Imaging device having two anti-reflection layers on a surface of silicon wafer

A method of making an imaging device having improved quantum efficiency in the range of 400 nanometers to 500 nanometers, comprising the steps of:

• preparing a silicon wafer of a first-type conductivity, said wafer having first and second major surfaces;
• forming a plurality of regions of a second-type conductivity in the second major surface of said wafer;
• implanting an element into the first major surface of said wafer so as to form a region of enhanced first-type conductivity, spaced from the first surface, for controlling blooming;
• forming a shallow region of second-type conductivity extending into said wafer from the first surface a distance less than said region of enhanced first-type conductivity;
• providing a first anti-reflection layer on the first surface of said wafer by depositing thereon, at ambient temperature, zinc sulfide having an index of refraction ranging from about 2.3 to 2.4, said first anti-reflection layer having a metric thickness of about 450 .ANG. and an optical thickness substantially equal to a quarter of the wavelength of light incident thereon;
• providing a second anti-reflection layer on said first anti-reflection layer by depositing thereon, at ambient temperature, a material selected from the group consisting of magnesium fluoride and cryolite and having an index of refraction ranging from about 1.32 to 1.35, said second anti-reflection layer having a metric thickness of about 850 .ANG. and an optical thickness substantially equal to a quarter of the wavelength of light incident thereon;
• and
• limiting the temperature of the device to about 125.degree. C. to prevent an increase in the index of refraction of said second anti-reflection layer.