An IR sensor comprises a heat sink substrate (10) having portions (12) of relatively high thermal conductivity and portions (14) of relatively low thermal conductivity and a planar thermocouple layer (16) having a hot junction (18) and a cold junction (20), with the hot junction (18) located on a portion (14) of the heat sink substrate with relatively low thermal conductivity. A low thermal conductivity dielectric layer (22) is provided over the thermocouple layer (16), and has a via (24) leading to the hot junction (18). An IR reflector layer (26) covers the low thermal conductivity dielectric layer (22) and the side walls of the via (24). An IR absorber (30; 30′) is within the via. This structure forms a planar IR microsensor which uses a structured substrate and a dielectric layer to avoid the need for any specific packaging. This design provides a higher sensitivity by providing a focus on the thermocouple, and also gives better immunity to gas conduction and convection.