Focused Ion Beam Technique in Nanofabrication

Transcription

1 The Meeting of National Graduate School of Nanoscience Focused Ion Beam Technique in Nanofabrication Antti Suutala Microelectronics and Materials Physics Laboratories University of Oulu // 1

2 Outline The Focused Ion Beam (FIB) Instrument Dual-Beam (FIB-SEM) Systems Ion Solid Interactions Gases for Deposition and Enhanced Etch Device Edits and Modifications FIB In-Situ Lift-Out TEM Specimen Technique Patterning and Deposition Examples // 2

3 The Focused Ion Beam (FIB) Instrument The basic FIB instrument consists of a vacuum system and chamber, a liquid metal ion source, an ion column, a sample stage, detectors, gas delivery system // 3

4 The Focused Ion Beam (FIB) Instrument The instrument is very similar to a scanning electron microscope (SEM) FIB instruments may be stand-alone single beam instruments or alternatively, FIB columns can be incorporated into other analytical instruments such as an SEM, TEM, or secondary ion mass spectrometry (SIMS) The most common of which is a FIB/SEM dual platform instrument // 4

5 The Focused Ion Beam (FIB) Instrument The capabilities of the FIB for small probe (diameter ~ 5 nm) sputtering are made possible by the liquid metal ion source (LMIS) Gallium is currently the most commonly used LMIS for FIB instruments for a number of reasons: Low melting point Low volatility Low vapour pressure Excellent mechanical, electrical, and vacuum properties Emission characteristics enable high angular intensity with a small energy spread // 5

6 The Focused Ion Beam (FIB) Instrument A schematic diagram of a Ga LMIS // 6

7 The Focused Ion Beam (FIB) Instrument Typical accelerating voltage in FIB systems ranges from 1 to 30 kev The ion column typically has two lenses; the condenser lens (1, probe forming) and the objective lens (2, focus) A set of apertures define the probe size and provides a range of ion currents (10 pa 30 na) Cylindrical octopole lenses perform multiple functions such as beam deflection, alignment, and stigmation correction // 7 Beam blankers are used to deflect the beam away from the centre of the column

8 The Focused Ion Beam (FIB) Instrument Large working distance (1.5-2 cm) permits the introduction of samples with varied topography without concern for field variations The sample stage typically has the ability to provide 5-axis movement (X, Y, Z, rotation and tilt) The bombardment of charged species to the surface of an insulator can cause sample charging Charge reduction methods: Good grounding of the specimen Coating the sample (C, W, Pt, Au, Cr, etc.) The use of an electron flood gun Imaging or milling with the SEM column turned on in dual platform system // 8

9 Dual-Beam (FIB-SEM) Systems A dual-beam system allows sample preparation, imaging, and analysis to be accomplished in one tool The ion beam and the electron beam complement each other in charge reduction, protective depositions, and imaging information The electron beam can be used to monitor the ion beam milling to endpoint precisely on the feature of interest The dual beam provides unparalleled flexibility in 3D structural analysis // 9

10 Dual-Beam (FIB-SEM) Systems The typical dual-beam column configuration is a vertical electron column with a tilted ion column. In this case, the sample will be tilted to 52 degrees for milling normal to the sample surface // 10

11 Dual-Beam (FIB-SEM) Systems The electron beam can be used for imaging without concern of sputtering the sample surface Electron beam deposition of materials can be used to produce very low energy deposition that will not affect the underlying surface of interest as dramatically as ion beam assisted deposition Non-destructive imaging of the sample may be accomplished with an integrated SEM Additional peripherals such as EDS and EBSD may be used for elemental or crystallographic information // 11

12 Ion-Solid Interactions The ability to mill, image, and deposit material using a FIB instrument depends critically on the nature of the ion beam solid interactions When the beam strikes the sample surface, many species are generated including sputtered atoms and molecules, secondary electrons, and secondary ions Sputtering occurs as the result of a series of elastic collisions where momentum is transferred from the incident ions to the target atoms within a collision cascade region // 12

13 Ion-Solid Interactions A schematic diagram below illustrate some of the possible ion beam - material interactions that can result from ion bombardment of a solid. Milling takes place as a result of physical sputtering of the target // 13

14 Ion-Solid Interactions If the ion is not backscattered out of the target surface, the ion will eventually come to rest, implanted within the target at some depth below the specimen surface The quality of the milled cuts or CVD regions depends critically on the interactions between the impinging ion beam and the target The response of a given target material to the ion beam is strongly dependent on factors such as beam current, incident ion energy, trench/feature geometry, raster pattern, and milling angle // 14

15 FIB Gases for Deposition and Enhanced Etch Gas delivery systems can be used in conjunction with the ion beam to produce site specific deposition of metals or insulators or to provide enhanced etching capabilities The gas molecules are adsorbed on the surface in the vicinity of the gas inlet, but decompose only where the ion beam strikes Repeated adsorption and decomposition result in the buildup of material in the ion scanned region The chemical precursors are obtained from a gas, liquid, or solid source that can be heated if required to produce the desired vapour pressure // 15

16 FIB Gases for Deposition and Enhanced Etch Figure below shows the spatial relationship of the gas source, the focused ion beam, the sample surface, and the volatilized and sputtered species Schematic drawing shows the deposition/controlled material removal principle. The enhanced etch process is shown. If adsorbed gas decomposes to non-volatile products, then deposition will take place // 16

17 FIB Gases for Deposition and Enhanced Etch Metals, such as W or Pt, are deposited by ion beam assisted chemical vapour deposition (CVD) of a precursor organometallic gas The ion beam assisted CVD process consists of a fine balance between sputtering and deposition. If the primary beam current density is too high for the deposition region, milling will occur Chemically enhanced sputtering is facilitated by the introduction of select species into the FIB chamber. For example, halogen-based species may enhance sputtering rates for specific substrate materials in the presence of the Ga + ion beam. Water has been shown to provide enhanced etching for carbonaceous materials // 17

18 Device Edits and Modifications One application that stands out is the ability of the FIB system to carry out device/circuit modifications on prototype chips Gallium beam interaction with the substrate is inherently destructive. The damage inflicted is essentially twofold: Impact damage due to heavier mass of the Ga + ions Surface charging effects Modifications carried out on prototype chips include: Making new connections using metal deposition ( W or Pt) Breaking connections Making probe points for tapping signals // 18

19 Device Edits and Modifications Modifications may be to fix design errors, or carry out design edits presented by customers Prototype chips can be worked on iteratively until the right result is achieved Implementation procedure Sample grounding Imaging On-Chip navigation Milling vias Filling vias Making connections and disconnecting lines Cleanup // 19

20 Device Edits and Modifications Sample Grounding Being grounded is vital to the health of the chip, as the beam striking the surface has a positive charge which has to be dissipated as it is being worked on Imaging The use of low beam current (30-50 pa) Apply a local platinum or global carbon coat On-Chip Navigation Dead reckoning; Visual references to reach the target location 3-point alignment; Stage of the FIB system is realigned using the coordinates from the chip obtained from the chip database CAD Navigation; Database information for the layout of the chip // 20

21 Device Edits and Modifications Milling vias Vias are typically milled using the same beam current used to image the sample (30-50 pa). Vias for top-level metal connections are usually a couple of microns on the side. On lower level metals, via size needs to increase because of the higher aspect ratio and to avoid metal to bridge on top of the via Larger contact areas ensure lower contact resistance The enhanced etch gas used to assist the milling of oxides enhances the milling rate about 7-9 times. Etch gas also essentially convert the sputtered material into a volatile compound and minimizes redeposition Filling vias Vias are filled by depositing a Pt or W plug The deposition box should be smaller than the mill box to make sure that the deposited metal makes a contact to the layer below and does not bridge across the top of the via and cause voiding Deposition time should not be too short or too long (~ 2.5 minutes) // 21

22 Device Edits and Modifications Making connections and disconnecting lines Typical connections run about um long, um wide and about um thick In order to avoid refilling of cuts with redeposition, lines are cut after all depositions are complete Typical beam currents for exposing the lines to be cut are similar to ones used for exposing lines (30-50 pa). There is a risk of exposing the underlying metal levels, when using a higher beam current. The use of enhancing etch gases produce cleaner cuts Clean-up Clean up is typically required if there are several modifications in close proximity to each other and only if the metal deposition over sprays overlap Milling around the deposition paths using individual mill boxes Imaging surface while the enhanced etch flow of gas is on One needs to be extremely careful so as not to mill too much into the top surface // 22

23 Device Edits and Modifications Milling and filling a via Deposited W lines // 23

24 Device Edits and Modifications Cleaned overspray Line cuts Deposited Milling and W filling line via Milling and filling via // 24

25 FIB In-Situ Lift-Out TEM Specimen Technique The objective of the FIB In-Situ Lift-Out (INLO) method is to produce a high quality electron transparent membrane to be imaged in the TEM A primary advantage is that specimens may be prepared from the starting bulk sample with little or no initial specimen preparation If an insulating material is to be FIB milled, a conductive coating may be applied to the sample to prevent charging INLO technique requires a vacuum compatible probe assembly // 25

26 FIB In-Situ Lift-Out TEM Specimen Technique TEM Specimen preparation begins by using the ion beam (or ebeam) CVD process to deposit a um thick metal line // 26

27 FIB In-Situ Lift-Out TEM Specimen Technique Next, high beam currents are used to mill large amounts of material away from the front and back of the region of interest. Then the bottom and the right edge are cut free leaving just a tab of material on the left side holding the specimen // 27

28 FIB In-Situ Lift-Out TEM Specimen Technique The manipulator probe is positioned to touch a FIB milled piece of sample. The FIB is then used to attach the probe to the sample using the FIB CVD capabilities // 28

29 FIB In-Situ Lift-Out TEM Specimen Technique Then the sample is lifted out of the bulk material // 29

30 FIB In-Situ Lift-Out TEM Specimen Technique and the probe/sample assembly is then positioned onto a TEM slotted grid // 30

31 FIB In-Situ Lift-Out TEM Specimen Technique The CVD operation is used to attach the sample to the grid. Once the sample is secured to the grid, the probe is FIB milled free from the sample // 31

32 FIB In-Situ Lift-Out TEM Specimen Technique The sample is then FIB milled to electron transparency thickness (<100 nm) and finely polished using conventional FIB milling practices // 32

33 FIB In-Situ Lift-Out TEM Specimen Technique Artefacts The high energy ion bombardment in the specimen can accumulate into several thousand volts of charge which can lead into large craters and local melting due to electrostatic discharge Since the rate of specimen material removal by the FIB is sensitive to the hardness, atomic number, and topology of the surface, ridges and grooves are cut into the surface of the specimen causing a so called waterfall effect when the beam encounters soft or low atomic number specimen surfaces The creation of amorphous layers are probably the greatest problem confronting TEM examination of thin TEM specimens Implanted ions in the specimen cause gallium contamination. Likewise some of material sputtered off the specimen by the ion beam in one portion of the specimen may land on another part of the specimen creating sputter-contamination artefacts // 33

34 Patterning and Deposition Examples Core structure for nano-aperture device FIB-milled circular corrugation pattern on quartz: Groove depth 100 nm, width 300 nm and, spacing 300 nm Focused electron beam deposited nano-post using tetraethyl orthosilicate (TEOS) (SiO 4 C 8 H 20 ) as precursor gas TEOS decomposes into a solid transparent dielectric material under electron irradiation Diameter ~ 100 nm Height several μm // 34

35 Patterning and Deposition Examples Focused electron beam deposited corkscrew Tetraethyl orthosilicate (TEOS) (SiO 4 C 8 H 20 ) as precursor gas TEOS decomposes into a solid transparent dielectric material under electron irradiation // 35

36 Patterning and Deposition Examples a) b) FESEM micrographs of the FIB-milled master moulds on Au-coated SiO2 substrate for nano imprint lithography (NIL). a) Binary grating: period 1050 nm, depth 400 nm, b) Blazed grating: period 1063 nm, depth (at the end) 1000 nm // 36

37 Patterning and Deposition Examples a) b) FESEM micrographs of the FIB-milled bitmap patterns on Au-coated SiO2 substrate to demonstrate the capability of the complex patterning in FIB system // 37

38 Questions? Nanoscale Patterning and Deposition Anyone? // 38