Method of etching a magnesium oxide film

1. A method of manufacturing a magnetoresistive device, the magnetoresistive device including: a first electrode and a second electrode located at a distance from each other; a magnetoresistive element including a metal layer and disposed between the first electrode and the second electrode; and an insulating portion in contact with the magnetoresistive element, wherein

the magnetoresistive element has a first surface in contact with the first electrode, a second surface in contact with the second electrode, and a third surface connecting the first and second surfaces to each other,

the metal layer includes the second surface, and

the insulating portion is made of magnesium oxide,

the method comprising the steps of:

forming the first electrode;

forming a magnetoresistive film on the first electrode, the magnetoresistive film becoming the magnetoresistive element later and having a top surface including a pre-second-surface-portion that later becomes the second surface;

patterning the magnetoresistive film so that the magnetoresistive film is provided with at least part of the third surface;

forming the insulating portion from magnesium oxide such that the insulating portion is in contact with the at least part of the third surface;

removing an unwanted magnesium oxide film that has been formed on the pre-second-surface portion by the magnesium oxide in the step of forming the insulating portion; and

forming the second electrode after the step of removing the unwanted magnesium oxide film, wherein

in the step of removing the unwanted magnesium oxide film, the unwanted magnesium oxide film is wet etched by using an etchant containing an aqueous ammonia solution.

2. The method according to claim 1 , wherein the etchant further contains ammonium persulfate.

3. The method according to claim 2 , wherein the etchant has a pH in the range of 9.3 to 9.6.

4. The method according to claim 1 , wherein

the magnetoresistive element includes a first magnetic layer, a second magnetic layer, and a magnesium oxide layer,

the first magnetic layer is located between the first surface and the magnesium oxide layer,

the second magnetic layer is located between the second surface and the magnesium oxide layer,

the magnesium oxide layer has an end located in the third surface, and

the insulating portion is in contact with the end of the magnesium oxide layer.

5. The method according to claim 1 , wherein

the magnetoresistive device is for use in a thin-film magnetic head having a medium facing surface facing a recording medium, and

the third surface of the magnetoresistive element includes a front end face located in the medium facing surface, a rear end face opposite to the front end face, and a first side surface and a second side surface opposite to each other in a track width direction.

6. The method according to claim 5 , wherein

in the step of patterning the magnetoresistive film, an etching mask is formed on the magnetoresistive film and the magnetoresistive film is dry etched by using the etching mask to thereby provide the magnetoresistive film with the first and second side surfaces, in the step of forming the insulating portion, the insulating portion is formed to be in contact with the first and second side surfaces with the etching mask left unremoved, and

in the step of removing the unwanted magnesium oxide film, the unwanted magnesium oxide film is wet etched after the etching mask is removed.

7. The method according to claim 6 , wherein the etching mask includes a first portion located above the pre-second-surface portion, and a second portion that is in contact with a part of the top surface of the magnetoresistive film other than the pre-second-surface portion and supports the first portion.

8. The method according to claim 6 , wherein

the magnetoresistive device further includes a first bias magnetic field applying layer and a second bias magnetic field applying layer that apply a bias magnetic field to the magnetoresistive element,

the first bias magnetic field applying layer is adjacent to the first side surface with the insulating portion interposed therebetween,

the second bias magnetic field applying layer is adjacent to the second side surface with the insulating portion interposed therebetween,

each of the first and second bias magnetic field applying layers has a top surface and includes a metal layer made of a magnetic alloy and exposed in the top surface, and

the method further comprises the step of forming the first and second bias magnetic field applying layers.

9. The method according to claim 5 , wherein

in the step of patterning the magnetoresistive film, an etching mask is formed on the magnetoresistive film and the magnetoresistive film is dry etched by using the etching mask to thereby provide the magnetoresistive film with the rear end face,

in the step of forming the insulating portion, the insulating portion is formed to be in contact with the rear end face with the etching mask left unremoved, and

in the step of removing the unwanted magnesium oxide film, the unwanted magnesium oxide film is wet etched after the etching mask is removed.

10. The method according to claim 9 , wherein the etching mask has an undercut.