Research Disclosure no longer supports Internet Explorer

1 Company Secret A METHOD, DEVICE, CHARGED PARTICLE-OPTICAL APPARATUS AND AN ASSESSMENT APPARATUS BACKGROUND [0001] During manufacturing processes of, for example, semiconductor integrated circuit (IC) 5 chips or displays, undesired defects may occur on a substrate (e.g. wafer or a mask). Such defects may reduce yield. Defects may occur as a consequence of all kinds of processing necessary to produce an integrated circuit or display, for example, lithography, etching, deposition or chemical mechanical polishing. Defects may include patterning defects, in which the created pattern lies outside the pattern tolerance for the process, and particles. Monitoring the extent of defects during the manufacturing 10 processes is therefore important. Such monitoring (or more generally assessment) includes the determination of the existence of a defect, but also the classification of the types of defects found. [0002] For the assessment of a sample, different types of inspection or metrology systems have been used, including charged particle systems such as electron microscopes. Such assessment for inspection may relates to defects, for example the existence and classification of such defects. Electron 15 microscopes typically generate a probe beam (also often referred to as primary beam) which may, for example, be scanned across a part of the substrate (such as in a scanning electron microscopes (SEM)). Collecting interaction products that result from the interaction of the primary beam with the part of the substrate, allows the electron microscope to collect data representing the probed part of the substrate. The data may be processed/rendered for example by the electron microscope to generate an image 20 representation of the part of the substrate. The collected data for example as a generated image representation allows for measuring structures on the part of the substrate, or allows for identifying defective structures by comparing the image representation with a reference. Such measurement may be referred to as metrology; the identification of defective structures may be referred to as (defect) inspection. The interaction products may contain charged particles which may be referred to as signal 25 particles (e.g. signal electrons), such as secondary electrons and backscattered electrons, and may contain other interaction products, such as X-ray radiation and even light. [0003] A multi-beam formed of multiple primary beams, e.g. an array of beams, may be used simultaneously. Such use of a multi-beam means that a sample (i.e. any inspection target, such as the substrate) can be assessed more quickly than when using a single-beam, as the multi-beam can scan 30 different parts of the sample simultaneously. For example, a higher throughput of assessment may be achieved using a multi-beam assessment apparatus than a single beam apparatus. [0004] In multi-beam systems (e.g. multi-beam SEM systems), it is desired to have at all times each beam focused on the sample. To achieve that goal, the focal surface of the multi-beam system may be calibrated, i.e. to optimize the Z position of the focal point of each e-beam to be as close as 35 possible to a surface of the sample. To perform such a calibration, a known method is to measure the defocus between the beams and a reference sample surface, and to optimize the SEM parameters so that the defocus is minimized for all beams (i.e. to minimize the spread around zero). However, there may 2 Company Secret be inaccuracies in the focal point of each beam (i.e. a focal surface) relative to the surface of the sample. Thus, although some mechanisms for improving beam focus are known, the known mechanisms leave room for improvement. In particular, as advances are made to pattern smaller features on the substrate, it is even more important to determine data relating to the focal surface (and the sample surface) with a 5 high level of accuracy. [0005] It is an object of the present invention to provide a method and/or multi-beam charged particle-optical device (and/or an apparatus comprising the same) which at least partially addresses one or more of the problems of the prior art, whether identified herein or elsewhere. DETAILED DESCRIPTION 10 [0006] There is a trend in the semiconductor industry (often known as “Moore’s law”) to reduce the physical dimensions of structures representing circuit components on a substrate and/or to increase the packing density of such structures, in order to reduce the physical size of electronic devices and/or enhance the computing power of electronic devices. The physical dimensions of such structures may be reduced and/or the packing density of such structures may be increased by increasing lithographic 15 resolution. Manufacturing processes of semiconductor IC chips can have 100s of individual steps. An error in any step of the manufacturing process has the potential to adversely affect the functioning of the electronic device. It is desirable to improve the overall yield of the manufacturing process. For example, to obtain a 75% yield for a 50-step manufacturing process (where a step may indicate the number of layers formed on a substrate), each individual step must have a yield greater than 99.4%. If 20 an individual step has a yield of 95%, the overall yield of the manufacturing process would be as low as 7-8%. It is desirable to determine defects quickly so as to maintain a high substrate throughput, defined as the number of substrates processed per hour. [0007] Figure 1 is a schematic diagram illustrating an exemplary assessment apparatus 100, e.g. a metrology apparatus or an inspection apparatus. The assessment apparatus 100 may be configured 25 to scan a sample with one or more beams of electrons. The sample may be a semiconductor substrate, a substrate made of other material, or a mask, for example. The electrons interact with the sample and generate interaction products. The interaction products comprise signal electrons, e.g. secondary electrons and/or backscattered electrons, and possibly X-ray radiation. The assessment apparatus 100 may be configured to detect the interaction products from the sample so that a data set may be 30 generated which may be processable into an image or any other data representation of the scanned area of the sample. For clarity, the description below focuses on embodiments in which the interaction products that are detected are signal electrons. The assessment apparatus 100 may comprise, for example during operation, a single beam or a plurality of beams, i.e. a multi-beam. The component beams of a multi-beam may be referred to as sub-beams or beamlets. A multi-beam may 35 be used to scan different parts of a sample simultaneously. When the assessment apparatus 100 uses a multi-beam, the assessment apparatus 100 may assess a sample more quickly than when the 3 Company Secret assessment apparatus 100 uses a single-beam. For example, a higher throughput of sample assessment may be achieved using a multibeam assessment apparatus compared to a single beam apparatus. [0008] The assessment apparatus 100 of Figure 1 comprises a vacuum chamber 110, a load lock 5 chamber 120, an electron-optical apparatus 140, an equipment front end module (EFEM) 130 and a controller 150. The electron-optical apparatus 140 (also known as an electron beam apparatus or an electron apparatus) may be within the vacuum chamber 110. The electron-optical apparatus 140 may comprise an electron-optical device (described in more detail below) and an actuatable stage. It should be appreciated that reference in the description to the electron-optical elements of the electron- 10 optical apparatus 140 can be considered to be a reference to the electron-optical device. [0009] The EFEM 130 includes a first loading port 130a and a second loading port 130b. The EFEM 130 may include additional loading port(s). The first loading port 130a and the second loading port 130b may, for example, receive substrate front opening unified pods that contain samples. One or more robot arms (not shown) in the EFEM 130 transport the samples to the load lock chamber 120. 15 [00010] The load lock chamber 120 is used to remove the gas around a sample. The load lock chamber 120 may be connected to a load lock vacuum pump system (not shown), which removes gas particles in the load lock chamber 120. The operation of the load lock vacuum pump system enables the load lock chamber to reach a first pressure below the atmospheric pressure. The vacuum chamber 110, which may be a main chamber of the assessment apparatus 100, is connected to a main chamber 20 vacuum pump system (not shown). The main chamber vacuum pump system removes gas molecules from the vacuum chamber 110 so that the pressure around the sample reaches a second pressure equal to or lower than the first pressure. Different parts of the electron-optical apparatus 140 may have different levels of pressure below the atmospheric pressure. After reaching the required pressure, the sample leaves the load lock chamber 120 and is transported to the electron-optical apparatus 140 by 25 which it may be assessed. The electron-optical apparatus 140 may use either a single beam or a multi- beam for the assessment. Alternatively, an electron-optical device array comprising a plurality of electron-optical devices may be used, further also referred to as a multi-column electron-array, in which each electron-optical device (or each column in the multi-column array) comprises, for example during operation, either a single beam or a multi-beam. 30 [00011] The controller 150 is electronically connected to the electron-o...