Sample Preparation

Sample Preparation Methods

	Prior to XPS Analysis Do NOT Analyze Area of Interest by SEM-EDX or Auger Prior to XPS Analysis To measure the true surface chemistry from your material or product, the region of interest to be analyzed by XPS must NOT be exposed to any electron beam methods, such as SEM-EDX or Auger. The area of interest for XPS analysis must not be analyzed by ToF-SIMS if the ToF-SIMS analysis time is more than 4-5 minutes. SEM-EDX, Auger, and ToF-SIMS provide nice images, but they degrade the true chemistry of the surface regions that you want to analyze by XPS. Why? SEM uses a 1-25 kV electron beam that causes low vacuum gases to break into reactive atoms or molecular fragments which attach to or react wtih the surface which was just activated by the electron beam wherever the electron beam strikes the sample. This is why you see Squares or Circles on samples because this is where the beam irradiated and paused at the edges or center more than it irradiated the middle region. The electron beam can cause carbon contamination that is thick enough to hide the true surface contamination. Please note that SEM-EDX element information is mainly due to bulk chemistry. Only at very low KE will SEM-EDX give you near surface chemistry. XPS gives chemistry from top 1-12 nm, but normal EDX at 5-10 kV gives chemistry from 0.5 – 2 microns (2,000 nm) below the surface. ToF-SIMS is even more surface sensitive than XPS. ToF-SIMS gives information from the top 1-3 nm. ToF-SIMS normally does not degrade the surface as long as the analysis time is less than 4-5 minutes. Auger (AES) is also more surface sensitive than XPS. Auger gives information from the top 1-5 nm. Unfortunately, the electron beam in Auger will do more damage than the SEM-EDX due to the much higher current of Auger electron beam. Do monochromatic XPS X-rays cause any degradation of surface chemistry? Yes, they can cause slight degradation to the area of analysis of certain materials if that area is exposed to the monochromatic X-ray beam for many hours or if the X-ray flux density is very high. Materials that are known to be truly damaged by many hour exposure to monochromatic X-rays include polymers such as: PVC, Poly-acrylic acid, and Teflon. Other materials include high oxidation state chemical (NaClO₄, CrO₃) and some catalysts. Some organics are sensitive, but amino acids are stable. We have studied sample degradation of many inorganic compounds and polymers and we can either decrease the power density during analysis or move half way during analysis to a clean fresh area. These methods allow us to maximize the quality and reliabiity of your XPS data. Non-monochromatic Mg or Al X-rays cause even more damage to materials due to the high temperature (200-250 C) of the non-monochromatic X-ray source which is very close (1-3 cm) from the sample.
	Sample Preparation – Treatments As-Received Condition After removing ~8 nm of surface material After removing ~25 nm of surface material to expose bulk Air-Plasma Corona treatment with Tesla Coil Bulk Chemistry Exposed Cleaned with Acetone Cleaned with 90% IPA Cleaned with 100% IPA Cleaned with Methanol (MeOH) Cleaned with Xylene Cleaned with Distilled Water Delamination of Laminar Crystal Fine Powder Pressed into Smooth Pellet Freshly cleaved/fractured in air Freshly Ground using Mortar & Pestle Fresh Fracture of Solid using Hammer in Air Hand Polish to Smooth Surface Heated in air 1 hour on hotplate at 300C Heated in low vacuum at 200C overnight Scraped with freshly cleaned single edge razor Surface Delamination of Laminar Crystal UV-Ozone cleaning (not yet available) Plasma cleaned (not yet available) Surface Treatments Available on Request ($50/treatment/sample) As-Received Condition: (no treatment, no charge) After Light Ion Etch: to remove ~8 nm of adventitious carbon contamination After Strong Ion Etch: to remove 25 nm of existing surface revealing bulk (oxidation state degradation may occur) Air-Plasma Corona treatment with Tesla Coil: insulators are exposed to air-plasma corona in air using a Laboratory Tesla Coil Bulk Chemistry Exposed: Bulk of sample exposed by cutting, fracturing, cleaving, hammer or delamination Cleaned with Acetone: to remove Acetone soluble organics and inorganics from surface Cleaned with 90% IPA: to remove IPA soluble organics and inorganics from surface Cleaned with 100% IPA: to remove IPA soluble organics from surface Cleaned with MeOH: to remove MeOH soluble organics and inorganics from surface Cleaned with Xylene: to remove Xylene soluble organics from surface Cleaned with Distilled Water: to remove Water soluble organics and inorganics from surface Delamination of Laminar Crystal Freshly Cleaved Wafer in Air: enables analysis of bulk material in wafer Freshly Ground: in mortar and pestile to expose bulk chemistry Fresh Fracture of Solid using Hammer in Air: enables analysis of bulk chemistry of solid Fine Powder Pressed into Pellet: increases signal counts and improve charge compensation, powder can be pressed into a smooth surface Heated in air 1 hour on hotplate at 300C Heated in low vacuum at 200C overnight Hand-Polish to Smooth Surface: to remove surface contamination and increase signal counts Scraped with freshly cleaned single edge razor Surface Delamination – using tape on laminar crystals Other: Simple Treatment Methods on Request (Not Yet available: plasma cleaning, UV-Ozone cleaning)
	Sample Names & Package Labelling Be certain to mark the region that will be analyzed by using a carbide tip to scribe a circle (2-3 mm) around the feature or region of interest, or use a Sharpie marking pen. The standard X-ray beam is 800 micron. Label the sample package with the name of the sample. Place sample inside clean Aluminum foil or a new paper envelop. Write the name or number of the sample on the outside of the foil or envelop. Describe the expected chemistry of the surface. Include a photo from an optical microscope. Do not use SEM.
	Sample Size, Shape, & Handling Real world samples come in many shapes and sizes. Many of them need to be reduced in size to fit on the instrument sample mount. Real world samples come in many shapes and sizes. Many of them need to be reduced in size to fit on the instrument sample mount. The largest sample mount is 8 cm (~3.3 inches) in diameter Because most materials outgas due to gases trapped on the surface, we will cut or cleave the sample to a smaller size. We wear clean nylon gloves and use freshly cleaned tools to reduce the size of your sample(s) Powders often require longer time in our load-lock chamber to outgas to <5×10^-5 torr. If longer than 20 minutes, we might add a small outgas surcharge. To minimize sample outgassing, you can degas the sample at your site, wrap sample in Al foil before shipping Note: If the sample is heated while degassing it, it is possible to change the chemistry of the surface. Do not heat during degas. Room temp. only.
	Simple Strategy Recommended for Routine Trouble Shooting Strategy Recommended: Analyze As-Received Good and Bad areas on the exact same sample if possible. Survey spectra only. (this equals 2 areas analyzed) Collect Only Survey Spectra: Analyze the Bad area (sample) first. Compare chemistry to expected chemistry, then analyze Good sample if more info is needed. Alternatively: Analyze As-Received Problem Area on Sample A, and Normal Area on Sample B (this equals 2 areas analyzed) Collect survey spectra only. Lightly Ion Etch the Bad Area: Collect survey spectrum or chemical state spectra of key signal Check Survey Spectra Peaks to Partially Resolve Chemical States with large Shifts: N (1s), S (2p), Si (2p) and various metals
	Sample Preparation Variables Angle lapping Ball cratering Blisters – break open with tweezers Chemical derivatization Chemical etch Clean with gas jet Fracture (in air, in LN2, under solvent) Glove bag (argon fill) Heating In air – oxidation LN2 fracturing in air Oil analysis Plasma clean Pressed pellet Sample fracturing Sample scraping Sample shape Scrape (in air, under hexane or other solvent) Scribe Analysis Area for transparent sample Surface derivatization	.

Analysis Conditions Used for XPS Measurements and Energy Resolution

Surface Condition: As-Received condition before any ion etching to allow detection of minor contaminants that might explain problem
X-ray Beam Size = Sample Area Analyzed: 500 x 800 um largest
(50 x 150 um smallest)
Survey Spectra BE Range: -10 to 1100 eV
(if As, Ga are expected, then -10 to 1400 eV will be used)
Electron Take-off-angle: 35 degrees maximizes surface information
(90 deg TOA requires use of Tilt Stage)
Carbon (1s) Chemical State Spectrum: five (5) scan C (1s) spectrum
included free of charge
Chemical State Spectra BE Range: 25 eV window for most XPS signals
(expanded to 50 eV when needed)
Detection Limits: 0.1 to 1.0 atom% depending on element and KE of peak
(1,000 ppm to 10,000 ppm)
Survey Spectrum Energy Resolution: FWHM (eV) for Ag (3d_5/2) = 1.5 eV
Chemical State Spectrum Energy Resolution: FWHM (eV) for Ag (3d_5/2) = 0.75 eV

Ultimate Energy Resolution Values

Ultimate Energy Resolution for a Conductor: FWHM (eV) for Re (4f_7/2) = 0.35 eV
Ultimate Energy Resolution of Hemispherical Analyzer: FWHM (eV) = 0.25 eV
FWHM (eV) of Monochromatic Al K_α X-rays: 0.16 eV

Sample Handling Guide