Thermal flow sensor

A thermal flow sensor comprising:

• a base member including a first principal surface extending in a first direction and a second direction perpendicular to the first direction;
• a cover fixed to the first principal surface;
• and
• a flow passage, wherein
• the base member includes a heater,
• the cover is formed by a silicon on insulator substrate including a silicon substrate, a silicon dioxide film provided on the silicon substrate, and a silicon film provided on the silicon dioxide film,
• the silicon film has a recessed portion defined therein, the recessed portion having a bottom surface defined by an exposed surface of the silicon dioxide film which is exposed from the silicon film, the recessed portion having a side surface defined by the silicon film,
• the flow passage includes a main flow passage portion extending along the first principal surface, and defined by the exposed surface of the silicon dioxide film which defines the bottom surface of the recessed portion, the silicon film defining the side surface of the recessed portion, and the first principal surface,
• in a plan view of the first principal surface, the main flow passage portion has a first longitudinal direction parallel to the first direction,
• the cover has an inlet and an outlet of the flow passage defined therein, and
• the heater is opposed to the main flow passage portion in a third direction perpendicular to both the first direction and the second direction.