A fluid ejection device comprising: a membrane (204, 304) including: a first column (206, 306) of firing chambers (202, 302); a second column (208, 308) of firing chambers; and a portioning wall (210, 310), wherein the portioning wall physically separates the first column of firing chambers from the second column of firing chambers; a plurality of actuators (220, 320), wherein an actuator of the plurality of actuators is provided in each of the firing chambers; and a substrate (230, 330) including a respective hole extending through the substrate from each of the firing chambers, wherein the firing chambers in the first column of firing chambers are physically separated from adjacent firing chambers in the first column of firing chambers by side walls (212, 312) and wherein the firing chambers in the second column of firing chambers are physically separated from adjacent firing chambers in the second column of firing chambers by side walls, and wherein the membrane, including the side walls, includes an epoxy-based negative photoresist; characterised in that the fluid ejection device further comprising: a fluid feed slot to supply fluid to the firing chambers (202, 302), wherein holes extending through the substrate (230, 330) comprise fluid feed holes that are in fluid communication with the fluid feed slot, wherein the substrate (230, 330) comprises a thickness that is between about 50 microns and about 150 microns. a membrane (204, 304) including: a first column (206, 306) of firing chambers (202, 302); a second column (208, 308) of firing chambers; and a portioning wall (210, 310), wherein the portioning wall physically separates the first column of firing chambers from the second column of firing chambers; a first column (206, 306) of firing chambers (202, 302); a second column (208, 308) of firing chambers; and a portioning wall (210, 310), wherein the portioning wall physically separates the first column of firing chambers from the second column of firing chambers; a plurality of actuators (220, 320), wherein an actuator of the plurality of actuators is provided in each of the firing chambers; and a substrate (230, 330) including a respective hole extending through the substrate from each of the firing chambers, wherein the firing chambers in the first column of firing chambers are physically separated from adjacent firing chambers in the first column of firing chambers by side walls (212, 312) and wherein the firing chambers in the second column of firing chambers are physically separated from adjacent firing chambers in the second column of firing chambers by side walls, and wherein the membrane, including the side walls, includes an epoxy-based negative photoresist; characterised in that the fluid ejection device further comprising: a fluid feed slot to supply fluid to the firing chambers (202, 302), wherein holes extending through the substrate (230, 330) comprise fluid feed holes that are in fluid communication with the fluid feed slot, wherein the substrate (230, 330) comprises a thickness that is between about 50 microns and about 150 microns. a fluid feed slot to supply fluid to the firing chambers (202, 302), wherein holes extending through the substrate (230, 330) comprise fluid feed holes that are in fluid communication with the fluid feed slot, wherein the substrate (230, 330) comprises a thickness that is between about 50 microns and about 150 microns.