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, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to Amtech Systems, Inc.	$ 7,317	$ 3,287	$ (1,420)	$ (53)	$ (285)	$ (1,209)	$ (1,499)	$ (4,015)	$ 9,131	$ (7,008)	$ (7,771)
Weighted average shares outstanding (in shares)	13,895	13,242	13,188	13,179	13,177	13,173	13,169	13,152	13,378	13,168	12,022
Basic loss per share attributable to Amtech shareholders (dollars per share)	$ 0.53	$ 0.25	$ (0.11)	$ 0.00	$ (0.02)	$ (0.09)	$ (0.11)	$ (0.31)	$ 0.68	$ (0.53)	$ (0.65)
Weighted average shares outstanding, common stock equivalents (in shares)									123	0	0
Weighted average shares outstanding, diluted shares (in shares)	14,294	13,398	13,188	13,179	13,177	13,173	13,169	13,152	13,501	13,168	12,022
Diluted loss per share attributable to Amtech shareholders (dollars per share)	$ 0.51	$ 0.25	$ (0.11)	$ 0.00	$ (0.02)	$ (0.09)	$ (0.11)	$ (0.31)	$ 0.68	$ (0.53)	$ (0.65)
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									1,364	1,840	1,640