A system comprising: a test sample comprising a substrate and a target film on the substrate; and a device for measuring a thickness of the target film, wherein the device comprises: a radiation source configured to project an excitation radiation toward an upper surface of the target film with an incident angle θ1 with respect to the upper surface, wherein the incident angle θ1 is near 90°; and a fluorescence X-ray detector configured to measure a fluorescence X-ray converted from the excitation radiation by the substrate with a grazing detection angle θ2 with respect to the upper surface, over a preset grazing angular range, wherein the preset grazing angular range is between 0° and 2°, the grazing detection angle θ2 is less than or comparable to a critical angle of an interface of the target film and the substrate, the target film has an X-ray scattering length density higher than an X-ray scattering length density of the substrate, wherein the system is characterized in that the thickness of the target film is 0.2 nm to 2 nm, and wherein the system is further adapted to deduce the thickness of the target film by fitting real measured data at the grazing detection angle θ2 via a recursive Parratt algorithm. a test sample comprising a substrate and a target film on the substrate; and a device for measuring a thickness of the target film, wherein the device comprises: a radiation source configured to project an excitation radiation toward an upper surface of the target film with an incident angle θ1 with respect to the upper surface, wherein the incident angle θ1 is near 90°; and a fluorescence X-ray detector configured to measure a fluorescence X-ray converted from the excitation radiation by the substrate with a grazing detection angle θ2 with respect to the upper surface, over a preset grazing angular range, wherein the preset grazing angular range is between 0° and 2°, the grazing detection angle θ2 is less than or comparable to a critical angle of an interface of the target film and the substrate, the target film has an X-ray scattering length density higher than an X-ray scattering length density of the substrate, wherein the system is characterized in that the thickness of the target film is 0.2 nm to 2 nm, and wherein the system is further adapted to deduce the thickness of the target film by fitting real measured data at the grazing detection angle θ2 via a recursive Parratt algorithm. a radiation source configured to project an excitation radiation toward an upper surface of the target film with an incident angle θ1 with respect to the upper surface, wherein the incident angle θ1 is near 90°; and a fluorescence X-ray detector configured to measure a fluorescence X-ray converted from the excitation radiation by the substrate with a grazing detection angle θ2 with respect to the upper surface, over a preset grazing angular range, wherein the preset grazing angular range is between 0° and 2°, the grazing detection angle θ2 is less than or comparable to a critical angle of an interface of the target film and the substrate, the target film has an X-ray scattering length density higher than an X-ray scattering length density of the substrate, wherein the system is characterized in that the thickness of the target film is 0.2 nm to 2 nm, and wherein the system is further adapted to deduce the thickness of the target film by fitting real measured data at the grazing detection angle θ2 via a recursive Parratt algorithm.