroxide) Alkali, metal: Na, Cs, Rb Orientation dependent: Miller indices become very important, Etch rates differ for different index planes • Isotropic etching Silicon: Hydrofluoric Nitric Acidic Silicon Nitride: Phosporic Acidic (H3PO4) Not orientation dependent