The present invention relates to a liquid concentration detecting apparatus which permits real-time and high-precision detection of concentration of a liquid used, for example, in a semiconductor plant, particularly for an etching or cleaning solution, and among others, a fluoric acid-based etching or cleaning solution.
In a semiconductor plant, for example, in order to perform the etching or cleaning of Al, Si or SiO.sub.2 as in a semiconductor process or an LSI producing process there are used aqueous solution of various substances such as hydrochloric acid, nitric acid, alkaline etching agents, chromic acid-based etching agents, phosphoric acid, ammonium hydroxide, hydrogen perioxide solution, and mixed water/organic liquid solution (for example, aqueous acetic acid solution), further including such etching or cleaning solutions (hereinafter, generically referred to as "etching solution(s)" as hydrofluoric acid (HF), buffered hydrofluoric acid (BHF), fluoronitric acid and sulfuric acid (H.sub.2 SO.sub.4). It is required to control the concentration of these etching solutions to .+-.0.1% in the case where the concentration of the etching solutions is 0 to 10%, and to .+-.0.01% in the case where the concentration of the etching solutions is 0 to 1%, with a view to maintaining etching performance.
As is disclosed in Japanese Patent Application Laid-Open No. H07-113,745, the present inventors have proposed a concentration detecting apparatus for an aqueous solution containing an inorganic chemical such as hydrofluoric acid. This apparatus comprises, as shown in FIG. 13, a detecting tube 200 running through partition walls 201 of a tubular main body 100, and a projecting section 202 and a receiving section 203 provided opposite to each other with the detecting tube 200 in between. As the apparatus detects water by a wavelength region near a wavelength of 1.48 .mu.m, the projecting section 202 has a light source 204 emitting near infrared rays in a specific wavelength zone absorbed by water, i.e., a light within a wavelength range of from 0.75 .mu.m to 2.5 .mu.m. An optical detector 205 for detecting a light having passed through the aqueous solution containing the organic chemical flowing in the detecting tube 200 from the projecting section 204 is arranged in the receiving section 203. The amount of light detected by the optical detector 205 is converted into an electric signal via a detecting circuit and a control section, and displayed on a display as a quantity of water or a concentration in this aqueous solution, or printed in a printer for output.
In the concentration detecting apparatus having the foregoing configuration, however, a light-emitting diode (LED) having an emitting wavelength region within a range of from 1.1 to 1.6 .mu.m or a semiconductor laser diode, within a range of from 1.1 to 1.67 .mu.m is used as the light source 204 of the projecting section 202, and a cylindrical tube, as the detecting tube 200. It is therefore impossible to detect the concentration of an etching solution (amount of water in the solution) at a high accuracy as .+-.0.1%.
There is at present available no concentration detecting apparatus, as far as the present inventors know, which permits inline real-time detection, which is a main object of the present invention, of the concentration of an etching solution such as hydrofluoric acid, sulfuric acid, fluoronitric acid or buffered hydrofluoric acid at an accuracy of .+-.0.1%.
As a result of extensive research and experimental efforts, the present inventors obtained the following findings. That is, it is possible to measure the concentration of an etching solution at a high accuracy from absorbance of the etching solution, by irradiating a light having a wavelength within the near infrared region ensuring clear occurrence of light absorption by the solution, and adopting a special configuration of the detecting section as described later.
The present invention has therefore an object to provide a low-cost liquid concentration detecting apparatus having a simple construction, which permits real-time detection at a high accuracy of the concentration of a liquid used in a semiconductor plant or the like, particularly the concentration of an etching solution (hydrofluoric acid-based etching solution among others).