Ion Photon Emission Microscopy Research Articles

Ion Photon Emission Microscope for Single Event Effect Testing in CIAE**Supported by the National Natural Science Foundation of China under Grant No 11690044.

Ion photon emission microscopy (IPEM) is a new ion-induced emission microscopy. It employs a broad ion beam with high energy and low fluence rate impinging on a sample. The position of a single ion is detected by an optical system with objective lens, prism, microscope tube and charge coupled device (CCD). A thin ZnS film doped with Ag ions is used as a luminescent material. Generation efficiency and transmission efficiency of photons in the ZnS(Ag) film created by irradiated Cl ions are calculated. A single Cl ion optical microscopic image is observed by high quantum efficiency CCD. The resolution of a single Cl ion given in this IPEM system is 6 μm. Several factors influencing the resolution are discussed. A silicon diode is used to collect the electrical signals caused by the incident ions. Effective and accidental coincidence of optical images and electronic signals are illustrated. A two-dimensional map of single event effect is drawn out according to the data of effective coincidence.

Time-between-photons method for measuring fluorescence lifetimes

We have applied a time-between-photons (TBP) method to measure fluorescence lifetimes in a pulsed-light excitation scheme. The TBP method has been originally proposed by Rossi and his coworkers in the field of ion photon emission microscopy (IPEM) for measuring lifetimes of ion-luminescent materials [Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 2193]. The TBP method has an advantage in that no reference signal is required in the instrumental setup. In the present paper, we demonstrate, for the first time to our knowledge, that the TBP method is also applicable to measure conventional fluorescence lifetimes. The TBP method suits for measuring fluorescent samples whose lifetimes are sufficiently long (≥10 ns) and intensities are moderately high (≤4 × 10−8 W): fluorescent samples with intermediate quantum yields for which the conventional time-correlated single-photon-counting (TC-SPC) method is somewhat difficult to employ as it is and the conventional analogue light-measurement method is still hard to use.

Ion beam characterization of advanced luminescent materials for application in radiation effects microscopy

The ion photon emission microscope (IPEM) is a technique developed at Sandia National Laboratories (SNL) to study radiation effects in integrated circuits with high energy, heavy ions, such as those produced by the 88” cyclotron at Lawrence Berkeley National Laboratory (LBNL). In this method, an ion-luminescent film is used to produce photons from the point of ion impact. The photons emitted due to an ion impact are imaged on a position-sensitive detector to determine the location of a single event effect (SEE). Due to stringent resolution, intensity, wavelength, decay time, and radiation tolerance demands, an engineered material with very specific properties is required to act as the luminescent film. The requirements for this material are extensive. It must produce a high enough induced luminescent intensity so at least one photon is detected per ion hit. The emission wavelength must match the sensitivity of the detector used, and the luminescent decay time must be short enough to limit accidental coincidences. In addition, the material must be easy to handle and its luminescent properties must be tolerant to radiation damage. Materials studied for this application include plastic scintillators, GaN and GaN/InGaN quantum well structures, and lanthanide-activated ceramic phosphors. Results from characterization studies on these materials will be presented; including photoluminescence, cathodoluminescence, ion beam induced luminescence, luminescent decay times, and radiation damage. Results indicate that the ceramic phosphors are currently proving to be the ideal material for IPEM investigations.

The ion photon emission microscope on SNL’s nuclear microprobe and in LBNL’s cyclotron facility

Radiation effects microscopy (REM) has evolved into an essential tool for the study, diagnostics and remedy of single event effects (SEE) in microelectronics devices, However, we are entering an era where the ion energies of the current systems are becoming inadequate for diagnosing SEE problems in modern ICs due to the great thickness of interlevel dielectric, metallization and passivation layers found on top of the active radiation-sensitive Si. Our solution is the ion photon emission microscope (IPEM), which eliminates the need to focus several GeV heavy ions. A tabletop IPEM is currently in use at Sandia National Laboratories (SNL), operating with alpha particles, and showing 4 μm resolution. We have recently developed a second system, and installed it on one of the SNL nuclear microprobe lines to demonstrate the principle and prove its potential as a portable radiation effects microscope that can be installed at the LBNL GeV cyclotron facility. The microprobe system is currently operating with ∼2 μm resolution. The determined advantages of installing a similar system at the LBNL cyclotron facility will be discussed, in addition to recently measured optical characteristics of the various phosphor materials being investigated.

Phosphors’ lifetime measurement employing the Time Between Photons method

The Time Between Photons theory (hereafter TBP) is applied to the evaluation of the lifetime of phosphors employed in the Ion Photon Emission Microscope (IPEM). IPEM allows Radiation Effects Microscopy (REM) without focused ion beams and appears to be the best tool for the radiation hardness assessment of modern integrated circuit at cyclotron energies. IPEM determines the impact point of a single ion onto the sample by measuring the light spot produced on a thin phosphor layer placed on the sample surface. The spot is imaged by an optical microscope and projected at high magnification onto a Position Sensitive Detector (PSD). Phosphors, when excited by an ion, emit photons with a particular lifetime, which is important to evaluate. We measured the statistical distribution of the Time Between consecutive detected Photons (TBP) for several phosphors and have been able to link it to their lifetime employing a theory that is derived in this paper. The single-photon signals are provided by the IPEM-PSD, or faster photomultipliers when high-speed materials had to be assessed.

Ion-luminescence properties of GaN films being developed for IPEM

Radiation effects microscopy (REM) for the next generation integrated circuits (ICs) will require GeV ions both to provide high ionization and to penetrate the thick overlayers in present day ICs. These ion beams can be provided by only a few cyclotrons in the world. Since it is extremely hard to focus these higher-energy ions, we have proposed the ion photon emission microscope (IPEM) that allows the determination of the ion hits by focusing the emitted photons to a position sensitive detector. The IPEM needs a thin luminescent foil that has high brightness, good spatial resolution and does not change the incident ion’s energy and direction significantly. Available organic-phosphor foils require a large thickness to produce enough photons, which results in poor spatial resolution. To solve this problem, we have developed thin, lightly doped n-type GaN films that are extremely bright. We have grown high quality GaN films on sapphire using metal organic chemical vapor deposition (MOCVD), detached the films from the substrate using laser ablation, and made them self-supporting. The smallest foils have 1mm2 area and 1μm thickness. The optical properties, such as light yield, spectrum and decay times were measured and compared to those of conventional phosphors, by using both alpha particles from a radioactive source and 250keV ions from an implanter. We found that the GaN performance strongly depends on composition and doping levels. The conclusion is that 1–2μm GaN film of a 1mm2 area may become an ideal ion position detector.

Performance of an alpha-IPEM

The ion photon emission microscope, or IPEM, is the first device that allows scientists to microscopically study the effects of single ions in air on semiconductors, microchips and even biological cells without having to focus the beam. Reported here is a prototype, the size of a conventional optical microscope, developed at Sandia. The alpha-IPEM, that employs alpha particles from a radioactive source, represents the first example of IBA imaging without an accelerator. The IPEM resolution is currently limited to ∼10μm, but we also report a gridded-phosphor approach that could improve this resolution to that of the optical microscope, or ∼1μm. Finally, we propose that a simple adaptation of the alpha-IPEM could be the only way to maintain the high utility of radiation effects microscopy into the future.

Fabrication of phosphor micro-grids using proton beam lithography

A new nuclear microscopy technique called ion photon emission microscopy or IPEM was recently invented. IPEM allows analysis involving single ions, such as ion beam induced charge (IBIC) or single event upset (SEU) imaging using a slightly modified optical microscope. The spatial resolution of IPEM is currently limited to more than 10μm by the scattering and reflection of ion-induced photons, i.e. light blooming or spreading, in the ionoluminescent phosphor layer. We are developing a “Microscopic Gridded Phosphor” (also called Black Matrix) where the phosphor nanocrystals are confined within the gaps of a micrometer scale opaque grid, which limits the amount of detrimental light blooming. MeV-energy proton beam lithography is ideally suited to lithographically form masks for the grid because of high aspect ratio, pattern density and sub-micron resolution of this technique. In brief, the fabrication of the grids was made in the following manner: (1) a MeV proton beam focused to 1.5–2μm directly fabricated a matrix of pillars in a 15μm thick SU-8 lithographic resist; (2) 7:1 aspect ratio pillars were then formed by developing the proton exposed area; (3) Ni (Au) was electrochemically deposited onto Cu-coated Si from a sulfamate bath (or buffered CN bath); (4) the SU-8 pillars were removed by chemical etching; finally (5) the metal micro-grid was freed from its substrate by etching the underlying Cu layer. Our proposed metal micro-grids promise an order-of-magnitude improvement in the resolution of IPEM.

Ion photon emission microscopy

A new ion-induced emission microscopy has been invented and demonstrated, which is called ion photon emission microscopy (IPEM). It employs a low current, broad ion beam impinging on a sample, previously coated or simply covered with a few microns of a fast, highly efficient phosphor layer. The light produced at the single ion impact point is collected with an optical microscope and projected at high magnification onto a single photon position sensitive detector (PSD). This allows maps of the ion strike effects to be produced, effectively removing the need for a microbeam. Irradiation in air and even the use of alpha particle sources with no accelerator are possible. Potential applications include ion beam induced charge collection studies of semiconducting and insulating materials, single event upset studies on microchips and even biological cells in radiobiological effectiveness experiments. We describe the IPEM setup, including a 60× OM-40 microscope with a 1.5 mm hole for the beam transmission and a Quantar PSD with 60 μm pixel. Bicron plastic scintillator blades of 10 μm were chosen as a phosphor for their nanosecond time resolution, homogeneity, utility and commercial availability. The results given in this paper are for a prototype IPEM system. They indicate a resolution of ∼12 μm, the presence of a spatial halo and a He-ion efficiency of ∼20%. This marks the first time that nuclear microscopy has been performed with a radioactive source.

Ion-induced emission microscopies

Abstract New emission-based MeV nuclear microscopies have been in the process of development for the past four years. These techniques all fall under the heading of ion-induced emission microscopy (I-IEM), and the first to be developed was ion-electron emission microscopy (IEEM). With I-IEM the ion beam is not focused, but instead, secondary particles emitted when a single-ion strikes the sample are projected at great magnification onto a high efficiency single particle detector generating position signals. These X and Y signals are then put into coincidence with other signals made by this same ion in a fashion completely analogous to traditional nuclear microprobe analysis. In this paper, we update the current state of I-IEMs, which currently includes IEEM and highly charged ion-secondary ion mass spectroscopy (HCI-SIMS or IIEM) and ion-photon emission microscopy. At the present time none of these microscopies have atomic resolution, but the potential exists for resolution adequate for many scientific and nanotechnology applications.

Ionoluminescence decay measured with single ions

A new ion beam analysis-based, single ion technique called the time to first photon has been developed to measure the decay of the luminescence signal of phosphors. Such measurements are currently needed to study luminescence decay mechanisms following high-density excitations and to identify strongly luminescent phosphor coatings with short lifetimes for ion photon emission microscopy (IPEM). The samples for this technique consist of thin phosphor layers placed or coated on the surface of PIN diodes. Single ions from an accelerator strike this sample and simultaneously create ion beam induced luminescence (IBIL) from the phosphor that is measured by a single-photon-detector, and an ion beam induced charge collection (IBICC) signal in the PIN diode. In this case, the IBICC signal provides the start pulse and the IBIL signal the stop pulse to a time to amplitude converter. It is straightforward to show that this approach also measures a signal proportional to activity versus time with an accuracy of 5% as long as the number of detected photons per ion is less than 0.1, which usually requires the use of absorbers for the IBIL detector or electronic discrimination for the IBIL signals. Details of the new analysis are given together with examples of luminescence decay measurements of several ceramic phosphors being considered to coat IPEM samples. IPEM is currently being developed at Sandia National Laboratory (SNL), the University of North Texas in Denton, and the Universities and INFN of Padova and Torino.

Luminescent layers for ion-photon emission microscopy

Ion beam induced luminescence (IBIL) combined with ion beam induced charge collection (IBICC) is applied in a quantitative study of the IBIL generation yield and detection efficiency for several plastic phosphor materials. The main purpose of this study is to search for strongly luminescence materials that can be used to easily coat samples to be studied with ion-photon emission microscopy (IPEM). A special focus is given to plastic scintillation materials because thin films are easily prepared, and such films have already been used for single event triggering. The emission yield was found to be low for typical Bicron plastic phosphors (only ∼70 photons/ion/μm). The total collection, transmission and photon detection efficiency of the optical microscope used in this study was determined to be only ∼0.00003. For thin film plastic phosphors ∼20 μm thick, the detection efficiency was only 0.04 photons/ion. This means that using these plastics, IPEM would need to be performed with ∼20× more beam fluence to obtain data, such as IBICC, similar to a standard scanned nuclear microprobe. Improvements are discussed.

Nuclear emission microscopies

Alternatives to traditional nuclear microprobe analysis (NMA) emerged two years ago with the invention of ion electron emission microscopy (IEEM). With nuclear emission microscopy (NEM) the ion beam is only partially focused so as to fill the field of view of a special emission particle microscope system fitted with a single particle position sensitive detector (PSD). When a single ion strikes the sample, the emitted secondaries (e.g. electrons, photons, ions, etc.) are projected at great magnification onto this PSD where position signals are generated. These X and Y signals are then put into coincidence with other signals made by this same ion in a fashion completely analogous to traditional nuclear microprobe analysis. In this paper, an update will be given on the state of NEMs, which currently includes IEEM and highly charged ion–secondary ion mass spectroscopy (HCI–SIMS). In addition, a new type of full-field nuclear imaging is proposed: ion photon emission microscopy or IPEM.