Earnings Per Share & Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to Amtech Systems, Inc.	$ (8,953)	$ 4,971	$ 2,835	$ 6,452	$ 7,317	$ 3,287	$ (1,420)	$ (53)	$ 5,305	$ 9,131	$ (7,008)
Weighted average shares outstanding (in shares)	14,730	14,925	14,891	14,781	13,895	13,242	13,188	13,179	14,833	13,378	13,168
Common stock equivalents (in shares)									232	123	0
Weighted average shares outstanding, diluted shares (in shares)	14,730	15,091	15,154	15,298	14,294	13,398	13,188	13,179	15,065	13,501	13,168
Basic income (loss) per share attributable to Amtech shareholders (dollars per share)	$ (0.61)	$ 0.33	$ 0.19	$ 0.44	$ 0.53	$ 0.25	$ (0.11)	$ 0.00	$ 0.36	$ 0.68	$ (0.53)
Diluted income (loss) per share attributable to Amtech shareholders (dollars per share)	$ (0.61)	$ 0.33	$ 0.19	$ 0.42	$ 0.51	$ 0.25	$ (0.11)	$ 0.00	$ 0.35	$ 0.68	$ (0.53)
Stock Options and Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of earnings per share									434	1,364	1,840