Liquid Cluster Ions Research Articles

Improved mass resolution and mass accuracy in TOF-SIMS spectra and images using argon gas cluster ion beams.

The popularity of argon gas cluster ion beams (Ar-GCIB) as primary ion beams in time-of-flight secondary ion mass spectrometry (TOF-SIMS) has increased because the molecular ions of large organic- and biomolecules can be detected with less damage to the sample surfaces. However, Ar-GCIB is limited by poor mass resolution as well as poor mass accuracy. The inferior quality of the mass resolution in a TOF-SIMS spectrum obtained by using Ar-GCIB compared to the one obtained by a bismuth liquid metal cluster ion beam and others makes it difficult to identify unknown peaks because of the mass interference from the neighboring peaks. However, in this study, the authors demonstrate improved mass resolution in TOF-SIMS using Ar-GCIB through the delayed extraction of secondary ions, a method typically used in TOF mass spectrometry to increase mass resolution. As for poor mass accuracy, although mass calibration using internal peaks with low mass such as hydrogen and carbon is a common approach in TOF-SIMS, it is unsuited to the present study because of the disappearance of the low-mass peaks in the delayed extraction mode. To resolve this issue, external mass calibration, another regularly used method in TOF-MS, was adapted to enhance mass accuracy in the spectrum and image generated by TOF-SIMS using Ar-GCIB in the delayed extraction mode. By producing spectra analyses of a peptide mixture and bovine serum albumin protein digested with trypsin, along with image analyses of rat brain samples, the authors demonstrate for the first time the enhancement of mass resolution and mass accuracy for the purpose of analyzing large biomolecules in TOF-SIMS using Ar-GCIB through the use of delayed extraction and external mass calibration.

Production and irradiation of ionic liquid cluster ions

We have developed a field-emission-type of cluster ion source using ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6). The current obtained was stable by placing a porous cap around the emitter. Time-of-flight (TOF) measurement showed that the peak mass number was approximately 5000 for positive and negative BMIM-PF6 ion beams. This indicated that BMIM-PF6 clusters with a size of a few tens of molecules were produced. With regard to the surface modification by BMIM-PF6 ion beams, positive and negative cluster ion beams were used to irradiate Si(100) and glass substrates. Scanning electron microscope (SEM) and atomic force microscope (AFM) observations showed that the surface roughness of substrates increased. Furthermore, X-ray photoelectron spectroscopy (XPS) measurement showed that the composition ratio of layers deposited by positive or negative cluster ion beams was similar to that of BMIM-PF6 solvent.

Surface Interaction and Processing Using Polyatomic Cluster Ions

We developed two types of polyatomic cluster ion sources, one of which was a liquid cluster ion source using organic materials with a high-vapor pressure. Vapors of liquid material such as ethanol and water were ejected through a nozzle into a vacuum region, and liquid clusters were produced by an adiabatic expansion phenomenon. Another type was a cluster ion source using ionic liquids with a relatively low-vapor pressure. Positive and negative cluster ions were produced by a high-electric field emission. In addition, the interaction of polyatomic cluster ions with solid surfaces such as Si(100), SiO2, glass, and PMMA surfaces was investigated, and chemical sputtering was predominant for the Si(100) surfaces irradiated by ethanol cluster ion beams. Also, the irradiation damage of the Si(100) surfaces by ethanol and water cluster ion beams was smaller than that by Ar monomer ion irradiation at the same acceleration voltage. With regard to surface modification, PMMA surfaces were chemically modified by water cluster irradiation. Also, glass surfaces changed to electrically conductive surfaces by ionic liquid cluster ion irradiation. Furthermore, to demonstrate engineering applications of high-rate sputtering and low-damage irradiation by ethanol cluster ion beams, micro-patterning was performed on the Si surfaces.

Chemical erosion and sputtering of solid surfaces with liquid cluster ions

The sputtering phenomena of solid surfaces such as Si(111) and SiO 2 surfaces were investigated using ethanol and water cluster ion beams. To be compared with Ar monomer ion irradiation, the sputtering yield of Si surfaces was approximately 100 times higher for ethanol cluster ion irradiation and approximately 10 times higher for water cluster ion irradiation. Furthermore, for the ethanol cluster ion irradiation, chemical erosion such as silicon hydride and hydro-carbide reaction occurred on the Si surface, which resulted in the high-rate sputtering of the surface. On the other hand, for the water cluster ion irradiation, oxidation occurred on the Si surface, and physical sputtering was performed on the surface. Based on these results, chemical reaction at a nano-scale area on the Si(111) surfaces was discussed from the thermodynamic approach, and the impact of cluster ions on the surface exhibited high temperature such as a few tens of thousands degrees, which resulted in the enhancement of the chemical reaction. Thus, liquid cluster ion irradiation exhibited unique erosion and sputtering even at room temperature, which were not obtained by a conventional wet process.

Characteristics of liquid cluster ion beam for surface treatment

A liquid cluster ion source, which is an ion source for the cluster beams produced with liquid materials, has been developed for the surface treatment of solid materials. The electrodes were designed for increasing the cluster beam intensity by a computer simulation of beam trajectories. The peaks of the cluster size distributions of the water and ethanol cluster ion beams of 3 atm vapor pressure were approximately at 2.4x10(3) and 1.6x10(3) molecules, respectively. The cluster size distributions of ethanol clusters were not sensitive to the variations of the acceleration voltages (V(e)) and currents (I(e)) of the electrons for ionization when the V(e) and I(e) were larger than approximately 200 V and 200 mA, respectively.

Dependence of size distribution for liquid cluster ions on retarding voltage

We have succeeded in producing liquid clusters such as ethanol and water clusters by an adiabatic expansion phenomenon, and the cluster size distribution was analyzed by using the time-of-flight method. For ethanol cluster ions, the size was distributed between a few hundreds and a few thousands, and the peak size was approximately 1200 molecules per cluster. For water cluster ions, the size distribution shifted to higher values, and the peak size was approximately 2500 molecules per cluster. Furthermore, the cluster size distribution was measured by changing the retarding voltage, and it was found that the minimum size increased with increase of the retarding voltage. In addition, the minimum size was larger than the critical size estimated by the theoretical calculation.

Irradiation effects of liquid cluster ion beams on silicon surfaces

We have developed a liquid cluster ion source, and produced liquid clusters such as ethanol and water clusters by an adiabatic expansion phenomenon. The average size was about 500 to 1000 molecules for the ethanol and about 2500 molecules for the water, respectively. The intensity of the cluster ion beams increased with increase of the vapor pressures. The irradiation effects of the liquid cluster ion beams on Si(100) surfaces were investigated. The sputtered depth increased with increase of an acceleration voltage for the ethanol and water cluster ions. In particular, the ethanol cluster ion irradiation sputtered the Si surface with the sputtering yield of approximately 100 times larger than that for Ar monomer ion irradiation. In addition, the Si surfaces after sputtering had an average roughness of less than 1 nm. Furthermore, the RBS channeling measurement showed that the irradiation damage of the Si surfaces by the ethanol and water cluster ions was smaller than that by the Ar monomer ion irradiation. This indicates that the low energy ion irradiation effect is predominant for the liquid cluster ion beams. As an engineering application of the liquid cluster ion beam process, the etching and patterning process were performed on the Si surfaces using metal masks. The mask patterns were formed very swiftly and finely by irradiation of the ethanol cluster ion beams.

Interactions of liquid cluster ion beams with metal surfaces

In order to investigate the interactions of ethanol and water cluster ion beams with metal surfaces, titanium (Ti) substrates were irradiated at different acceleration voltages. The sputtered depth increased with increase of the acceleration voltage. The sputtering yield of Ti at an acceleration voltage of 9kV for the ethanol cluster ions was approximately 100 times larger than that by argon (Ar) ion beams. On the other hand, for the case of the water cluster ion irradiation, the sputtering yield was a few tens times larger than that for Ar monomer ion irradiation. In addition, the AFM observation showed that the sputtered surfaces by the ethanol and water cluster ion beams had an average roughness of less than 1nm. For other metal surfaces such as Al, Ni, Cu, Ag and Au surfaces, the sputtering yield by the liquid cluster ion beams was also much larger than that by Ar monomer ion beams at the same acceleration voltage. Furthermore, the wettability of the Ti surfaces was changed drastically by the species of the cluster ions as well as the acceleration voltage. For the Ti surfaces irradiated by the ethanol cluster ion beams, the contact angle decreased with increase of the acceleration voltage, and it became about 10° at higher acceleration voltages. However, for the case of the water cluster ion irradiation, the contact angle was almost the same at various acceleration voltages, and it was about 80° which was larger than that of unirradiated surface.

Size analysis of water cluster ions and their irradiation effects on solid surfaces

AbstractThis article reports the development of a liquid cluster ion source. When water vapor was injected through a nozzle into a high‐vacuum region, water clusters were produced at vapor pressures greater than 1 atm. The size of the water cluster ions was measured by the time‐of‐flight (TOF) method, and it was distributed between a few hundreds and a few tens of thousands of molecules. The intensity of the water cluster ions increased with increase of the vapor pressure, and the peak size was about 2500 molecules per cluster. The effects of irradiation by water cluster ions on solid surfaces such as Si(100) and SiO2 surfaces were investigated. The sputtered depth of the Si and SiO2 surfaces increased with increase of the acceleration voltage, and the SiO2 surfaces were sputtered more than the Si surfaces. The sputtering yields at an acceleration voltage of 9 kV were 17.8 atoms per ion for Si and 17.1 molecules per ion for SiO2, which were approximately ten times larger than that for Ar monomer ion irradiation. Rutherford backscattering spectroscopy (RBS) measurement showed that the number of displacement atoms for irradiation by water cluster ions on the Si surfaces at ion doses larger than 1 × 1015 ions/cm2 was less than that for Ar monomer ion irradiation at the same acceleration voltage. In addition, the ellipsometry and X‐ray photoelectron spectroscopy (XPS) measurements showed that the thickness of the oxide layer formed on the Si surface increased with increase of the acceleration voltage, and it was between 10 nm and 15 nm at an acceleration voltage of 9 kV. Copyright © 2006 John Wiley & Sons, Ltd.

Characterization of liquid cluster ion source by the time-of-flight method

A liquid cluster ion source, which offers many possibilities for new and advanced applications in the surface processing of materials, was developed, and the cluster size distribution was measured by using time-of-flight method. When the vapor pressure of liquid materials such as ethanol and water was larger than 1atm, the liquid clusters could be produced by an adiabatic expansion phenomenon. The peak size of the cluster ions increased with the increase of the vapor pressures, and decreased with the increase of the electron voltage and the electron current for ionization. In another case of producing liquid cluster ions at lower vapor pressures, He gas was used to mix up with vapors of liquid materials. Even if a vapor pressure was less than 0.1atm, the liquid cluster ions with smaller size could be produced at the He gas pressures larger than 0.5atm. The vapors of the liquid materials were cooled down by the collision with the He gas, which resulted in the effective formation of the liquid clusters even at lower vapor pressures.

Interactions of ethanol cluster ion beams with silicon surfaces

The liquid cluster ion beam system was developed, in which cluster ions of organic molecules such as ethanol could be produced. In order to investigate the interactions of ethanol cluster ion beams with solid surfaces, SiO2 substrates and SiO2 films were irradiated at different acceleration voltages. The sputtered depth increased exponentially with the acceleration voltage. When the acceleration voltage was 9kV, the sputtered depths of Si and SiO2 at a dose of 1×1016ions/cm2 were 344.6nm and 47.2nm, respectively. The sputtering yield for the Si surface was approximately 100 times larger than that by Ar ion beams. With regards to the sputtering ratio of Si to SiO2, the ratio increased with decreasing acceleration voltage. This suggests that chemical reactions between Si and ethanol produced silicon hydride which was the dominant etching material for the Si surfaces.In addition, the AFM observation showed that the sputtered surface had an average roughness of less than 1nm. With regards to the crystalline state of the sputtered surface, the RBS and ellipsometry measurements were performed, and they showed that the Si surface damage induced by ethanol cluster ion irradiation was less than that by Ar monomer ion irradiation. Thus, Si surfaces etched at high sputtering speed by ethanol cluster ion beams had a lower damage and an atomically flat surface.

Production of liquid cluster ions and their application to surface etching

A new type of cluster ion beam system using organic liquid materials such as ethanol has been developed, and it has several advantages for surface etching and chemical modification based on the different properties of liquid cluster ions. Ethanol vapors were ejected through a nozzle into a high-vacuum region, and ethanol clusters were produced by an adiabatic expansion phenomenon at the vapor pressures larger than 1atm. In another case of producing ethanol clusters at a lower vapor pressure, He gas was used to mix up with ethanol vapors, and the mixed gases were ejected into a high vacuum region. Even if a vapor pressure of ethanol was 0.1atm, ethanol clusters were produced at the He gas pressure larger than 1atm.The ethanol clusters produced were ionized by an electron bombardment method, and the cluster ions were accelerated toward a substrate by applying an acceleration voltage. For the case of ethanol cluster ion irradiation at an acceleration voltage of 9kV, the sputtering yields for Al, Cu, Ag and Au films, which were used as a substrate, were about ten times larger than that by Ar monomer ion irradiation. In addition, the surface flatness of the metal films was improved by irradiation of ethanol cluster ions.

Fundamental characteristics of liquid cluster ion source for surface modification

We have developed a cluster ion source using liquid organic materials, which has several advantages for surface modification based on the different properties of liquid cluster ions. When vapors of liquid materials such as ethanol were ejected through a nozzle into a high-vacuum region, clusters were produced by an adiabatic expansion phenomenon. The clusters produced were ionized by an electron bombardment method, and the cluster ion current increased with increase of an electron current for ionization. In order to achieve the effective transport of cluster ion beams, a unipotential or einzel lens was used. The current density was much larger than that without using the lens. Furthermore, the beam diameter could be controlled by adjusting the lens voltage, and it was changed between 5mm and 20mm. In addition, several kinds of substrates such as Si, SiO2, PC substrates were irradiated at different acceleration voltages with ethanol cluster ion beams, and chemical and physical sputtering with a high sputtering yield was achieved. The sputtered surfaces were very smooth at an atomic level. The liquid cluster ion beams have a high potential for surface etching with the atomic level flatness, which is not obtained by a conventional wet process.