Alternating current (AC) stress test circuit, method for evaluating AC stress induced hot carrier injection (HCI) degradation, and test structure for HCI degradation evaluation

1. An alternating current (AC) stress test circuit for hot carrier injection (HCI) degradation evaluation in semiconductor devices, comprising:

a ring oscillator circuit having a plurality of stages electrically connected in series between an input terminal and an output terminal electrically connected with the input terminal and whereby forming a loop, wherein each of the stages comprises a first node and a second node, a designated one of the stages serves as device under test (DUT);

a first isolating switch and a second isolating switch, respectively electrically connecting the first node and the second node of the DUT;

an input means being electrically connected to the first isolating switch and for applying a gate on voltage to the first node through the first isolating switch to bias the DUT; and

an output means being electrically connected to the second isolating switch and for measuring a current flowing from the second node through the second isolating switch.

2. The AC stress test circuit as claimed in claim 1 , wherein the DUT comprises a first transistor and a second transistor having different conductivity, the gates of the first and second transistors are electrically connected to the first node, the drains/sources of the first and second transistors are electrically connected to the second node, the first isolating switch and the second isolating switch both have the same conductivity as the second transistor.

3. The AC stress test circuit as claimed in claim 1 , wherein the DUT comprises a first transistor and a second transistor having different conductivity, the gates of the first and second transistors are electrically connected to the first node, the drains/sources of the first and second transistors are electrically connected to the second node, the first isolating switch and the second isolating switch both have the same conductivity as the first transistor.

4. The AC stress test circuit as claimed in claim 1 , wherein the first isolating switch comprises a first transistor and a second transistor having different conductivity and electrically connected in parallel and between the first node and the first pad, the second isolating switch comprises a third transistor and a fourth transistor having the different conductivity and electrically connected in parallel and between the second node and the second pad.

5. The AC stress test circuit as claimed in claim 1 , further comprising a control pad being electrically connected to the gates of the first and second isolating switches and for receiving a control signal to switch ON/OFF states of the first and second isolating switches.

6. The AC stress test circuit as claimed in claim 1 , wherein each of the stages is an inverter.

7. The AC stress test circuit as claimed in claim 1 , wherein the DUT and each of the input terminal and the output terminal have at least one of the remained stages arranged therebetween.

8. A method for evaluating AC stress induced HCI degradation in semiconductor devices, wherein the method is adapted to be performed on an AC stress test circuit, the AC stress test circuit comprises a ring oscillator circuit, a first pad, a second pad, a first isolating switch and a second isolating switch, the ring oscillator circuit having a plurality of stages electrically connected in series between an input terminal and an output terminal electrically connected with the input terminal and whereby forming a loop, each of the stages comprising a first node and a second node, a designated one of the stages serving as device under test (DUT), the first isolating switch and the second isolating switch respectively electrically connecting the first pad and the second pad to the first node and the second node of the DUT, the method comprises steps of:

during a stress mode of the AC stress test circuit, enabling the AC stress test circuit to apply an internal AC stress to the DUT while the first and second isolating switches both being switched-off; and

during a measurement mode of the AC stress test circuit, disabling the AC stress test circuit, switching on both the first and second isolating switches, applying a gate-on voltage to the first node through the first pad and measuring a current flowing from the second node through the second pad.

9. The method as claimed in claim 8 , wherein the steps of switching on both the first and second isolating switches and applying the gate-on voltage to the first node through the first pad comprise:

switching on both the first and second isolating switches by a logic “1” voltage; and

applying a logic “0” voltage to the first node through the first pad.

10. The method as claimed in claim 8 , wherein the steps of switching on both the first and second isolating switches and applying the gate-on voltage to the first node through the first pad comprise:

switching on both of the first and second isolating switches by a logic “0” voltage; and

applying a logic “1” voltage to the first node through the first pad.

11. A test structure for HCI degradation evaluation in semiconductor devices, the test structure comprising: an AC stress test circuit comprising: a ring oscillator circuit having a plurality of stages connected in series between an input terminal and an output terminal electrically connected with the input terminal and whereby forming a loop, wherein each of the stages comprises a first node and a second node, a designated one of the stages serves as a first DUT; a first probing pad and a second probing pad; and a first isolating switch and a second isolating switch, respectively electrically connecting the first probing pad and the second probing pad to the first node and the second node of the first DUT, wherein the first probing pad applies a gate on voltage to the first node through the first isolating switch to bias the first DUT and the second probing pad measures a current flowing from the second node through the second isolating switch; and a direct current (DC) stress test circuit independent from the AC stress test circuit, the DC stress test circuit comprising: a second DUT having a same circuit configuration as the first DUT and comprising a third node and a fourth node; and a third pad and a fourth pad, respectively electrically connected to the third node and the fourth node.

12. The test structure as claimed in claim 11 , wherein the first DUT comprises a first transistor and a second transistor having different conductivity, the gates of the first and second transistors are electrically connected to the first node, the drains/sources of the first and second transistors are electrically connected to the second node, the first isolating switch and the second isolating switch both have the same conductivity as the second transistor.

13. The test structure as claimed in claim 11 , wherein the first DUT comprises a first transistor and a second transistor having different conductivity, the gates of the first and second transistors are electrically connected to the first node, the drains/sources of the first and second transistors are electrically connected to the second node, the first isolating switch and the second isolating switch both have the same conductivity as the first transistor.

14. The test structure as claimed in claim 11 , wherein the first isolating switch comprises a second transistor and a first transistor having different conductivity and electrically connected in parallel and between the first node and the first pad, the second isolating switch comprises a fourth transistor and a third transistor having the different conductivity and electrically connected in parallel and between the second node and the second pad.

15. The test structure as claimed in claim 11 , wherein the AC stress test circuit further comprises a control pad being electrically connected to the gates of the first and second isolating switches and for receiving a control signal to switch ON/OFF states of the first and second isolating switches.

16. The test structure as claimed in claim 11 , wherein each of the stages is an inverter.

17. The test structure as claimed in claim 11 , wherein the DC stress test circuit further comprises:

a fifth pad and a sixth pad; and

a third isolating switch and a fourth isolating switch, respectively electrically connecting the fifth pad and the sixth pad to the third node and the fourth node;

wherein the third and fourth isolating switches both are always switched-off during the DC stress test circuit in operation.

18. The test structure as claimed in claim 11 , wherein the first DUT and each of the input terminal and the output terminal have at least one of the remained stages arranged therebetween.