Module 4 Diffusion and Ion Implantation Part 2- S K Pradhan.pptx

IC technology module 4 class notes

1. Implantation Technology 1

2. A gas is ionized, and the ions are accelerated by a high electric field, and injected into the target wafer to hundreds of nm depth. Ion implantation and its history Typical ion implantation parameters: Io n : P , As, S b, B , In, O Dose : 10 11 - 10 18 cm -2 Ion energy : 1 - 400 keV Uniformity and reproducibility : ± 1% Temperature : room temperature Ion flux : 10 12 -10 14 cm -2 s -1 Dose (  ) = # of atoms/cm 2 . Concentration (C) = # of atoms/cm 3 . The idea was proposed by Shockley in 1954, but used for mass production only after late 1970s. Modern ion implanters were originally developed from particle accelerator technology. Their energy range spans 100eV to several MeV (a few nm’s to several microns in depth range ). The implantation is always followed by a thermal activation (600-1100 o C). 2

3. Advantages of ion implantation Very precise dose control . The ion implanter forms a simple electrical circuit. By monitoring the current in the circuit (or by a monitoring circuit with Faraday cups), significant accuracy in the implanted dose can be maintained. Assuming a current sensitivity of nA, and a minimum required implantation time of 10 seconds, it can be shown that doses as low as 10 11 cm -2 , can be measured. On the contrary, in chemical source predeposits, dose values less than 5  10 13 /cm 2 are not achievable. High dose introduction is not limited to solid solubility limit values. Excellent doping uniformity is achieved across the wafer (< 1% variation across 12” wafer) and from wafer to wafer. Less dopant lateral diffusion , good for small device (short channel). Done in high vacuum , it is a very clean process step. Besides precise dose control, one can also control the profile (peak depth and spread range) better than diffusion (peak concentration always near surface). (top) Doping by diffusion and “drive-in”. (bottom) Doping by ion implantation with or without “drive-in”.

4. Advantages/disadvantage of ion implantation 4 Advantage: Low-temperature process (can use photoresist as mask) Wide selection of masking materials, e.g. photoresist, oxide, poly-Si, metal Less sensitive to surface cleaning procedures. Very fast (6" wafer can take as little as 6 seconds for a moderate dose) Complex profiles can be achieved by multi-energy implants. Disadvantage: Very expensive equipment ( $1M or more). At high dose values, throughput is less than diffusion (chemical source pre- deposition on surface). Ions damage the semiconductor lattice. Not all the damage can be corrected by annealing. Very shallow and very deep doping are difficult or impossible. Masking materials can be “knocked” into the wafer creating unwanted impurities, or even destroying the quality of the interface.

5. Application of ion implantation in CMOS fabrication 5 9-10 different implantations!

6. SIMOX (Separation by IMplantation of OXygen) for SOI wafer SOI wafers provide better performance for high speed circuits than conventional wafers. SOI wafer by SIMOX has better yield, but more expensive than wafer bonding and CMP process. (CMP = chemical mechanical polishing) SIMOX depends on a peaked implant profile. Sharp interface are formed during annealing. 400nm buried oxide requires a high oxygen dose of 2  10 18 /cm 2 , which is a slow process. 6 SOI wafer = silicon on insulator

7. Implantation equipment IBS research implanter 7

8. Implantation equipment 8

9. Variable extraction voltage (typically  30KV ) Positive ions are attracted to the exit side of the source chamber, which is biased at a large negative potential with respect to the filament. Plasma ion source and ion extraction 9 Filament emits electrons, which are accelerated to gain enough energy. The electrons collide with the molecules or atoms, and ionize them. The ions are extracted, rough-focused, then travel toward the magnetic analyzer. ( e xt r ac t ion)

10. Schematic of an ion implanter to produce uniform implantation of desired dose. The beam is bended to prevent the neutral particles from hitting the target . 10 Ion source: operates at a high voltage(25kV) and convert the electrically neutral dopant atoms in the gas phase into plasma ions and undesired species. Some sources: Arsine, Phosphine, Diborane, … Solid can be sputtered in special ion sources. Mass spectrometer: a magnet bend the ion beam through right angle, and select the desired impurity ion and purge undesired species. Selected ion passes through an aperture. Accelerator: add energy to beam up to 5MeV. (contained, to shield possible x-ray). Scanning system: x and y axis deflection plates are used to scan the beam across the wafer

11. Practical implantation dosimetry (dose measurement) The implant dose  is the number of ions implanted per unit area (cm 2 ) of the wafer. If a beam current I is scanned for a time t , the total implanted charge Q = ( I x t ). 11

12. Gaussian distribution for first order approximation . R p = projected range , is a function of ion energy and mass, and atomic number of impurity as well as target material.  R p = straggle = standard deviation . C p = peak concentration at x=R p . Dose Q=  C(x)dx=(2  ) 1/2 C p  R p . Dopant (impurity) concentration profile The impurity is shown implanted completely below the wafer surface (x=0). 12 * * Use C for concentration instead of N used here.

13. Projection range (depth) and straggle (standard deviation) M i and M t are the masses for incident and target ion. 13

14. Example 14

15. Junction depth in Si Junction formation by impurity implantation. Two pn junctions are formed at x j1 and x j2 .  B 15 p p j p B p B p N  x   N e N N p x j  R p   R 2 ln 2  R 2 N N N  x  R  N p 2  R 2 p 2  R 2 ln 2  B  e N p  x j  R p  2   Implant into Si already doped at N B . E.g. implant P into B-doped Si. P: n-type doping; B: p-type.  x j  R p  2

16. Example calculations 16

17. In many application doping profiles other than the simple Gaussian are required. Composite doping profile using multiples implants. 17

18.      4 Dt ex p    C  x , t   x 2 2  Dt Q C  x            2  ex p    p 2  R p     x  R Q 2   R p 2 ) 2 p  D t (  R C  x , t        Q P P 2   R  2 D t   exp  2  x  R P  2 2 2    R  2 D t  D t  D t ＋ Dt Diffusion during subsequent anneals During high temperature steps after implant (most commonly an activation anneal), the implanted impurities will begin to diffuse, broadening the implantation profile. For implantations far away from the surface and for reasonable short characteristic diffusion lengths, the new profile can be approximated by: 18

19. Light ions such as B experience a relatively large amount of backward scattering and fill in the distribution on the front side of the peak. Heavy atoms such as antimony, experience a large amount of forward scattering and tend to fill in the profile on the substrate side of the peak. This asymmetry is usually expressed in terms of the skewness moment. The different skewness can be visualized by thinking of forward momentum. A more accurate distribution can be obtained by including kurtosis distribution. A number of model has been proposed to explain this behavior. The most common one is known as Pearson type – IV (complicated). Real impurity distribution Boron Implanted into Silicon 19

20. 20 Channeling

21. 110 111 100 Random tilt and rotation Looking at Si at different orientations 21

22. Occur when ion velocity is parallel to a major crystal orientation. Some ions may travel considerable distances with little energy loss. Once in a channel, ion will continue in that direction, making many glancing internal collisions that are nearly elastic (their stopping is then dominated by electronic drag only), until it comes to rest or finally dechannels. The latter may be result of a crystal defect or impurity. Channeling effect S i S i S i Si S i S i S i Si Si Si Si Si Si Si Si Si 22

23. Critical Angle: defined as the maximum angle between the ion and the channel for a glancing collision to occur. Where Z 1 is the incident ion atomic number, Z 2 is the target atom atomic number, E is the acceleration energy in keV (voltage), and d is the atomic spacing in the direction of the ion path in angstroms. Note: channeling is more likely for heavy ions and lower energies. But another source says the opposite: The effect is particularly pronounced when implanting light atoms on axis into a heavy matrix sine the ion’s atomic radius is much less than the crystal spacing. Channeling effect: critical angle 23

24. Phosphorus impurity profiles for 40keV ion implantations to silicon at various angles from the <110> axis. Impurity distribution due to channeling effect Even (the complicated) Pearson profile fails in the case of crystalline silicon where ion channeling may occur. The resultant profile can be described by a “Dual-Pearson” distribution. long “tail” 24

25. A thin screen oxide which is amorphous is often used, causing some randomization of incident beam before it enters the lattice. Most IC implantation is done off axis. A typical tilt angle is 7 o . Methods to reduce channeling effect 25

26. Destroy the lattice before implantation Methods to reduce channeling effect High dose Si + implantation to convert the surface layer into amorphous Si. Implantation of desired dopant 26

27. Example (channeling) 27

28. 28 Lattice Damage and Annealing

29. EOR damage Damage at end of range (EOR) The main disadvantage of ion implantation is the production of lattice damage (vacancies and interstitials, or V/I) which may evolve from simple point defects into complex dislocations or voids. Eventually, implantation leads to an amorphous silicon structure (can be used for channeling reduction). Most damage can be repaired by annealing. However, annealing cycles of 30 min at close to 29 1000 degree C can cause considerable spreading of the implant by diffusion.

30. (Si) Si  Si I + Si V Damage to the lattice: target atom displacement Energetic incident ions collide with target atoms, leading to their displacement. The result is an interstitial atom and a vacancy, V-I pair = Frankel defect. The displaced atoms may have energy high enough to further displace other target atoms along its path. I: interstitial; V: vacancy Vacancy 30 Self i nt e r s tit i al

31. Number of displaced target atoms 2 E d 31 An implanted ion can increase the number of recoil atoms only if it possesses an energy greater than 2E d , where E d is the minimum energy required to break four covalent bonds and dislodge a lattice atom. E d is called threshold energy or displacement energy (for Si, E d  15eV). When the energy of the incident ion or secondary knocked-on atom reach E d , they can be considered stopped, because if they do damage to transfer all their energy to a lattice atom, they can cause a single displacement but remain at rest in the lattice position themselves. Thus the number of displaced atoms created by an energetic particle can be estimated by N=E n /2E d , where E n is the energy lost in nuclear collision. For example, 30keV As ion will lead to roughly 30000/(2  15)=1000 displaced atoms. The number will be less for 30keV light ions, whose energy is mainly lost by electronic stopping. N ( E )  E

32. If the dose is high enough, the implanted layer will become amorphous. The dose required to produce an amorphous silicon layer is called critical implant dose. The heavier the impurity, the lower the dose that is required to create an amorphous layer. Amorphization Critical dose 32

33. Damage distribution More crystalline damage at end of range, S n > S e Less crystalline damage, S n < S e Most damage is done by nuclear interactions (nuclear stopping) 33

34. Damage distribution 34 For light ion (lighter than target), small energy transfer to target atom for each collision, generate few displaced target atoms, and ion scatted at large angle. Low density non-overlapping damage, but over large area with a saw-tooth shape. For heavy ion , large energy transfer for each collision, small scatter angle. The displaced atom can further displace other target atoms. Small range, large damage density over small volume.

35. Damage density distribution 35

36. As ion Si atom Annealing repair damage and activate dopants After implantation, we need an annealing step, usually under Ar, N 2 or vacuum. A typical  900 o C, 30min will: Restore silicon lattice to its perfect crystalline state - silicon atoms can move back into lattice sites at these temperatures. Put dopants into Si substitution sites for electrical activation - nearly all of the implanted dose becomes electrically active except for impurity concentrations exceeding 10 19 /cm 3 . Restore the electron and hole mobility – now that the lattice becomes perfect again. 36

37. 37 Mathematical Model for Ion Implantation

38. 38 Selective Implantation

39. Moments description 39

40. Ion implantation: Pearson IV profile Measured boron impurity distributions compared with four moments (i.e. Pearson IV) distribution functions. The boron was implanted into amorphous silicon without annealing. Very good “curve fitting”. 40