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® Sputtering Eyal Ginsburg WW46/02. ® Sputtering Contents Metallization structure Metallization structure PVD System Overview PVD System Overview Sputtering:

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Presentation on theme: "® Sputtering Eyal Ginsburg WW46/02. ® Sputtering Contents Metallization structure Metallization structure PVD System Overview PVD System Overview Sputtering:"— Presentation transcript:

1 ® Sputtering Eyal Ginsburg WW46/02

2 ® Sputtering Contents Metallization structure Metallization structure PVD System Overview PVD System Overview Sputtering: yield, conditioning, methods Sputtering: yield, conditioning, methods Film nucleation and growth Film nucleation and growth

3 ® Sputtering Contact & Metal Lines - SEM M3 M2 M1 W Plug Via 2 Silicon substrate

4 ® Sputtering Glue Layer (Cont. 1)

5 ® Sputtering Aluminum - General Al-alloys thin films were selected for the first 30 years of the IC industry. Al-alloys thin films were selected for the first 30 years of the IC industry. They continue to be the most widely used materials, although copper. They continue to be the most widely used materials, although copper. Al has low resistivity ( =2.7 -cm), and its compatibility with Si and SiO 2. Al has low resistivity ( =2.7 -cm), and its compatibility with Si and SiO 2. Al forms a thin native oxide (Al 2 O 3 ) on its surface upon exposure to oxygen, and affect the contact resistance. Al forms a thin native oxide (Al 2 O 3 ) on its surface upon exposure to oxygen, and affect the contact resistance.

6 ® Sputtering Aluminum - General (cont.) Al thin films can also suffer from corrosion (ex. Al dry etch may leave chlorine residues on Al surface and lead to formation of HCl and then attack the Al). Al thin films can also suffer from corrosion (ex. Al dry etch may leave chlorine residues on Al surface and lead to formation of HCl and then attack the Al).

7 ® Sputtering Aluminum interconnects The material used in interconnects is not pure aluminum, but an aluminum alloy. Usually with Cu (0.5-2%), sometimes with Si. The material used in interconnects is not pure aluminum, but an aluminum alloy. Usually with Cu (0.5-2%), sometimes with Si. The Cu in Al-alloy slows the electromigration (EM) phenomenon. Si slows EM slightly, used in contact level to prevent spiking. The Cu in Al-alloy slows the electromigration (EM) phenomenon. Si slows EM slightly, used in contact level to prevent spiking. Al-alloys decrease the melting point, increase the resistivity and need to be characterized (ex. Dry etch). Al-alloys decrease the melting point, increase the resistivity and need to be characterized (ex. Dry etch).

8 ® Sputtering Metal line – stack Usually the metal line contains 4-5 layers: Al - This layer makes the contacts with the Tungsten plugs. It is the primary current carrier. Al - This layer makes the contacts with the Tungsten plugs. It is the primary current carrier. TiN Layer - Creates a barrier between the Al/Cu and the Titanium layers because of the increasing temperature at a downstream process will increase the rate of the reaction of Al with Ti. TiN Layer - Creates a barrier between the Al/Cu and the Titanium layers because of the increasing temperature at a downstream process will increase the rate of the reaction of Al with Ti.

9 ® Sputtering Metal stack (Cont. 1) Titanium Layer - Provides an alternate current path (shunt) around flaws in the primary current carrier. And thus improves electromigration characteristics. Titanium Layer - Provides an alternate current path (shunt) around flaws in the primary current carrier. And thus improves electromigration characteristics.

10 ® Sputtering Metal stack (Cont. 2) TiN ARC Layer - This is an anti reflecting coating which aides lithography to keep control of critical dimensions and to absorb light during the resist exposure. It also functions as a hillock suppressant. TiN ARC Layer - This is an anti reflecting coating which aides lithography to keep control of critical dimensions and to absorb light during the resist exposure. It also functions as a hillock suppressant.

11 ® Sputtering Metal stack - SEM ILD Metal line W- Via2 Metal line Al TiN Ti TiN

12 ® PVD System Overview (Endura)

13 ® Sputtering Endura PVD system

14 ® Sputtering Endura standard mainframe

15 ® Sputtering Mainframe Components Preclean Ch. – Applies a light. Non selective plasma etch to the wafer before the PVD process. Preclean Ch. – Applies a light. Non selective plasma etch to the wafer before the PVD process. Cooldown Ch. – Cools the wafer after the PVD process. Cooldown Ch. – Cools the wafer after the PVD process. Expansion Ch. (C&D) – Optionally configured for PVD or other processes such as etch. Expansion Ch. (C&D) – Optionally configured for PVD or other processes such as etch. Wafer orienter/degas Ch. – Orients the wafer flat to a designated angle and degasses the wafer to remove water vapor before the preclean process. Wafer orienter/degas Ch. – Orients the wafer flat to a designated angle and degasses the wafer to remove water vapor before the preclean process. PVD Ch. – DC magnetron sputter deposition chambers for depositing materials used in interconnects metalization (ex. Al, Ti, TiN, TiW). PVD Ch. – DC magnetron sputter deposition chambers for depositing materials used in interconnects metalization (ex. Al, Ti, TiN, TiW). Cassette loadlocks – The starting point for wafer transfers. Accept 1 cassette with 25 wafers. Cassette loadlocks – The starting point for wafer transfers. Accept 1 cassette with 25 wafers.

16 ® Sputtering Vacuum system PVD system uses Ultra-High Vacuum (UHV) to reduce particulates and provide purer film qualities. PVD system uses Ultra-High Vacuum (UHV) to reduce particulates and provide purer film qualities. The tool uses staged vacuum regimes to achieve UHV. The tool uses staged vacuum regimes to achieve UHV.

17 ® Sputtering Pressure regions and vacuum stages

18 ® Sputtering PVD chambers and pumps

19 ® Sputter deposition for ULSI

20 ® Sputtering Sputtering – General Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic particles. Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic particles. Alternative to evaporation. Alternative to evaporation. First discovered in 1852, and developed as a thin film deposition technique by Langmuir in 1920. First discovered in 1852, and developed as a thin film deposition technique by Langmuir in 1920. Metallic films: Al-alloys, Ti, TiW, TiN, Tantalum, Nickel, Cobalt, Gold, etc. Metallic films: Al-alloys, Ti, TiW, TiN, Tantalum, Nickel, Cobalt, Gold, etc.

21 ® Sputtering Reasons for sputtering Use large-area-targets which gives uniform thickness over the wafer. Use large-area-targets which gives uniform thickness over the wafer. Control the thickness by Dep. time and other parameters. Control the thickness by Dep. time and other parameters. Control film properties such as step coverage (negative bias), grain structure (wafer temp), etc. Control film properties such as step coverage (negative bias), grain structure (wafer temp), etc. Sputter-cleaned the surface in vacuum prior to deposition. Sputter-cleaned the surface in vacuum prior to deposition.

22 ® Sputtering Sputtering steps 1. Ions are generated and directed at a target. 2. The ions sputter targets atoms. 3. The ejected atoms are transported to the substrate. 4. Atoms condense and form a thin film.

23 ® Sputtering Sputtering Coating process that involves the transport of material from the target to the wafer. Atoms from the target are ejected as a result of momentum transfer between incident ions and the target. The particles traverse the vacuum chamber and are deposited on the wafer.

24 ® Sputtering Application of Sputtering Thin film deposition: Thin film deposition: – Microelectronics – Decorative coating – Protective coating Etching of targets: Etching of targets: – Microelectronics patterning – Depth profiling microanalysis Surface treatment: Surface treatment: – Hardening – Corrosion treatment

25 ® Sputtering The billiard ball model There is a probability that atom C will be ejected from the surface as a result of the surface being stuck by atom A. There is a probability that atom C will be ejected from the surface as a result of the surface being stuck by atom A. In oblique angle (45º-90º) there is higher probability for sputtering, which occur closer to the surface. In oblique angle (45º-90º) there is higher probability for sputtering, which occur closer to the surface.

26 ® Sputtering Sputter yield Defined as the number of atoms ejected per incident ion. Defined as the number of atoms ejected per incident ion. Typically, range 0.1-3. Typically, range 0.1-3. Determines the deposition rate. Determines the deposition rate. Depends on: Depends on: 1. Target material. 2. Mass of bombarding ions. 3. Energy of the bombarding ions. 4. Direction of incidence of ions (angle). 5. Pressure

27 ® Sputtering 1 2 Sputter yield (Cont. 1) Target materials: Al/Cu(0.5%) Grain size: 45 m Grain size: 200 m

28 ® Sputtering Sputter yield (Cont. 2) Molecule size – need to be about the same size as the sputtered material: Molecule size – need to be about the same size as the sputtered material: – too big cause layer deformation and yield a lot of material. – too small cause layer deformation w/o ejecting atoms. Target deformation = Less uniform dep.

29 ® Sputtering Sputter yield (Cont. 3) Ion energy Vs. sputter yield:

30 ® Sputtering Sputter yield (Cont. 4) Sputter yield peaks at <90º. Sputter yield peaks at <90º. Atoms leave the surface with cosine distribution. Atoms leave the surface with cosine distribution.

31 ® Sputtering Sputter yield (Cont. 5) Pressure reduction – allow better deposited atoms/molecules flux flow towards the substrate. Expressed by Mean free path which is the average distance an atom can move, in one direction without colliding at another atom. Pressure reduction – allow better deposited atoms/molecules flux flow towards the substrate. Expressed by Mean free path which is the average distance an atom can move, in one direction without colliding at another atom.

32 ® Sputtering Process conditions Type of sputtering gas. In purely physical sputtering (as opposed to reactive sputtering) this limits to noble gas, thus Argon is generally the choice. Type of sputtering gas. In purely physical sputtering (as opposed to reactive sputtering) this limits to noble gas, thus Argon is generally the choice. Pressure range: usually 2-3 mTorr (by glow discharge). Pressure range: usually 2-3 mTorr (by glow discharge). Electrical conditions: selected to give a max sputter yield (Dep rate). Electrical conditions: selected to give a max sputter yield (Dep rate).

33 ® Sputtering Sputter deposition film growth Sputtered atoms have velocities of 3-6E5 cm/sec and energy of 10-40 eV. Sputtered atoms have velocities of 3-6E5 cm/sec and energy of 10-40 eV. Desire: many of these atoms deposited upon the substrate. Desire: many of these atoms deposited upon the substrate. Therefore, the spacing is 5-10 mm. Therefore, the spacing is 5-10 mm. The mean free path is usually <5-10 mm. The mean free path is usually <5-10 mm. Thus, sputtered atoms will suffer one or more collision with the sputter gas. Thus, sputtered atoms will suffer one or more collision with the sputter gas.

34 ® Sputtering Sputter dep. film … (Cont. 1) The sputter atoms may therefore: The sputter atoms may therefore: 1. Arrive at surface with reduce energy (1-2 eV). 2. Be backscattered to target/chamber. The sputtering gas pressure can impact on film deposition parameters, such as Dep rate and composition of the film. The sputtering gas pressure can impact on film deposition parameters, such as Dep rate and composition of the film.

35 ® Sputtering Sputtering – additional methods Reactive sputtering Reactive sputtering RF sputtering RF sputtering Magnetron sputtering Magnetron sputtering Collimated sputtering Collimated sputtering Hot sputtering Hot sputtering

36 ® Sputtering Reactive sputtering Reactive gas is introduced into the sputtering chamber in addition to the Argon plasma. Reactive gas is introduced into the sputtering chamber in addition to the Argon plasma. The compound is formed by the elements of that gas combining with the sputter material (Ex. TiN). The compound is formed by the elements of that gas combining with the sputter material (Ex. TiN). The reaction is usually occurs either on the wafer surface or on the target itself. The reaction is usually occurs either on the wafer surface or on the target itself. As you add more reactive gas at some point the reaction rate exceeds the sputtering rate. As you add more reactive gas at some point the reaction rate exceeds the sputtering rate. At this point the target surface switches from clean metal to compound over a short time. At this point the target surface switches from clean metal to compound over a short time.

37 ® Sputtering Reactive sput. (Cont. 1) The transition in target chemistry changes sputtering conditions dramatically ! The transition in target chemistry changes sputtering conditions dramatically !

38 ® Sputtering Reactive sput. (Cont. 2) Typical compounds deposited by reactive sputtering: Target Reactive Gas Compound Al O2O2O2O2 Al 2 O 3 Al N2N2N2N2AlN Ti O2O2O2O2 TiO 2 Ti N2N2N2N2TiN Si N2N2N2N2 Si 3 N 4 Ta O2O2O2O2 Ta 2 O 5 Zn O2O2O2O2ZnO In-Sn O2O2O2O2 In 2 O 3 -SnO 2

39 ® Sputtering RF sputtering DC sputter deposition is not suitable for insulator deposition, because the positive charge on the target surface rejects the ion flux and stop the sputtering process. DC sputter deposition is not suitable for insulator deposition, because the positive charge on the target surface rejects the ion flux and stop the sputtering process. RF voltages can be coupled capacitively through the insulating target to the plasma, so conducting electrodes are not necessary. RF voltages can be coupled capacitively through the insulating target to the plasma, so conducting electrodes are not necessary. The RF frequency is high enough to maintain the plasma discharge. The RF frequency is high enough to maintain the plasma discharge.

40 ® Sputtering RF sputtering (Cont. 1) During the first few complete cycles more electrons than ions are collected at each electrode (high mobility), and cause to negative charge to be buildup on the electrodes. During the first few complete cycles more electrons than ions are collected at each electrode (high mobility), and cause to negative charge to be buildup on the electrodes. Thus, both electrodes maintain a steady- state DC potential that is negative with respect to plasma voltage, V p. Thus, both electrodes maintain a steady- state DC potential that is negative with respect to plasma voltage, V p. A positive V p aids the transport of the slower positive ions and slow down the negative electrodes. A positive V p aids the transport of the slower positive ions and slow down the negative electrodes.

41 ® Sputtering RF sputtering (Cont. 2) The induced negative biasing of the target due to RF powering means that continuous sputtering of the target occurs throughout the RF cycle. The induced negative biasing of the target due to RF powering means that continuous sputtering of the target occurs throughout the RF cycle. But it is also means that this occurs at both electrodes. But it is also means that this occurs at both electrodes.

42 ® Sputtering RF sputtering (Cont. 3) The wafer will be sputtered at the same rate as the target since the voltage drops would be the same at both electrodes for symmetric system. The wafer will be sputtered at the same rate as the target since the voltage drops would be the same at both electrodes for symmetric system. It would thus be very difficult to deposit any material in that way. It would thus be very difficult to deposit any material in that way. Smaller electrode requires a higher RF current density to maintain the same total current as the larger electrode. Smaller electrode requires a higher RF current density to maintain the same total current as the larger electrode.

43 ® Sputtering RF sputtering (Cont. 4) By making the area of the target electrode smaller than the other electrode, the voltage drop at the target electrode will be much greater than at the other electrode. By making the area of the target electrode smaller than the other electrode, the voltage drop at the target electrode will be much greater than at the other electrode. Therefore almost all the sputtering will occur at the target electrode. Therefore almost all the sputtering will occur at the target electrode.

44 ® Sputtering RF sputtering (Cont. 5) We also use RF sputtering to clean out bottoms of Contacts and Vias before the actual deposition. We also use RF sputtering to clean out bottoms of Contacts and Vias before the actual deposition. – Remove native oxides and etch residues from Contacts/Vias. During this step, a controlled thickness of surface material is sputtered off the wafer, removing any contaminants or native oxide. During this step, a controlled thickness of surface material is sputtered off the wafer, removing any contaminants or native oxide. A film can then be sputter deposited immediately afterward without breaking the vacuum. A film can then be sputter deposited immediately afterward without breaking the vacuum. This process was done in the pre-clean chamber. This process was done in the pre-clean chamber. This may also be done by BIAS SPUTTERING (reversing the electrical connections). This may also be done by BIAS SPUTTERING (reversing the electrical connections).

45 ® Sputtering Magnetron sputtering Here magnets are used to increase the percentage of electrons that take part in ionization events, increase probability of electrons striking Ar, increase electron path length, so the ionization efficiency is increased significantly. Here magnets are used to increase the percentage of electrons that take part in ionization events, increase probability of electrons striking Ar, increase electron path length, so the ionization efficiency is increased significantly. Another reasons to use magnets: Another reasons to use magnets: – Lower voltage needed to strike plasma. – Controls uniformity. – Reduce wafer heating from electron bombardment. – Increased deposition rate

46 ® Sputtering Magnetron sputtering (Cont. 1) Lower voltage: Lower voltage: – Magnets produce magnetic field – Magnetic field make an electron go in curved path (helix) – Curved paths are longer more collisions – More collisions make more ions easier to strike plasma. Controls uniformity: Controls uniformity: – Electrons paths are more curved near stronger magnetic field. – More ions collide with target in regions of high magnetic field. – More ion collisions lead to more target atoms sputtering. – More magnets near edge/center makes edge/center thick deposition.

47 ® Sputtering Magnetron sputtering (Cont. 2) A magnetic field is applied at right angle to electric field by placing large magnets behind the target. A magnetic field is applied at right angle to electric field by placing large magnets behind the target. This traps the electrons near the target surface, and causes them to move in spiral motion until the collide with an Ar atom. This traps the electrons near the target surface, and causes them to move in spiral motion until the collide with an Ar atom. Dep rate increases up to 10-100 times faster than without magnetron configuration. Dep rate increases up to 10-100 times faster than without magnetron configuration.

48 ® Sputtering Magnetron sput (Cont. 3) Magnetron sputtering can be done in either DC or RF modes, but the former is more common. Magnetron sputtering can be done in either DC or RF modes, but the former is more common. Target erodes rapidly in the ring region resulting in a deep groove in the target face, which cause to non-uniformity film. Target erodes rapidly in the ring region resulting in a deep groove in the target face, which cause to non-uniformity film.

49 ® Sputtering Collimated sputtering During the PVD process, metal atoms are sputtered at all angles. The standard process deposits metal on all areas of the process kit and at various angles on the wafer. During the PVD process, metal atoms are sputtered at all angles. The standard process deposits metal on all areas of the process kit and at various angles on the wafer. A small range of arrival angles during deposition can cause nonuniform film. A small range of arrival angles during deposition can cause nonuniform film. This leads to poor bottom coverage of small geometry, high aspect ratio contacts and vias as the holes seal off at the top before filling up at the bottom. This leads to poor bottom coverage of small geometry, high aspect ratio contacts and vias as the holes seal off at the top before filling up at the bottom.

50 ® Sputtering Collimated sput. (Cont. 1) One way to improve this by having a narrow range of arrival angles, while atoms arriving perpendicularly to the wafer. One way to improve this by having a narrow range of arrival angles, while atoms arriving perpendicularly to the wafer. This method called collimated sputtering (first proposed in 1992). This method called collimated sputtering (first proposed in 1992). A hexagonal holes plate is placed between the target and the wafer. A hexagonal holes plate is placed between the target and the wafer.

51 ® Sputtering Collimated sput. (Cont. 2) As the sputtered atoms travel through the collimator toward the wafer, only those with nearly normal incidence trajectory will continue to strike the wafer. As the sputtered atoms travel through the collimator toward the wafer, only those with nearly normal incidence trajectory will continue to strike the wafer. The collimator thus acts as a physical filter to low angle sputter atoms. The collimator thus acts as a physical filter to low angle sputter atoms.

52 ® Sputtering Collimated sput. (Cont. 3) 70-90% of atoms are filtered and therefore the Dep rate is significantly reduced. 70-90% of atoms are filtered and therefore the Dep rate is significantly reduced. In addition the collimator should be cleaned and replaced, resulting additional downtime of the tool = COST. In addition the collimator should be cleaned and replaced, resulting additional downtime of the tool = COST. Suitable for contact and barrier layers where lot of material is not needed to be deposited. Suitable for contact and barrier layers where lot of material is not needed to be deposited. Benefit with cover the bottom of Vias. Benefit with cover the bottom of Vias.

53 ® Sputtering Collimated sput. (Cont. 4) The next figure shows the bottom coverage of collimated sputtering compared to conventional versus contact aspect ratio. The next figure shows the bottom coverage of collimated sputtering compared to conventional versus contact aspect ratio.

54 ® Sputtering Hot sputtering Hot sputtering is a method used to fill spaced during deposition as well as to improve overall coverage. Hot sputtering is a method used to fill spaced during deposition as well as to improve overall coverage. The basic idea is to heat the substrate to 450-500ºC during deposition. The basic idea is to heat the substrate to 450-500ºC during deposition. Surface diffusion is significantly increased so that filling in spaces, smoothing edges and planarization are accomplished, driven by surface energy reduction. Surface diffusion is significantly increased so that filling in spaces, smoothing edges and planarization are accomplished, driven by surface energy reduction. The temperature in Via planarization processes is generally lower than that in contact to protect previously deposited Al layers. The temperature in Via planarization processes is generally lower than that in contact to protect previously deposited Al layers.

55 ® Sputtering Hot sputtering (Cont. 1) The lower power in the hot aluminum step increases the length of time that the Al atoms can diffuse, increasing the distance that they travel before they stop. The lower power in the hot aluminum step increases the length of time that the Al atoms can diffuse, increasing the distance that they travel before they stop. Usually, a thin cold deposition is done first with substrate at room temperature, which has better adhesion to the underlying material. Usually, a thin cold deposition is done first with substrate at room temperature, which has better adhesion to the underlying material. Then is followed by hot PVD deposition. Then is followed by hot PVD deposition. Main drawbacks is the relatively high temp. (reaction, thermal-budget, etc). Main drawbacks is the relatively high temp. (reaction, thermal-budget, etc).

56 ® Film Nucleation and Growth

57 ® Sputtering Things affect film structure The things that control grain structure are: The things that control grain structure are: – Substrate – Base pressure (or contamination level) – Deposition temperature – Deposition rate – Later processing temperature – Process pressure (#collisions)

58 ® Sputtering Film microstructure The film microstructure gives a graphic representative of how changing process pressure and wafer temperature affects the structure of a PVD film. The film microstructure gives a graphic representative of how changing process pressure and wafer temperature affects the structure of a PVD film.

59 ® Sputtering Grain size Al grains - AFM photos. What is the reason for the differences between these pictures ? A.B.

60 ® Sputtering What happened to this Ti target ?

61 ® Sputtering Target malfunction Ti target was warped near the edge of the target Ti target was warped near the edge of the target The root cause: the flatness of the backing plates, being out of specification. The epoxy did not adhere to the blank. During sputtering, the area where the epoxy did not adhere to the blank experienced high temperatures that could no longer be dissipated by the backing plate due to the minimal contact to the blank. Thus, as the area in question became hotter, the more likely that assembly warped. The root cause: the flatness of the backing plates, being out of specification. The epoxy did not adhere to the blank. During sputtering, the area where the epoxy did not adhere to the blank experienced high temperatures that could no longer be dissipated by the backing plate due to the minimal contact to the blank. Thus, as the area in question became hotter, the more likely that assembly warped.

62 ® Sputtering The crystal structure of Ti: HCP up to 882 °C HCP up to 882 °C BCC above 882 °C BCC above 882 °C

63 ® Sputtering Dep rate Vs. KWHR

64 ® Sputtering Where to Get More Information S. Wolf, Silicon Processing for the VLSI era, Vol 1-2. S. Wolf, Silicon Processing for the VLSI era, Vol 1-2. Peter Van Zant, Microchip Fabrication. Peter Van Zant, Microchip Fabrication. Stephen A. Campbell, The science and engineering of microelectronic fabrication. Stephen A. Campbell, The science and engineering of microelectronic fabrication. J. D. Plummer, M. D. Deal and P.B. Griffin, Silicon VLSI technology. J. D. Plummer, M. D. Deal and P.B. Griffin, Silicon VLSI technology. J.L. Vossen and W. Kern, Thin film processing II. J.L. Vossen and W. Kern, Thin film processing II.


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