J. David Casey

Gas-assisted etching with focused ion beam technology

Abstract The role of focused ion beam (FIB) systems in device failure analysis has been well documented 1 . FIB etching is used to cross-section features and to prepare TEM samples with better than 0.1 micron (μm) accuracy 2 , to open probe holes for mechanical and electron beam probing of circuitry, and to rewire circuits by cutting or adding connections. FIB is used to image cross sections with either secondary ions or secondary electrons, to measure metal grain size distributions through channeling contrast, and to measure process control parameters. System limitations include a relatively slow removal rate by beam sputtering of large volumes of material. Also, redeposition of sputtered material on the sidewalls of holes limits the achievable sidewall angle, and thus limits the aspect ratios of holes to approximately 6:1. With decreasing geometry sizes and multiple planarized metal layers, device modifications require cuts and interconnect holes of up to 10:1 aspect ratio, which up to now have been difficult with FIB. We will discuss recent progress in using gas-assisted etching (GAE) to enhance the FIB etching, to sharpen the etch profile, and to etch high aspect ratio holes. The hardware and GAE process will be briefly described. We will discuss the relative etch rates and selectivity of GAE with typical device materials, such as aluminum (Al), tungsten (W), silicon dioxide (SiO 2 ) and silicon (Si), using two halogen-based etchant gases, xenon difluoride (XeF 2 ) and chlorine (Cl 2 ). Images of sidewall profiles with and without GAE will be compared, and we will demonstrate several applications of GAE for e-beam and optical analysis.

Chemically enhanced FIB repair of opaque defects on molybdenum silicide photomasks

The characteristics of an ideally repaired opaque defect on a molybdenum silicide (MoSiaObNc) photomask are: (1) the total removal of the MoSiaObNc defect, leaving no residual MoSiaObNc; (2) a smooth, level quartz surface (no over-etch) after the MoSiaObNc is removed; (3) minimal riverbedding of the quartz at the perimeter of the MoSiaObNc defect; and (4) maximum light transmission (%T) at the i-line (365 nm) and DUV (248 nm) lithographic wavelengths. Achieving these ideal repair characteristics is becoming increasingly difficult as the patterned features become smaller, as the lithographic wavelength becomes shorter and as phase shifting mechanisms are implemented. A chemical process has been developed to enhance the FIB (focused ion beam) etching of MoSiaObNc defects. Using this chemical process, a FIB protocol has been developed which enhances the removal of a MoSiaObNc defect while inhibiting the removal of quartz. AFM (atomic force microscopy) indicates that (1) MoSiaObNc is totally removed, (2) the quartz remains smooth and level (no over-etch), and (3) the riverbends are, at this time, 10 - 45 nm; our target is 1 - 15 nm. The MoSiaObNc etch process reduces optical staining due to implanted gallium

Focused ion beam induced deposition of low-resistivity copper material

Focused ion beam (FIB) induced processes for material etching and deposition have proven successful in integrated circuit device modification applications. Current FIB metal deposition processes are typically limited to resistivities in the range of 150–200μΩcm due to included impurities; however, today’s high-frequency devices require very low interconnect resistivity. The organometallic precursor material copper (I) hexafluoroacetylacetonate trimethylvinylsilane, or Cu(hfac)TMVS for FIB-assisted metal deposition was investigated. 50kV Ga+ ions were scanned over a defined area of an Al∕SiO2 resistivity test substrate in the presence of the precursor vapor, using two different 50kV FIB column designs with beam currents from 49to2070pA and current densities of 13–36Acm−2. Resistivity was measured by the four-point probe method. This study verifies prior reported resistivities of ⩽50μΩcm at room T across all deposition parameters for film growth yields ⩽0.18μm3nC−1 ion dose. Depositing on a heated substrate...

Focused ion beam deposition of new materials: dielectric films for device modification and mask repair and tantalum films for x-ray mask repair

Two processes have been developed to enable both focused ion beam (FIB) repair of advanced masks and FIB device modification. Silicon dioxide- based films can be deposited by rastering a focused ion beam across a surface onto which a combination of siloxane and oxygen gases have been adsorbed. The deposited material exhibits sufficient dielectric strength to be used for FIB modification of devices. Applications of FIB dielectric deposition include: (1) Local passivation. (2) Backfilling vias to allow for probing buried metal layers without contacting exposed metal layers. (3) Electrical isolation between crossed metal lines. (4) Optically transparent films for phase shift mask repair. In the first half of this paper we discuss the gas delivery system, and the material and electrical characteristics of the films, as well as describing typical device modifications using FIB dielectric films. In the second half of the paper we describe a process for deposition of tantalum- containing films using a tantalum-based organometallic precursor for repair of clear defect on X-ray masks. Although FIB gold films are adequate for repair of gold-absorber, silicon-membrane X-ray masks, gold films are not acceptable in the fab line, and tantalum is preferred for repair of either tungsten or tantalum absorber X-ray masks.

Chemically enhanced focused-ion-beam (FIB) repair of opaque defects on chrome photomasks

The characteristics of an ideally repaired opaque defect on a chrome (Cr) photomask are: (1) the total removal of the Cr defect, leaving no residual Cr; (2) a smooth, level quartz surface (no over-etch) after the Cr is removed; (3) minimal riverbedding of the quartz at the perimeter of the Cr defect and (4) maximum light transmission (%T) at the lithographic wavelength. Achieving these ideal repair characteristics is becoming increasingly difficult as the patterned features become smaller, as the lithographic wavelength becomes shorter and as phase shifting mechanisms are implemented. A chemical process has been developed to enhance the FIB (focused ion beam) etching of Cr defects. This chemical process enhances the FIB removal of a Cr defect 2.0 - 2.2 fold while inhibiting the removal of quartz by 60 - 80%. AFM (atomic force microscopy) indicates that (1) Cr is totally removed, (2) the quartz remains smooth and level (no over-etch) and (3) the riverbeds are 5 - 25 nm. If necessary, a second FIB-induced chemical process is used following the chrome etch process to reduce optical staining due to implanted gallium (a gallium ion beam is used in commerical FIB systems) such that the %T of the repaired areas at i-line(365nm) and DUV(248nm) wavelengths is 95%. In general, this second process is required at 248 nm but not at 365 nm. AIMS evaluations indicate a critical dimension variation between repaired and reference patterns of 10% at 35% light intensity at UV and DUV wavelengths. In summary: a. an FIB etch process has been developed which repairs opaque Cr defects, b. a second FIB etch process removes implanted gallium so that the %T is above 95% at i-line (if neccessary) and DUV wavelengths; c. these two etch processes are done sequentially, while the defect is positioned under the FIB column (post treatment processes are not required); d. clear defects can also be repaired at the same time by FIB-induced deposition of opaque carbon. Keywords: Mask repair, opaque defects, chrome defects, FIB

Focused ion beam in-situ cross sectioning and metrology in lithography

We demonstrate an approach to cross-section and measure sub- 0.25 micrometer photoresist profiles in both a manual and an automated fashion. This approach includes the use of a focused ion beam (FIB) system to cut small trenches through photoresist lines, leaving a clean, vertical face to measure. We demonstrate the advantage of using this process over existing techniques in the semiconductor industry. A FIB can locally cross-section the photoresist, resulting in a side- wall that is comparable to that of a mechanical cleave. It can then measure the profile of the photoresist at multiple points using a 5 nm gallium probe. The system accomplishes the entire process inside one vacuum chamber with a limited number of steps. In contrast, when using a SEM to measure profiles, the sample must be mechanically cleaved outside of the vacuum chamber, potentially destroying the entire part and leaving a slightly distorted viewing face. Also, a SEM probe can cause swelling of the photoresist due to higher currents and penetration depths than a FIB probe and must therefore be used at low accelerating voltages. When operated at these low accelerating voltages, the SEM has degraded resolution with a spot size near 10 nm. A scanning probe microscope (SPM), on the other hand, can non-destructively measure profiles, but it is slow and less automated than the FIB or SEM. Unlike a FIB, the SPM lacks the ability to image the material transition directly beneath the photoresist. We also address concerns of sample damage, gallium contamination, and image quality.

Current focused-ion-beam repair strategies for opaque defects and clear defects on advanced phase-shifting masks

Over the past several years, advanced photolithography has moved from 0.35 micrometers technology to 0.25 micrometers as the standard. Soon the technology will move into the 0.18 micrometers generation. Due to the ever-shrinking feature sizes on advanced photolithographic masks, phase shifting technology has been incorporated to improve resolution on the exposed wafer. On such masks the minimum phase error and the maximum percent transmission must be dealt with. These requirements have challenged the ability to repair masks with opaque and clear defects. The Micrion focused ion beam system currently repairs opaque defects found on advanced phase shifting chrome and molybdenum silicide masks. In this paper, Micrion discusses advanced repair techniques and strategies used to address the stringent requirements of matching phase and percent transmission at the repaired defect sites. Difficulties in opaque defect and clear defect repair strategies will be discussed.

Edge-placement accuracy of opaque and clear defect repairs using focused ion beam technology

On the standard Micrion 8000 PM Repair System platform, the repair accuracy for clear defect repair and opaque defect repair is plus or minus 75 nm. Incorporation of a new ion beam column has pushed the repair accuracy for clear and opaque defect repairs to smaller values. This new system can image isolated defects less than 200 nm in size. To characterize the repair accuracy of the system, experiments on edge placement accuracy were performed. This paper presents data on the accuracy of defect repairs using the Micrion 8000 PSM Repair System on Chrome masks. The study specifically looks at the edge placement of opaque defect and clear defect repairs on masks coated with a conductive layer versus masks not coated with a conductive layer. We also explore the edge placement accuracy of the repair due to the directionality of the repair scan. Finally we examine the shape of the distribution function of the repair measurements and also investigate differences in the measured edge placement accuracy of repairs using different measuring techniques.