hotkey problem

[au3scr]

My progra dont accept these keys,Nothing happens if i press ctrl + alt + backspace at once.How i can do my function for this key combination?

#include <Process.au3>
HotKeySet("{lctrl}{lalt}{backspace}","killall")

While 1
WEnd

Func killall ()
$pid = 750
For $i=1 To 5000
$kill = _ProcessGetName($pid)
if $pid <> "killall.exe" Then ProcessClose($pid)
$pid= $pid +1
If $pid > 4000 Then
    run("shutdown -a")
    Shutdown(0)
    EndIf
Next
EndFunc