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. <EOS>