Each Chief State's Attorney, deputy chief state's attorney and state's attorney who has completed not less than ten years of service as a prosecutorial official, shall receive semiannual longevity payments based on such service completed as of the first day of July and the first day of January of each year, as follows:
(1) Each Chief State's Attorney, deputy chief state's attorney or state's attorney who has completed ten or more years but less than fifteen years of service shall receive one-quarter of three per cent of the annual salary of his office.
(2) Each Chief State's Attorney, deputy chief state's attorney or state's attorney who has completed fifteen or more years but less than twenty years of service shall receive one-half of three per cent of the annual salary of his office.
(3) Each Chief State's Attorney, deputy chief state's attorney or state's attorney who has completed twenty or more years but less than twenty-five years of service shall receive three-quarters of three per cent of the annual salary of his office.
(4) Each Chief State's Attorney, deputy chief state's attorney or state's attorney who has completed twenty-five or more years of service shall receive three per cent of the annual salary of his office.